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Special Sensory
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O
f all the ancient civilizations, it was the Greeks who led the
way in understanding the body and its workings. One of the earliest
Greek anatomists was Alcmaeon of Croton (c. 500 BC). Through his
animal dissections, Alcmaeon came to recognize many structures and
was the first to mention the eye in his writings. He described the
optic nerve and decided that three things were necessary for vision—
external light, the “fire” in the eye (he assumed there must be fire in
the eye because a blow to the eye produces sparks,
or stars), and the liquid in the eyeball.
The Greeks also developed early surgical procedures, among them
techniques for the removal of cataracts. However, it was the Roman
encyclopedist, Aulus Cornelius Celsus (1st century AD), whose most
important surviving works are concerned with medicine, who provided a vivid description of the procedure:
The needle is to be sharp enough to penetrate, yet not too fine;
and this to be inserted straight through . . . at a spot between the
pupil of the eye and the angle adjacent to the temple, away from the
middle of the cataract, in such a way that no vein is wounded. The
needle, however, should not be inserted timidly. When the spot is
reached, the needle is to be sloped against the colored area [lens]
itself and rotated gently, guiding it little by little below the pupil;
when the cataract has passed below the pupil, it is pressed upon
more firmly in order that it may settle below.
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CHAPTER
54
Disorders of Visual Function
Edward W. Carroll
Carol M. Porth
Robin L. Curtis
DISORDERS OF THE ACCESSORY STRUCTURES
OF THE EYE
Disorders of the Eyelids
Eyelid Weakness
Eyelid Inflammation
Disorders of the Lacrimal System
Dry Eyes
Dacryocystitis
DISORDERS OF THE CONJUNCTIVA, CORNEA,
AND UVEAL TRACT
Disorders of the Conjunctiva
Allergic Conjunctivitis
Infectious Conjunctivitis
Ophthalmia Neonatorum
Disorders of the Cornea
Corneal Trauma
Keratitis
Abnormal Corneal Deposits
Corneal Transplantation
Disorders of the Uveal Tract
Uveitis
The Pupil and Pupillary Reflexes
INTRAOCULAR PRESSURE AND GLAUCOMA
Control of Intraocular Pressure
Glaucoma
Angle-Closure Glaucoma
Open-Angle Glaucoma
Congenital and Infantile Glaucoma
DISORDERS OF THE LENS AND LENS FUNCTION
Disorders of Refraction and Accommodation
Disorders of Refraction
Disorders of Accommodation
Cataracts
Causes and Types of Cataracts
Manifestations
Diagnosis and Treatment
DISORDERS OF THE VITREOUS AND RETINA
Disorders of the Vitreous
Disorders of the Retina
The Neural Retina
Photoreceptors
Disorders of Retinal Blood Supply
Retinopathies
Retinal Detachment
Macular Degeneration
DISORDERS OF NEURAL PATHWAYS AND
CORTICAL CENTERS
Optic Pathways
Visual Cortex
Visual Fields
Visual Field Defects
Disorders of the Optic Pathways
Disorders of the Visual Cortex
Testing of Visual Fields
DISORDERS OF EYE MOVEMENT
Extraocular Eye Muscles and Their Innervation
Eye Movements and Gaze
Strabismus
Paralytic Strabismus
Nonparalytic Strabismus
Diagnosis and Treatment
Amblyopia
A
bout 70% of all sensory receptors are in the eyes. The
photoreceptors sense and encode the patterns made
by light in our surroundings. Neural pathways carry
the coded information from the eyes to the brain where
the signals gain meaning, fashioning images of the world
around us.
Almost 17.3 million persons in the United States have
some degree of visual impairment; of these, 1.1 million
are legally blind.1 The prevalence of vision impairment
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Special Sensory Function
increases with age. An estimated 26% of persons 75 years of
age and older report visual impairment severe enough to interfere with recognizing a friend across the room or reading
newspaper print even when wearing glasses. At the other
end of the age spectrum, an estimated 95,100 children
younger than 18 years of age are severely visually impaired.1
Alterations in vision can result from disorders of the
eyelids and optic globe (conjunctiva, cornea, and uvea),
intraocular pressure (glaucoma), lens (cataract), vitreous
humor and retina, visual pathways and visual cortex, and
extraocular muscles and eye movement.
Disorders of the Accessory Structures
of the Eye
VISION
➤ Vision is a special sensory function that incorporates the
visual receptor functions of the eyeball, the optic nerve,
and visual pathways that carry and distribute sensory information from the optic globe to the central nervous system,
and the primary and visual association cortices that
translate the sensory signals into visual images.
➤ The eyeball is a hollow spherical structure that functions in
the reception of the light rays that provide the stimuli for
vision. The refractive surface of the cornea and accommodative properties of the lens serve to focus the light signals
from near and far objects on the photoreceptors in the
retina.
➤ Visual information is carried to the brain by axons of the
retinal cells that form the optic nerve. The two optic nerves
fuse in the optic chiasm, where axons of the nasal retina of
each eye cross to the contralateral side and travel with
axons of the ipsilateral temporal retina to form the fibers
of the optic radiations that travel to the visual cortex.
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ State the cause of eyelid weakness
✦ Define entropion and ectropion
✦ Explain the differences between marginal blepharitis, a
➤ Binocular vision depends on the coordination of three pairs
of extraocular nerves that provide for the conjugate eye
movements, with optical axes of the two eyes maintained
parallel with one another as the eyes rotate in their sockets.
hordeolum, and a chalazion as to causes and
manifestations
✦ State the causes and treatment of dry eye
The optic globe, commonly called the eyeball, is a remarkable, mobile, nearly spherical structure contained
in a pyramid-shaped cavity of the skull called the orbit
(Fig. 54-1). Only the anterior one fifth of the orbit is occupied by the eyeball; the remainder is filled with muscles,
nerves, the lacrimal gland, and adipose tissue that supports the normal position of the optic globe. Exposed surfaces of the eyes are protected by the eyelids, which are
mucous membrane–lined skin flaps that provide a means
for shutting out most light. Tears bathe the anterior sur-
face of the eye; they prevent friction between it and the
lid, maintain hydration of the cornea, and protect the eye
from irritation by foreign objects.
Three distinct layers form the wall of the eyeball: the
sclera or outer supporting layer, the choroid or middle vascular layer, and the retina, which is composed of the neuronal retinal layer and the outer pigmented layer (Fig. 54-2).
Retina
Choroid
Superior
rectus
Sclera
Bulbar and
palpebral
conjunctiva
Cornea
Levator
palpebrae
superioris
Lens
Superior tarsal
plate
Iris
Optic
nerve
Meibomian
gland in
tarsal plate
Inferior
rectus
Inferior
oblique
muscle
Orbicularis
oculi muscle
Ciliary body
FIGURE 54-1 Lateral view of the eye and its
appendages.
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Disorders of Visual Function
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Pupil
Anterior chamber
Iris
Cornea
Conjunctiva
Canal of Schlemm
Posterior chamber
Ciliary body
Lens
Cillary processes
Vitreous body
Retina
Choroid
Retinal
blood vessel
Optic
disk
Sclera
Lateral rectus
muscle
Medial rectus
muscle
Optic nerve
Fovea
centralis
FIGURE 54-2 Transverse section of the eyeball.
The outer layer of the eyeball consists of a tough, opaque,
white, fibrous layer called the sclera. It is strong yet elastic,
and maintains the shape of the globe. The sclera is continuous with the cornea anteriorly and with the cranial
dural sheath that surrounds and protects the optic nerve
posteriorly.
DISORDERS OF THE EYELIDS
The upper and lower eyelids, the palpebrae, are modified
folds of skin that protect the eyeball. The palpebral fissure
is the oval opening between the upper and lower eyelids.
At the corners of the eye, where the upper and lower lids
meet, is an angle called the canthus; the lateral canthus is
the outer, or temporal, angle, and the medial canthus is the
inner, or nasal, angle. In each lid, a tarsus, or plate of dense
connective tissue, gives the lid its shape (see Fig. 54-1).
Each tarsus contains modified sebaceous glands, called meibomian glands, the ducts of which open onto the eyelid
margins. Sebaceous secretions of the meibomian glands enable airtight closure of the lids and prevent rapid evaporation of tears.
Two striated muscles, the levator palpebrae superioris and the orbicularis oculi, provide for movement of
the eyelids (Fig. 54-3). The levator palpebrae, which is innervated by the oculomotor cranial nerve (cranial nerve
[CN] III), raises the upper lid. Encircling the eye is the orbicularis oculi muscle, which is supplied by the facial
nerve (CN VII). When this muscle contracts, it closes the
eyelids.
Eyelid Weakness
Drooping of the eyelid is called ptosis. It can result from
weakness of the levator muscle that elevates the upper lid
in conjunction with the unopposed action of the orbicularis oculi that forcefully closes the eyelids. Weakness of
the orbicularis oculi causes an open eyelid, but not ptosis.
Neurologic causes of eyelid weakness include damage to
the innervating cranial nerves or to the nerves’ central nuclei in the midbrain and the caudal pons.
Normally, the edges of the eyelids, or palpebrae, are in
such a position that the palpebral conjunctiva that lines
the eyelids is not exposed and the eyelashes do not rub
against the cornea. Turning in of the lid is called entropion.
It is usually caused by scarring of the palpebral conjunctiva
or degeneration of the fascial attachments to the lower lid
that occurs with aging. Corneal irritation may occur as the
eyelashes turn inward. Ectropion refers to eversion of the
lower lid. The condition is usually bilateral and caused by
relaxation of the orbicularis oculi muscle because of CN VII
weakness or the aging process. Ectropion causes tearing
and ocular irritation and may lead to inflammation of the
cornea.
Entropion and ectropion can be treated surgically.
Electrocautery penetration of the lid conjunctiva also can be
used to treat mild forms of ectropion. After electrocautery,
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Special Sensory Function
Superior rectus
muscle
Superior tarsal
plate
Tendon of superior
oblique muscle
Lacrimal gland
Superior lacrimal duct
Lacrimal sac
Lateral palpebral
ligament
Medial palpebral
ligament
Inferior lacrimal duct
Inferior tarsal plate
Nasolacrimal duct
Inferior rectus
muscle
Opening of duct into
inferior nasal meatus
Inferior oblique
muscle
FIGURE 54-3 The eye and its appendages: anterior view.
contraction of the resulting scar tissue usually draws the lid
up to its normal position.
Eyelid Inflammation
Blepharitis is a common bilateral inflammation of the anterior or posterior structures of eyelid margins. Anterior
blepharitis involves the eyelid skin, eyelashes, and associated
glands. Two main types of anterior blepharitis occur: seborrheic and staphylococcal.2,3 The seborrheic form is usually
associated with seborrhea (i.e., dandruff) of the scalp or
brows. Staphylococcal blepharitis may be caused by Staphylococcus epidermidis or Staphylococcus aureus, in which case
the lesions are often ulcerative. The chief symptoms of
anterior blepharitis are irritation, burning, redness, and
itching of the eyelid margins. Treatment includes careful
cleaning with a wet applicator or clean washcloth to remove the scales. When the disorder is associated with a
microbial infection, an antibiotic ointment is prescribed.
Posterior blepharitis is inflammation of the eyelids that
involves the meibomian glands. It may result from a bacterial infection, particularly with staphylococci, or dysfunction of the meibomian glands, in which there is a strong
association with acne rosacea.2,3 The meibomian glands
and their orifices are inflamed, with dilation of the glands,
plugging of the orifices, and abnormal secretions. The lid
margins are often rolled inward to produce mild entropion, and the tears may be frothy and abnormally greasy
from the meibomian secretions. Treatment of posterior
blepharitis is determined by associated conjunctival and
corneal changes. Long-term, low-dose systemic antibiotic
therapy guided by results of bacterial cultures, along with
short-term topical steroids, may be needed.
A hordeolum, or stye, is caused by infection of the sebaceous glands of the eyelid and can be internal or exter-
nal (Fig. 54-4A). The main symptoms are pain, redness,
and swelling. The treatment is similar to that of abscesses
in other parts of the body. Heat, such as a warm compress,
is applied, and antibiotic ointment may be used. Incision
or expression of the infectious contents of the abscess may
be necessary.
A chalazion is a granulomatous inflammation of a
meibomian gland that may follow an internal hordeolum.
It is characterized by a small nontender nodule on the
upper or lower lid (see Fig. 54-4B). The conjunctiva around
the chalazion is red and elevated. If the chalazion is large
enough, it may press on the eyeball and distort vision.
Treatment consists of surgical excision.
DISORDERS OF THE LACRIMAL SYSTEM
The lacrimal system includes the major lacrimal gland,
which produces the tears; the puncta and tear sac, which
collect the tears; and the nasolacrimal duct, which empties
the tears into the nasal cavity. The lacrimal gland lies in the
orbit, superior and lateral to the eyeball (see Fig. 54-3).
Approximately 12 small ducts connect the lacrimal gland
to the superior conjunctival fornix. Tears contain approximately 98% water, 1.5% sodium chloride, and small
amounts of potassium, albumin, and glucose. The function
of tears is to make a smooth optical surface by abolishing
minute surface irregularities. Tears also wet and protect the
delicate surface of the cornea and conjunctiva. They flush
and remove irritating substances and microorganisms, and
they provide the cornea with necessary nutrient substances.
Tears also contain lysozymes and immunoglobulin A (IgA),
IgG, and IgE, which synergistically act to protect against infection. Although IgA predominates, IgE concentrations
are increased in some allergic conditions.
CHAPTER 54
A
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1295
filtrate the lacrimal and parotid glands. The disorder is
associated with diminished salivary and lacrimal secretions, resulting in keratoconjunctivitis sicca (dry eye syndrome) and xerostomia (i.e., dry mouth). The syndrome
occurs mainly in women near menopause and is often associated with connective tissue disorders such as rheumatoid
arthritis.
Persons with dry eyes complain of a dry or gritty sensation in the eye, burning, itching, inability to produce
tears, photosensitivity, redness, pain, and difficulty in moving the eyelids. Dry eyes and the absence of tears can cause
keratinization of the cornea and conjunctival epithelium.
In severe cases, corneal ulcerations can occur. Consequent
corneal scarring can cause blindness.
The treatment of dry eyes includes frequent instillation
of artificial tear solutions into the conjunctival sac. More
prolonged duration of action can be obtained from topical
preparations containing methylcellulose or polyvinyl alcohol. An ointment is useful for prolonged lubrication. Overall, these artificial tear preparations are safe and without
side effects. However, the preservatives necessary to maintain their sterility can be irritating to the cornea.2,5
Dacryocystitis
B
FIGURE 54-4 (A) Stye (acute hordeolum). (B) Chalzion. (Bickley L.S.
(2003). Bates’ guide to physical examination and history taking (8th ed.,
p. 178). Philadelphia: Lippincott Williams & Wilkins)
Dry Eyes
The thin film of tears that covers the cornea is essential in
preventing drying and damage of the outer layer of the
cornea. This tear film is composed of three layers: the
superficial lipid layer, derived from the meibomian glands
and thought to retard evaporation; the aqueous layer, secreted by the lacrimal glands; and the mucinous layer,
which overlies the cornea and epithelial cells. Because the
epithelial cell membranes are hydrophobic and cannot be
wetted by aqueous solutions alone, the mucinous layer
plays an essential role in wetting these surfaces. Periodic
blinking of the eyes is needed to maintain a continuous
tear film over the ocular surface.
Several conditions reduce the functioning of the lacrimal glands. With aging, the lacrimal glands diminish their
secretion, and as a result, many older persons awaken from
a night’s sleep with highly irritated eyes. Dry eyes also result from loss of reflex lacrimal gland secretion because of
congenital defects, infection, irradiation, damage to the
parasympathetic innervation of the gland, and medications
such as antihistamines and drugs with an anticholinergic
action. Wearing contact lenses contributes to eye dryness
through decreased blinking. Sjögren’s syndrome is a systemic disorder in which lymphocytes and plasma cells in-
Dacryocystitis is an infection of the lacrimal sac. It occurs
most often in infants and in persons older than 54 years
of age.2 It is usually unilateral and most often occurs secondary to obstruction of the nasolacrimal duct. Often, the
cause of the obstruction is unknown, although there may
be a history of severe trauma to the midface. The symptoms include tearing and discharge, pain, swelling, and
tenderness. The treatment includes application of warm
compresses and antibiotic therapy. In chronic forms of the
disorder, surgical repair of the tear duct may be necessary.
In infants, dacryocystitis is usually caused by failure
of the nasolacrimal ducts to open spontaneously before
birth. When a duct fails to open, a secondary dacryocystitis may develop. These infants are usually treated with
gentle massage of the tear sac, instillation of antibiotic
drops into the conjunctival sac, and, if that fails, probing
of the tear duct.
In summary, the eyelids serve to protect the eye. Ptosis refers
to drooping of the upper lid, which is caused by injury to CN III.
Entropion, which refers to turning in the upper eyelid and eyelashes, is discomforting and causes corneal irritation. Ectropion,
or eversion of the lower eyelid, causes tearing and may lead to
corneal inflammation. Marginal blepharitis is the most common disorder of the eyelids. It commonly is caused by a staphylococcal infection or seborrhea (i.e., dandruff).
The lacrimal system includes the major lacrimal gland,
which produces the tears, the puncta and tear sac, which collect the tears, and the nasolacrimal duct, which empties the
tears into the nasal cavity. Tears protect the cornea from drying and irritation. Impaired tear production or conditions that
prevent blinking and the spread of tears produce drying of the
eyes and predispose them to corneal irritation and injury.
Dacryocystitis is an infection of the lacrimal sac.
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UNIT XIII
Special Sensory Function
Disorders of the Conjunctiva, Cornea,
and Uveal Tract
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ Compare symptoms associated with red eye caused by
conjunctivitis, corneal irritation, and acute glaucoma
✦ List at least four causes of red eye
✦ Describe the appearance of corneal edema
✦ Characterize the manifestations, treatment, and possible
complications of bacterial, Acanthamoeba, and herpes
keratitis
✦ Describe the structures of the uveal tract
✦ Describe tests used in assessing the pupillary reflex and
cite the possible causes of abnormal pupillary reflexes
DISORDERS OF THE CONJUNCTIVA
The conjunctiva is a thin layer of mucous membrane that
lines the anterior surface of both eyelids as the palpebral
conjunctiva and folds back over the anterior surface of the
optic globe as the ocular or bulbar conjunctiva. The ocular
conjunctiva covers only the sclera or white portion of the
optic globe, not the cornea. When both eyes are closed,
the conjunctiva lines the closed conjunctival sac. Although
the conjunctiva protects the eye by preventing foreign objects from entering beyond the conjunctival sac, its main
function is the production of a lubricating mucus that
bathes the eye and keeps it moist.
Conjunctivitis, or inflammation of the conjunctiva
(i.e., red eye or pink eye), is one of the most common forms
of eye disease.4,5 It may result from bacterial or viral infection, allergens, chemical agents, physical irritants, or radiant energy. Infections may extend from areas adjacent to
the conjunctiva or may be blood-borne, such as in measles
or chickenpox. Newborns can contract conjunctivitis during the birth process. Infectious forms of conjunctivitis are
often bilateral and may involve other family members and
close associates. Unilateral disease suggests sources of irritation such as foreign bodies or chemical irritation.
Depending on the cause, conjunctivitis can vary in
severity from a mild hyperemia (redness) with tearing to severe conjunctivitis with purulent drainage. The conjunctiva is extremely sensitive to irritation and inflammation.
Important symptoms of conjunctivitis are a foreign body
sensation, a scratching or burning sensation, itching, and
photophobia. Severe pain suggests corneal rather than conjunctival disease. Itching is common in allergic conditions.
A discharge, or exudate, may be present with all types of
conjunctivitis and may cause transient blurring of vision.
It is usually watery when the conjunctivitis is caused by
allergy, a foreign body, or viral infection and mucopurulent in the presence of bacterial or fungal infection. A
characteristic of many forms of conjunctivitis is papillary
hypertrophy. This occurs because the palpebral conjunctiva is bound to the tarsus by fine fibrils. As a result, inflammation that develops between the fibrils causes the
conjunctiva to be elevated in mounds called papillae.
When the papillae are small, the conjunctiva has a smooth,
velvety appearance. Red papillary conjunctivitis suggests
bacterial or chlamydial conjunctivitis. In allergic conjunctivitis, the papillae often become flat topped, polygonal,
and milky in color and have a cobblestone appearance.
The diagnosis of conjunctivitis is based on history,
physical examination, and microscopic and culture studies to identify the cause. Because a red eye may be the sign
of several eye conditions, it is important to differentiate
between redness caused by conjunctivitis and that caused
by more serious eye disorders, such as corneal lesions and
acute glaucoma. In contrast to corneal lesions and acute
glaucoma, conjunctivitis produces injection (i.e., enlargement and redness) of the peripheral conjunctival blood
vessels rather than those radiating around the corneal limbus. Conjunctivitis also produces only mild discomfort, as
compared with the moderate to severe discomfort associated
with corneal lesions or the severe and deep pain associated
with acute glaucoma. Infectious forms of conjunctivitis are
usually bilateral and may involve other family members and
associates. Unilateral disease suggests sources of irritation
such as foreign bodies or chemical irritation.
Allergic Conjunctivitis
Allergic conjunctivitis encompasses a spectrum of conjunctival conditions usually characterized by itching. The
most common of these is seasonal allergic rhinoconjunctivitis, or hay fever. Seasonal allergic conjunctivitis is an
IgE-mediated hypersensitivity reaction precipitated by small
airborne allergens such as pollens.6 It typically causes bilateral tearing, itching, and redness of the eyes.
The treatment of seasonal allergic rhinoconjunctivitis
includes allergen avoidance, the use of cold compresses,
oral antihistamines, and vasoconstrictor eye drops. Allergic
conjunctivitis also has been successfully treated with topical mast cell stabilizers, histamine H1 receptor antagonists,
and topical nonsteroidal antiinflammatory drugs.6 All three
types of agents are well tolerated and have a rapid onset of
action. In severe cases, a short course of topical corticosteroids may be required to afford symptomatic relief.
Infectious Conjunctivitis
The agents of infectious conjunctivitis include bacteria,
viruses, and chlamydiae. Infections may spread from areas
adjacent to the conjunctiva or may be blood-borne, such
as in measles or chickenpox. Newborns can contract conjunctivitis during the birth process.
Bacterial Conjunctivitis. Bacterial conjunctivitis may present as a hyperacute, acute, or chronic infection. Hyperacute
conjunctivitis is a severe, sight-threatening ocular infection.
The infection has an abrupt onset and is characterized by a
copious amount of yellow-green drainage. The symptoms,
which typically are progressive, include conjunctival redness and chemosis, lid swelling, and tender, swollen preaurical lymph nodes. The most common causes of hyperacute purulent conjunctivitis are Neisseria gonorrhoeae and
Neisseria meningitidis, with N. gonorrhoeae being the most
common.3–5 Gonococcal ocular infections left untreated result in corneal ulceration with ultimate perforation, and
CHAPTER 54
sometimes permanent loss of vision. Diagnostic methods
include immediate Gram staining of ocular specimens and
special cultures for Neisseria species. Treatment includes
systemic antibiotics supplemented with ocular antibiotics.
Because of the increasing prevalence of penicillin-resistant
N. gonorrhoeae, antibiotic choice should be determined by
current information regarding antibiotic sensitivity.
Acute bacterial conjunctivitis typically presents with
burning, tearing, and mucopurulent or purulent discharge.
Common agents of bacterial conjunctivitis are Streptococcus
pneumoniae, S. aureus, and Haemophilus influenzae.4 The eyelids are sticky, with possible excoriation of the lid margins.
Treatment may include local application of antibiotics. The
disorder is usually self-limited, lasting approximately 10
to 14 days if untreated. Scrupulous eyelid hygiene and
prompt adequate treatment of infected persons and their
contacts are effective.
Chronic bacterial conjunctivitis most commonly is caused
by Staphylococcus species, although other bacteria may be
involved. It is often associated with blepharitis and bacterial colonization of eyelid margins. The symptoms of
chronic bacterial conjunctivitis vary and can include itching, burning, foreign body sensation, and morning eyelash crusting. Other symptoms include flaky debris and
erythema along the lid margins, eyelash loss, and eye redness. Some people with chronic bacterial conjunctivitis
also have recurrent styes and chalazia of the lid margins.
Treatment includes good eyelid hygiene and application
of topical antibiotics.
Viral Conjunctivitis. Etiologic agents of viral conjunctivitis
include adenoviruses, herpesviruses, and enteroviruses. Adenovirus type 3 infection is usually associated with pharyngitis, fever, and malaise.4,5 It causes generalized hyperemia,
copious tearing, and minimal exudate. Children are affected more often than adults. Swimming pools contaminated because of inadequate chlorination are common
sources of infection. Infections due to adenovirus types 4
and 7 are often associated with acute respiratory disease.
These viruses are rapidly disseminated when large groups
mingle with infected individuals (e.g., military recruits).
Adenovirus type 8 epidemics are associated with contamination of ophthalmic products and equipment. No specific treatment for this type of viral conjunctivitis exists; it
usually lasts 7 to 14 days. Preventive measures include
scrupulous eyelid hygiene and avoiding shared use of eyedroppers, eye makeup, goggles, and towels.
Herpes simplex virus conjunctivitis is characterized by
unilateral infection, irritation, mucoid discharge, pain, and
mild photophobia. Herpetic vesicles may develop on the
eyelids and lid margins. Although the infection is usually
caused by the type 1 herpesvirus, it also can be caused by
the type 2 virus that causes genital herpes. It is often associated with herpes simplex virus keratitis, in which the
cornea shows discrete epithelial lesions. Treatment involves the use of systemic or local antiviral agents. Topical
antiviral agents such as vidarabine, trifluridine, or idoxuridine usually provide prompt relief.4 Local corticosteroid
preparations increase the activity of the herpes simplex
virus, apparently by enhancing the destructive effect of
Disorders of Visual Function
1297
collagenase on the collagen of the cornea. The use of these
medications should be avoided in those suspected of having herpes simplex conjunctivitis or keratitis.
Chlamydial Conjunctivitis. Chlamydial conjunctivitis is
usually a benign suppurative conjunctivitis transmitted by
the type of Chlamydia trachomatis (serotypes D through K)
that causes venereal infections (see Chapter 48). It is spread
by contaminated genital secretions and occurs in newborns of mothers with C. trachomatis infections of the birth
canal. It also can be contracted through swimming in unchlorinated pools. The incubation period varies from 5 to
12 days, and the disease may last for several months if untreated. The infection is usually treated with appropriate
oral antibiotics.
A more serious form of infection is caused by a different strain of C. trachomatis (serotypes A through C). This
form of chlamydial infection affects the conjunctiva and
causes ulceration and scarring of the cornea. It is the leading cause of preventable blindness in the world. Although
the agent is widespread, it is seen mostly in developing
countries, particularly those of Africa, Asia, and the Middle
East.7 It is transmitted by direct human contact, contaminated objects (fomites), and flies.
Ophthalmia Neonatorum
Ophthalmia neonatorum is a form of conjunctivitis that
occurs in newborns younger than 1 month of age and is
usually contracted during or soon after vaginal delivery.
Many causes are known, including N. gonorrhoeae, Pseudomonas, and C. trachomatis.8 Epidemiologically, these infections reflect those sexually transmitted diseases most
common in a particular area. Once the most common form
of conjunctivitis in the newborn, gonococcal ophthalmia
neonatorum has an incidence of 0.3% of live births in the
United States; C. trachomatis has an incidence of 8.2% of
live births.8 Drops of 0.5% erythromycin or 1% silver nitrate are applied immediately after birth to prevent gonococcal ophthalmia. Silver nitrate instillation may cause
mild, self-limited conjunctivitis.
Signs of ophthalmia neonatorum include redness and
swelling of the conjunctiva, swelling of the eyelids, and
discharge, which may be purulent. The conjunctivitis
caused by silver nitrate occurs within 6 to 12 hours of birth
and clears within 24 to 48 hours.8 The incubation period
for N. gonorrhoeae is 2 to 5 days and for C. trachomatis, 5 to
14 days. Infection should be suspected when conjunctivitis develops 48 hours after birth.7 Ophthalmia neonatorum is a potentially blinding condition, and it can cause
serious and potentially systemic manifestations. It requires
immediate diagnosis and treatment.
DISORDERS OF THE CORNEA
At the anterior part of the eyeball, the outer covering of
the eye is modified to form the transparent cornea,
which bulges anteriorly from its junction with the sclera
(Fig. 54-5). A major part of the refraction (i.e., bending) of
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Epithelium
Bowman’s
membrane
Cornea
Stroma
Descemet’s
membrane
Trabecular meshwork
Canal of Schlemm
Endothelium
Conjunctiva
Ciliary process
Sphincter muscle
Iris
Pigment layer
Ciliary muscle
Dilatory muscle
Sclera
Lens
Zonular fibers
Ciliary epithelium
FIGURE 54-5 Anterior chamber angle
and surrounding structures.
Ora serrata
light rays and focusing of vision occurs in the cornea. Three
layers of tissue form the cornea: an extremely thin outer epithelial layer, which is continuous with the bulbar conjunctiva; a middle layer called the substantia propria or stroma;
and an inner endothelial layer, which lies next to the aqueous humor of the anterior chamber. The substantia propria
is composed of regularly arranged collagen bundles embedded in a mucopolysaccharide matrix. This organization of
the collagen fibers, which makes the substantia propria
transparent, is necessary for light transmission. Hydration
within a limited range is necessary to maintain the spacing
of the collagen fibers and transparency.
The cornea is avascular and obtains its nutrient and
oxygen supply by diffusion from blood vessels of the adjacent sclera, from the aqueous humor at its deep surface, and
from tears. The corneal epithelium is heavily innervated by
sensory neurons (trigeminal nerve [CN V], ophthalmic division [CN V1]). Epithelial damage causes discomfort that
ranges from a foreign body sensation and burning of the
eyes to severe, stabbing or knifelike, incapacitating pain. Reflex lacrimation is common. Disorders of the cornea include trauma and infections, abnormal corneal deposits,
and arcus senilis.
Corneal Trauma
Trauma that causes abrasions of the cornea can be extremely painful, but if minor, the abrasions usually heal in
a few days. The epithelial layer can regenerate, and small
defects heal without scarring. If the stroma is damaged,
healing occurs more slowly, and the danger of infection is
increased. Injuries to Bowman’s membrane and the stro-
mal layer heal with scar formation and permanent opacification. Opacities of the cornea impair the transmission
of light. A minor scar can severely distort vision because it
disturbs the refractive surface.
The integrity of the epithelium and the endothelium is
necessary to maintain hydration of the cornea within a limited range. Damage to either structure leads to edema and
loss of transparency. Among the causes of corneal edema is
the prolonged and uninterrupted wearing of hard contact
lenses, which can deprive the epithelium of oxygen, disrupting its integrity. The edema disappears spontaneously
when the cornea contacts the atmosphere. Corneal edema
also occurs after a sudden rise in intraocular pressure. With
corneal edema, the cornea appears dull, uneven, and hazy.
Visual acuity decreases, and iridescent vision (i.e., rainbows
around lights) occurs. Iridescent vision results from epithelial and subepithelial edema, which splits white light into
its component parts, with blue in the center and red on the
outside.
Keratitis
Keratitis refers to inflammation of the cornea. It can be
caused by infections, hypersensitivity reactions, ischemia,
defects in tearing, trauma, and interruption in sensory innervation, as occurs with local anesthesia. Scar tissue formation due to keratitis is the leading cause of blindness
and impaired vision throughout the world. Most of this
vision loss is preventable if the condition is diagnosed
early and appropriate treatment is begun.
Diagnosis of keratitis is based on history of trauma,
medication use, and signs and symptoms associated with
CHAPTER 54
corneal irritation and disease.9 Because the cornea has
many pain fibers, superficial or deep corneal abrasions
cause discomfort, photophobia, and lacrimation. The discomfort may range from a foreign body sensation to severe
pain. Defective vision results from the changes in transparency and curvature of the cornea that occur. Because of
the discomfort involved, examination of the eye is often
eased by instillation of a local anesthetic agent. Fluorescein
staining can be used to outline an ulcerated area. The biomicroscope (slit lamp) is used for proper examination of the
cornea. In cases of an infectious etiology, scrapings from
the ulcer are obtained for staining and culture studies.
Keratitis can be divided into two types: nonulcerative,
in which all the layers of the epithelium are affected but
the epithelium remains intact, and ulcerative, in which
parts of the epithelium, stroma, or both are destroyed.
Nonulcerative or interstitial keratitis is associated with
many diseases, including syphilis, tuberculosis, and lupus
erythematosus. It also may result from a viral infection entering through a small defect in the cornea. Treatment is
usually symptomatic.
Causes of ulcerative keratitis include infectious agents
such as those causing conjunctivitis (e.g., Staphylococcus,
S. pneumoniae, Chlamydia), exposure trauma, and use of
extended-wear contact lenses. Bacterial keratitis is aggressive and demands immediate care. Exposure trauma may
result from deformities of the lid, paralysis of the lid muscles, or severe exophthalmos. Mooren’s ulcer is a chronic,
painful, indolent ulcer that occurs in the absence of infection. It is usually seen in older persons and may affect both
eyes. Although the cause is unknown, an autoimmune origin is suspected.
Acanthamoeba is a free-living protozoan that thrives
in contaminated water. Acanthamoeba keratitis is an increasingly serious and sight-threatening complication of
wearing a soft contact lens, particularly when homemade
saline solutions are used for cleaning.9 It also may occur in
non–contact lens wearers after exposure to contaminated
water or soil. It is characterized by pain that is disproportionate to the clinical manifestations, redness of the eye,
and photophobia. The disorder commonly is misdiagnosed as herpes keratitis. Diagnosis is confirmed by scrapings and culture with specially prepared medium. In the
early stages of infection, epithelial débridement may be
beneficial. Treatment includes intensive use of topical
antibiotics. However, the organism may encyst within the
corneal stroma, making treatment more difficult. Keratoplasty may be necessary in advanced disease to arrest the
progression of the infection.
Herpes Simplex Keratitis. Herpes simplex virus (HSV) keratitis is the most common cause of corneal ulceration in
the United States. Most cases are caused by HSV type 1 infections. However, in neonatal infections acquired during
passage through the birth canal, approximately 80% are
caused by HSV type 2. The disease can occur as a primary
or recurrent infection.9 Primary infections cause follicular
conjunctivitis and blepharitis, characterized by a rounded
cobblestone pattern of avascular lesions. Epithelial keratitis
may develop. After the initial primary infection, the virus
Disorders of Visual Function
1299
may persist in a quiescent or latent state that remains in the
trigeminal ganglion and possibly in the cornea without
causing signs of infection. During childhood, mild primary
herpes simplex virus infection may go unnoticed.
Recurrent infection may be precipitated by various
poorly understood, stress-related factors that reactivate the
virus. Involvement is usually unilateral. The first symptoms are irritation, photophobia, and tearing. Some reduction in vision may occur when the lesion affects the
central part of the cornea. Because corneal anesthesia occurs early in the disease, the symptoms may be minimal,
and the person may delay seeking medical care. A history
of fever blisters or other herpetic infection is often noted,
but corneal lesions may be the only sign of recurrent herpes infection. Most typically, the corneal lesion involves
the epithelium and has a typical branching pattern. These
epithelial lesions heal without scarring.
Herpetic lesions that involve the stromal layer of the
cornea produce increasingly severe corneal opacities. They
are thought to have an immune rather than an infectious
cause. The most common cause of corneal blindness in
the Western world is stromal scarring from herpes simplex
keratitis.9
The treatment of HSV keratitis focuses on eliminating
viral replication within the cornea while minimizing the
damaging effects of the inflammatory process. It involves
the use of epithelial debridement and drug therapy.
Debridement is used to remove the virus from the corneal
epithelium. Topical antiviral agents such as trifluridine
(Viroptic) drops, idoxuridine (IDU) drops, or vidarabine
(Vira-A) ointment are used to promote healing. Corticosteroid drugs increase viral replication. With a few exceptions, their use is usually contraindicated.
Varicella Zoster Ophthalmicus. Herpes zoster or shingles
is a relatively common infection caused by herpesvirus
type 3, the same virus that causes varicella (chickenpox).10
It occurs when the varicella virus, which has remained
dormant in the neurosensory ganglia since the primary infection, is reactivated. Herpes ophthalmicus, which represents 10% to 25% of all cases of herpes zoster, occurs when
reactivation of the latent virus occurs in the ganglia of the
ophthalmic division of the trigeminal nerve.9,10 Immunocompromised persons, particularly those with human immunodeficiency disease (HIV) infection, are at higher risk
for developing herpes zoster ophthalmicus than those
with a normally functioning immune system.
Herpes zoster ophthalmicus usually presents with
malaise, fever, headache, and burning and itching of the
periorbital area. These symptoms commonly precede the
eruption by 1 or 2 days. The rash, which is initially vesicular, becomes pustular and then crusting (see Chapter 61).
Involvement of the tip of the nose and lid margins indicates a high likelihood of ocular involvement. Ocular signs
include conjunctivitis, keratitis, and anterior uveitis, often
with elevated intraocular pressure. Persons with corneal
disease present with varying degrees of decreased vision,
pain, and light insensitivity.
Treatment includes the use of high-dose oral antiviral drugs (acyclovir, valacyclovir). Initiation of treatment
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within the first 72 hours after the appearance of the rash
reduces the incidence of ocular complications but not the
postherpetic neuralgia (see Chapter 50).
Abnormal Corneal Deposits
The cornea frequently is the site for deposition of abnormal metabolic products. In hypercalcemia, calcium salts
can precipitate in the cornea, producing a cloudy band
keratopathy. Cystine crystals are deposited in cystinosis,
cholesterol esters in hypercholesterolemia, and a golden
ring of copper (i.e., Kayser-Fleischer ring) in hepatolenticular degeneration due to Wilson’s disease. Pharmacologic
agents, such as chloroquine, can result in crystal deposits
in the cornea.
Arcus senilis is an extremely common, bilateral, benign
corneal degeneration that may occur at any age but is
more common in the elderly. It consists of a grayish-white
infiltrate, approximately 2 mm wide, that occurs at the periphery of the cornea. It represents an extracellular lipid
infiltration and commonly is associated with hyperlipidemia. Arcus senilis does not produce visual symptoms,
and there is no treatment for the disorder.
Corneal Transplantation
Advances in ophthalmologic surgery permit corneal transplantation using a cadaver cornea. Unlike kidney or heart
transplantation procedures, which are associated with considerable risk for rejection of the transplanted organ, the
use of cadaver corneas entails minimal danger of rejection
because this tissue is not exposed to the vascular system
and therefore the immunologic defense system. Instead,
the success of this type of transplantation operation depends on the prevention of scar tissue formation, which
would limit the transparency of the transplanted cornea.
DISORDERS OF THE UVEAL TRACT
The middle vascular layer, or uveal tract, is an incomplete
ball with gaps at the pupil and the optic nerve (see
Fig. 54-3). The pigmented uveal tract has three distinct regions: the choroid, ciliary body, and iris. The choroid is a
highly vascular, dark-brown membrane that forms the
posterior five sixths of the uveal tract. Its blood vessels provide nourishment for the other layers of the eyeball. Its
brown pigment, produced by melanocytes, absorbs light
within the eyeball and light that penetrates the retina. The
light absorptive function prevents the scattering of light
and is important for visual acuity, particularly with high
background illumination levels. The ciliary body is a thickened ring of tissue that encircles the lens. It has smooth
muscle and secretory functions. Its smooth muscle function contributes to changes in lens shape and its secretory
function to the production of aqueous humor.
The iris is an adjustable diaphragm that permits
changes in pupil size and in the light entering the eye. The
posterior surface of the iris is formed by a two-layer epithelium continuous with those layers covering the ciliary
body. The anterior layer contains the dilator or radial muscles of the iris. Just anterior to these muscles is a layer of
highly vascular connective tissue. Embedded in this layer
are concentric rings of smooth muscle that compose the
sphincter muscle of the pupil. The anterior layer of the iris
forms an irregular anterior surface, containing many fibroblasts and melanocytes. Eye color differences result
from the density of the pigment. The amount of pigment
decreases from that found in dark brown eyes through
shades of brown and gray to that found in blue eyes.
Several mutations affect the pigment of the uveal tract,
including albinism. Albinism is a genetic (autosomal recessive trait) deficiency of tyrosinase, the enzyme needed for
the synthesis of melanin by the melanocytes. Tyrosinasenegative albinism, also called classic albinism, is characterized by an absence of tyrosinase; affected persons have
white hair, pink skin, and light blue eyes. In these persons,
excessive light penetrates the unpigmented iris and choroid, and, to some extent, the anterior sclera. Their photoreceptors are flooded with excess light, and visual acuity is
markedly reduced. Excess stimulation of the photoreceptors at normal or high illumination levels is experienced
as painful photophobia.
Uveitis
Inflammation of the entire uveal tract, which supports the
lens and neural components of the eye, is called uveitis. It
is one of several inflammatory disorders of ocular tissue
with clinical features in common and an immunologically
based cause.5 A serious consequence of uveitis can be the
involvement of the underlying retina. Parasitic invasion of
the choroid can result in local atrophic changes that usually involve the retina; examples include toxoplasmosis
and histoplasmosis. Sarcoid deposition in the form of
small nodules results in irregularities of the underlying
retinal surface.
THE PUPIL AND PUPILLARY REFLEXES
Changes in pupil size are controlled by contraction or relaxation of the dilator sphincter muscle of the iris. The
pupillary reflex, which controls the size of the pupillary
opening, is controlled by the autonomic nervous system,
with the parasympathetic nervous system producing pupillary constriction or miosis and the sympathetic nervous system producing pupillary dilation or mydriasis. The
sphincter muscle that produces pupillary constriction is
innervated by postganglionic parasympathetic neurons of
the ciliary ganglion and other scattered ganglion cells between the scleral and choroid layers. Part of the oculomotor (CN III) nucleus is called the Edinger-Westphal nucleus.11
This autonomic nucleus, found in the midbrain, provides
the preganglionic innervation for these parasympathetic
axons. Pupillary dilation is provided by sympathetic innervation under excitatory descending control from the
hypothalamus. Innervation is derived from preganglionic
neurons in the upper thoracic cord, which send axons
along the sympathetic chain to synapse with postganglionic neurons in the superior cervical ganglion. Postganglionic fibers travel along the surfaces of the carotid and
smaller arteries to reach the eye.
The pupillary reflex is controlled by a region in the
midbrain called the pretectum. Pretectal areas on each side
CHAPTER 54
of the brain are connected, explaining the binocular aspect
of the light reflex. The afferent stimuli for pupillary constriction arise in the ganglionic cells of the retina and are
transmitted to the pretectal nuclei at the junction of the thalamus and the midbrain, and from there to preganglionic
neurons in the oculomotor (CN III) nuclei (Fig. 54-6).
Normal function of the pupillary reflex mechanism
can be tested by shining a penlight into one eye of the person being tested. To avoid a change in pupil size due to accommodation, the person is asked to stare into the distance.
A rapid constriction of the pupil exposed to light should
occur; this is called the direct pupillary light reflex. Because
the reflex is normally bilateral, the contralateral pupil also
should constrict, a reaction called the consensual pupillary
light reflex. The circuitry of the light reflex is partially separated from the main visual pathway. This is illustrated by
the fact that the pupillary reflex remains unaffected when
lesions to the optic radiations or the visual cortex occur.
The cortically blind person retains direct and consensual
light reflexes.
Integrity of the dual autonomic control of pupillary
diameter is vulnerable to trauma, tumor enlargement, or
vascular disease. With diffuse damage to the forebrain involving the thalamus and hypothalamus, the pupils are
typically small but respond to light. Damage to the CN III
nucleus or nerve eliminates innervation of four of the six
extraocular muscles and the levator muscle of the upper
lid, and it results in permanent pupillary dilation in the af-
Optic nerve
Ciliary ganglion
Red nucleus
Optic tract
Edinger-Westphal
nucleus
IIIn
Disorders of Visual Function
1301
fected eye. Lesions affecting the cervical spinal cord or the
ascending sympathetic ganglionic chain in the neck or internal carotid artery (e.g., Horner’s syndrome) can interrupt the sympathetic control of the iris dilator muscle,
resulting in permanent pupillary constriction. Tumors of
the orbit that compress structures behind the eye can eliminate all pupillary reflexes, usually before destroying the
optic nerve.
Pupillary size can also be differentially affected by
pharmacologic agents. Bilateral pupillary constriction is
characteristic of opiate usage. Pupillary dilation results
when topical parasympathetic blocking agents such as
atropine are applied and sympathetic pupillodilatory
function is left unopposed. These medications are used by
ophthalmologists to facilitate the examination of the
transparent media and fundus of the eye. Miotic drugs (e.g.,
pilocarpine), which are used in the treatment of narrowangle glaucoma, produce pupil constriction and in that
manner facilitate aqueous humor circulation.
In summary, the conjunctiva lines the inner surface of the
eyelids and covers the optic globe to the junction of the cornea
and sclera. Conjunctivitis, also called red eye or pink eye, may
result from bacterial or viral infection, allergens, chemical
agents, physical agents, or radiant energy. It is important to
differentiate between redness caused by conjunctivitis and
that caused by more serious eye disorders, such as acute glaucoma or corneal lesions.
Keratitis, or inflammation of the cornea, can be caused by
infections, hypersensitivity reactions, ischemia, trauma, or defects in tearing. Trauma or disease that involves the stromal
layer of the cornea heals with scar formation and permanent
opacification. These opacities interfere with the transmission
of light and may impair vision.
The uveal tract is the middle vascular layer of the eye. It
contains melanocytes that prevent diffusion of light through
the wall of the optic globe. Inflammation of the uveal tract
(uveitis) can affect visual acuity.
The pupillary reflex, which controls the size of the pupil, is
controlled by the autonomic nervous system. The parasympathetic nervous system controls pupillary constriction, and
the sympathetic nervous system controls pupillary dilation.
Lateral
geniculate
nucleus
Intraocular Pressure and Glaucoma
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ Describe the formation and outflow of aqueous humor
Pretecto-oculomotor
tract
Posterior
commissure
Pulvinar
from the eye and relate to the development of glaucoma
✦ Compare closed-angle and open-angle glaucoma
✦ Explain why glaucoma leads to blindness
Pretectal nucleus
FIGURE 54-6 Diagram of the path of the pupillary light reflex. (Reproduced with permission from Walsh F.B., Hoyt W.F. [1969]. Clinical neuro-ophthalmology [3rd ed., vol. 1]. Baltimore: Williams &
Wilkins)
Glaucoma includes a group of conditions that produce an elevation in intraocular pressure. If left untreated,
the pressure may increase sufficiently to cause ischemia
and degeneration of the optic nerve, leading to progressive
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blindness. An estimated 60 million people worldwide currently have glaucoma. About 6 million people worldwide
are blind from glaucoma, including 100,000 Americans,
making it the leading cause of blindness in the United
States.12 African Americans are three to four times more
likely to have open-angle glaucoma as whites, and even
greater racial disparities exist in terms of blindness from
the disease. The condition is often asymptomatic, and a
significant loss of peripheral vision may occur before medical attention is sought.
CONTROL OF INTRAOCULAR PRESSURE
The intraocular pressure is largely regulated by the aqueous humor, which occupies the anterior segment of the
optic globe. The aqueous humor serves to maintain the
intraocular pressure and provides for the nutritive needs
of the lens and posterior cornea. It contains a low concentration of protein and high concentrations of ascorbic
acid, glucose, and amino acids. It also mediates the exchange of respiratory gases.
The aqueous humor is produced by the ciliary epithelium in the posterior chamber and flows between the anterior surface of the lens and posterior surface of the iris,
through the pupil and into the anterior chamber. Aqueous
humor, which is an ultrafiltrate of plasma, is secreted at an
average rate of 2 to 3 microliters per minute.13 Essentially
all of the aqueous humor is secreted by the ciliary processes, which are linear folds that project from the ciliary
body into the space behind the iris where the lens ligaments and ciliary muscle attach to the eyeball.
Aqueous humor leaves through the iridocorneal angle
between the iris and the sclera. Here it filters through the
trabecular meshwork and enters the canal of Schlemm for
return to the venous circulation (see Fig. 54-5). The canal
of Schlemm is actually a thin-walled vein that extends circumferentially around the eye. Its endothelial membrane
is so porous that even large protein molecules up to the
size of a red blood cell can pass from the posterior chamber into the canal of Schlemm.
The interior pressure of the eye must exceed atmospheric pressure to prevent the eyeball from collapsing.
Hydrostatic pressure of the aqueous humor results from a
balance of several factors: the rate of secretion, the resistance to flow between the iris and the ciliary body, and the
resistance to resorption at the trabeculated region of the
sclera at the iridocorneal angle. Normally, the rate of aqueous production is equal to the rate of aqueous outflow, and
the intraocular pressure is maintained within a normal
range of 9 to 21 mm Hg. However, data suggest that the
value may be closer to 12 ±1 mm Hg in young, healthy
adults during daylight hours. The mean value increases by
approximately 1 mm Hg per decade after 40 years of age.14
GLAUCOMA
Glaucoma is an optic neuropathy characterized by optic
disk cupping and visual field loss. It usually results from an
increase in intraocular pressure that results from abnor-
malities in the balance between aqueous production and
outflow. The most common cause is an interference with
aqueous outflow from the anterior chamber, rather than
overproduction of aqueous humor. Glaucoma is commonly classified as angle-closure (i.e., narrow-angle) or
open-angle (i.e., wide-angle) glaucoma, depending on the
location of the compromised aqueous humor circulation.
Glaucoma may occur as a congenital or an acquired condition, and it may manifest as a primary or secondary
disorder. Primary glaucoma occurs without evidence of
preexisting ocular or systemic disease. Secondary glaucoma can result from inflammatory processes that affect the
eye, from tumors, or from the blood cells of traumaproduced hemorrhage that obstruct the outflow of aqueous humor.
In glaucoma, temporary or permanent impairment of
vision results from pressure-induced degenerative changes
in the retina and optic nerve and from corneal edema and
opacification. Damage to optic nerve axons in the region
of the optic nerve can be recognized on ophthalmoscopic
examination. The normal optic disk has a central depression called the optic cup. With progressive atrophy of axons
caused by increased intraocular pressure, pallor of the optic
disk develops, and the size and depth of the optic cup increase. Because changes in the optic cup precede the visual
field loss, regular ophthalmoscopic examination is important for detecting eye changes that occur with increased intraocular pressure. Stereoscopic viewing during slit-lamp
examination improves the accuracy of evaluation.15
Advances in computer technology allow detection
and quantification of visual changes due to glaucoma.
These tests of vision include color vision analysis, blueon-yellow visual field testing and testing of contrast sensitivity, dark adaptation, and other tests of retinal function. In the future, these tests are likely to be used in
detecting visual field defects not currently detected by standard means.
Angle-Closure Glaucoma
Angle-closure glaucoma results from occlusion of the anterior chamber angle by the iris (Fig. 54-7). It is most likely to
develop in eyes with preexisting narrow anterior chambers.
An acute attack is often precipitated by pupillary dilation,
which causes the iris to thicken, blocking the circulation
between the posterior and anterior chambers.5,12,15 Approximately 5% to 10% of all cases of glaucoma fall into this
category. Angle-closure glaucoma usually occurs as the result of an inherited anatomic defect that causes a shallow
anterior chamber. It is seen more commonly in people of
Far-Eastern, Asian, or Inuit (Eskimo) descent and in people
with hypermetropic eyes.12,15 This defect is exaggerated by
the anterior displacement of the peripheral iris that occurs
in older persons because of the increase in lens size that
occurs with aging.
The depth of the anterior chamber can be evaluated
by transillumination or by a technique called gonioscopy.
Gonioscopy uses a special contact lens and mirrors or
prisms to view and measure the angle of the anterior
chamber. The transillumination method uses only a penlight. The light source is held at the temporal side of the
CHAPTER 54
Iridocorneal angle
Anterior
chamber
Posterior chamber
FIGURE 54-7 Narrow anterior chamber.
eye and directed horizontally across the iris. In persons
with a normal-sized anterior chamber, the light passes
through the chamber to illuminate both halves of the iris.
In persons with a narrow anterior chamber, only the half
of the iris adjacent to the light source is illuminated.
Symptoms of acute angle-closure glaucoma are related
to sudden, intermittent increases in intraocular pressure.
These occur after prolonged periods in the dark, emotional
upset, and other conditions that cause extensive and prolonged dilation of the pupil. Administration of pharmacologic agents such as atropine that cause pupillary dilation
(mydriasis) also can precipitate an acute episode of increased intraocular pressure in persons with the potential
for angle-closure glaucoma. Attacks of increased intraocular
pressure are manifested by ocular pain and blurred or iridescent vision caused by corneal edema.12 The pupil may be
enlarged and fixed. Symptoms are often spontaneously relieved by sleep and conditions that promote pupillary
constriction. With repeated or prolonged attacks, the eye
becomes reddened, and edema of the cornea may develop,
giving the eye a hazy appearance. A unilateral, often excruciating, headache is common. Nausea and vomiting may
occur, causing the headache to be confused with migraine.
Some persons with congenitally narrow anterior chambers never develop symptoms, and others develop symptoms only when they are elderly. Because of the dangers of
vision loss, those with narrow anterior chambers should be
warned about the significance of blurred vision, halos, and
ocular pain. Sometimes, decreased visual acuity and an unreactive pupil may be the only clue to angle-closure glaucoma in the elderly.
The treatment of acute angle-closure glaucoma is primarily surgical. It involves creating an opening between
the anterior and posterior chambers with laser or incisional iridectomy to allow aqueous humor to bypass the
pupillary block. The anatomic abnormalities responsible
for angle-closure glaucoma are usually bilateral, and prophylactic surgery is often performed on the other eye.
Disorders of Visual Function
1303
Open-Angle Glaucoma
Primary open-angle glaucoma is the most common form
of glaucoma. It tends to manifest after 35 years of age, with
an incidence of 0.5% to 2% among persons 40 years of age
and older. The condition is characterized by an abnormal
increase in intraocular pressure that occurs without obstruction at the iridocorneal angle, hence the name openangle glaucoma. Instead, it usually occurs because of an
abnormality of the trabecular meshwork that controls the
flow of aqueous humor into the canal of Schlemm.12,16
Primary open-angle glaucoma is usually asymptomatic and chronic, causing progressive damage to the optic nerve and visual field loss unless it is appropriately
treated. Elevated intraocular pressure is the major risk factor for open-angle glaucoma, but it is not the only diagnostic factor. Some people maintain a higher intraocular
pressure without evidence of optic nerve or visual field
loss. It is suggested that these people be described as glaucoma suspects or ocular hypertensives.15
The etiology of primary open-angle glaucoma remains
unclear. There is question as to whether damage to the
optic nerve is the result of excessive intraocular pressure,
decreased blood flow to the optic nerve, or both factors.
The only known causative risk factors are elevated intraocular pressure and insults to the eye, including trauma,
uveitis, and corticosteroid therapy. Risk factors for this disorder include: an age of 40 years and older, black race, a
positive first-degree family history, diabetes mellitus, and
myopia.15 In some persons, the use of moderate amounts
of topical or inhaled corticosteroid medications can cause
an increase in intraocular pressure. Sensitive persons also
may sustain an increase in intraocular pressure with the
use of systemic corticosteroid drugs.
Diagnostic methods include applanation tonometry
(measurement of intraocular pressure), ophthalmoscopic
visualization of the optic nerve, and central visual field
testing. Measurement of intraocular pressures provides a
means of assessing glaucoma risk. Because the condition is
usually asymptomatic, persons at risk for open-angle glaucoma should have regular direct ophthalmoscopic examinations, on both eyes, concentrating on the optic disk.
Optic disk changes frequently are noted before visual field
defects become apparent.
The elevation in intraocular pressure in persons with
open-angle glaucoma is usually treated pharmacologically
or, in cases where pharmacologic treatment fails, by increasing aqueous outflow through a surgically created pathway. Drugs used in the long-term management of glaucoma
fall into five classes: β-adrenergic antagonists, prostaglandin
analogs, adrenergic agonists, carbonic anhydrase inhibitors,
and cholinergic agonists.17 Most glaucoma drugs are applied
topically. However, systemic side effects may occur.
Topical β-adrenergic antagonists are usually the drugs
of first choice for lowering intraocular pressure. The βadrenergic antagonists are thought to lower intraocular
pressure by decreasing aqueous humor production in the
ciliary body. β-adrenergic antagonists decrease the production of aqueous humor by approximately one third.17
Prostaglandins are locally acting hormones, found in most
tissues. At low concentrations, prostaglandin F2α increases
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Special Sensory Function
uveoscleral outflow through the iris root and ciliary body,
either by decreasing the extracellular matrix or by relaxing
the ciliary musculature. A topical prostaglandin analog, latanoprost (Xalatan), is the only drug of this class currently
available in the United States.17
Adrenergic agonists cause an early decrease in production of aqueous humor by constricting the vessels supplying the ciliary body. Nonselective adrenergic agents such
as epinephrine stimulate both α and β receptors and tend
to cause more systemic side effects such as tachycardia
than nonselective drugs. Apraclonidine, an α-adrenergic
agonist, reduces intraocular pressure by decreasing aqueous humor formation. Dipivefrin is a prodrug converted to
epinephrine in the eye and seldom causes side effects.
Carbonic anhydrase inhibitors reduce the secretion of
aqueous humor by the ciliary epithelium. Until recently,
these drugs had to be taken orally, and systemic side effects
were common. A topical carbonic anhydrase inhibitor
(dorzolamide), which acts locally, is now available. Acetylcholine is the postganglionic neuromediator for the parasympathetic system; it increases aqueous outflow through
contraction of the ciliary muscle and pupillary constriction
(miosis). Cholinergic drugs exert their effects by increasing
the effects of acetylcholine. Acetylcholine is broken down
by the enzyme acetylcholinesterase. The most commonly
used miotic drug is pilocarpine, which functions as a direct
cholinergic agonist. Echothiophate, another miotic agent,
acts indirectly by inhibiting the breakdown of acetylcholine by acetylcholinesterase.
When a reduction in intraocular pressure cannot be
maintained through pharmacologic methods, surgical treatment may become necessary. Until recently, the main surgical treatment for open-angle glaucoma was a filtering
procedure in which an opening was created between the
anterior chamber and the subconjunctival space. An argon
or neodymium–yttrium-aluminum-garnet (Nd:YAG) laser
technique, in which multiple spots are applied 360 degrees
around the trabecular meshwork, has been developed.12
The microburns resulting from the laser treatment scar
rather than penetrate the trabecular meshwork, a process
thought to enlarge the outflow channels by increasing the
tension exerted on the trabecular meshwork. Cryotherapy,
diathermy, and high-frequency ultrasound may be used in
some cases to destroy the ciliary epithelium and reduce
aqueous humor production.
Congenital and Infantile Glaucoma
There are several types of childhood glaucoma including
congenital glaucoma that is present at birth and infantile
glaucoma that develops during the first 2 to 3 years of life.
As with glaucoma in adults, childhood glaucoma can occur
as a primary or secondary disorder.
Congenital glaucoma is caused by a disorder in which
the anterior chamber retains its fetal configuration, with
aberrant trabecular meshwork extending to the root of the
iris, or is covered by a membrane. In general, it has a much
poorer prognosis than infantile glaucoma. Primary infantile
glaucoma occurs in approximately 1 in 10,000 live births
but accounts for 2% to 15% of persons in institutions for
the blind.18 It is bilateral in 65% to 80% of cases and occurs more commonly in males than females. About 10% of
cases have a familial origin, and the rest are either sporadic
or possibly multifactorial with reduced penetrance. The
familial cases are usually transmitted as an autosomal dominant trait with potentially high penetrance. Recent studies
suggest a mutation in chromosome 2 (2p21 region). This
gene is expressed in the tissues of the anterior chamber of
the eye and its protein product plays an important role in
the metabolism of molecules that are used in signaling
pathways during the terminal stages of anterior chamber
development.18
The earliest symptoms of congenital or infantile glaucoma are excessive lacrimation and photophobia. Affected
infants tend to be fussy, have poor eating habits, and rub
their eyes frequently. Diffuse edema of the cornea usually
occurs, giving the eye a grayish-white appearance. Chronic
elevation of the intraocular pressure before the age of 3
years causes enlargement of the entire globe (i.e., buphthalmos). Early surgical treatment is necessary to prevent
blindness.
In summary, glaucoma is a leading cause of blindness in the
United States. It is characterized by conditions that cause an increase in intraocular pressure and that, if untreated, can lead to
atrophy of the optic disk and progressive blindness. The aqueous humor is formed by the ciliary epithelium in the posterior
chamber and flows through the pupil to the angle formed by
the cornea and the iris. Here, it filters through the trabecular
meshwork and enters the canal of Schlemm for return to the
venous circulation. Glaucoma results from overproduction or
the impeded outflow of aqueous humor from the anterior
chamber of the eye.
Two major forms of glaucoma exist: angle closure and
open angle. Angle-closure glaucoma is caused by a narrow anterior chamber and blockage of the outflow channels at the
angle formed by the iris and the cornea. This occurs when the
iris becomes thickened during pupillary dilation. Open-angle
glaucoma is caused by microscopic obstruction of the trabecular meshwork. Open-angle glaucoma is usually asymptomatic,
and considerable loss of the visual field often occurs before
medical treatment is sought. Routine screening by applanation
tonometry provides one of the best means for early detection
of glaucoma before vision loss has occurred. Congenital glaucoma is caused by a disorder in which the anterior chamber retains its fetal configuration, with aberrant trabecular meshwork
extending to the root of the iris, or is covered by a membrane.
It occurs in approximately 1 in 10,000 live births but accounts
for 2% to 15% of persons in institutions for the blind.
Disorders of the Lens and Lens Function
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ Describe the changes in eye structure that occur with
cataract
CHAPTER 54
Disorders of Visual Function
1305
✦ Cite risk factors associated with cataract
✦ Characterize the visual changes that occur with cataract
✦ Describe the treatment of persons with cataracts
The function of the eye is to transform light energy
into nerve signals that can be transmitted to the cerebral
cortex for interpretation. Optically, the eye is similar to a
camera. It contains a lens system that inverts an image, an
aperture (i.e., the pupil) for controlling light exposure, and
a retina that corresponds to the film and records the image.
A
DISORDERS OF REFRACTION
AND ACCOMMODATION
The lens is an avascular, transparent, biconvex body, the
posterior side of which is more convex than the anterior
side. A thin, highly elastic lens capsule is attached to the
surrounding ciliary body by delicate suspensory radial ligaments called zonules, which hold the lens in place (see
Fig. 54-5). In providing for a change in lens shape, the
tough elastic sclera acts as a bow, and the zonule and the
lens capsule act as the bowstring. The suspensory ligaments
and lens capsule are normally under tension, causing the
lens to have a flattened shape for distant vision. Contraction of the muscle fibers of the ciliary body narrows the
diameter of the ciliary body, relaxes the fibers of the suspensory ligaments, and allows the lens to relax to a more
spherical or convex shape for near vision.
When light passes from one medium to another, its
velocity is decreased or increased, and the direction of
light transmission is changed. This change in direction of
light rays is called refraction. When light rays pass through
the center of a lens, their direction is not changed; however, other rays passing peripherally through a lens are
bent (Fig. 54-8). The refractive power of a lens is usually
described as the distance (in meters) from its surface to the
point at which the rays come into focus (i.e., focal length).
Usually, this is reported as the reciprocal of this distance
(i.e., diopters).13 For example, a lens that brings an object
into focus at 0.5 m has a refractive power of 2 diopters
(1.0/0.5 = 2.0). With a fixed-power lens, the closer an object is to the lens, the further behind the lens is its focus
point. The closer the object, the stronger and more precise
the focusing system must be.
In the eye, the major refraction of light begins at the
convex corneal surface. Further refraction occurs as light
moves from the posterior corneal surface to the aqueous
humor, from the aqueous humor to the anterior lens surface, from the anterior lens surface to the posterior lens
surface, and from the posterior lens surface to the vitreous
humor.
Disorders of Refraction
A perfectly shaped optic globe and cornea result in optimal visual acuity, producing a sharp image in focus at all
points on the retinal surface in the posterior part, or fundus, of the eye (see Fig. 54-8). Unfortunately, individual
differences in formation and growth of the eyeball and
cornea frequently result in inappropriate focal image formation. If the anteroposterior dimension of the eyeball is
B
C
FIGURE 54-8 (A) Accommodation. The solid lines represent rays of
light from a distant object, and the dotted lines represent rays from
a near object. The lens needs to be flatter for the former and more
convex for the latter. In each case, the rays of light are brought to
a focus on the retina. (B) Hyperopia corrected by a biconvex lens,
shown by the dotted lines. (C) Myopia corrected by a biconcave lens,
shown
too short, the image is focused posterior to (behind) the
retina. This is called hyperopia or farsightedness. In such
cases, the accommodative changes of the lens can bring
distant images into focus, but near images become blurred.
Hyperopia is corrected by appropriate biconvex lenses. If the
anteroposterior dimension of the eyeball is too long, the
focus point for an infinitely distant target is anterior to
the retina. This condition is called myopia or nearsightedness (see Fig. 54-8). Persons with myopia can see close objects without problems because accommodative changes
in their lens bring near objects into focus, but distant objects
are blurred. Myopia can be corrected with an appropriate
biconcave lens. Radial keratotomy, a form of refractive corneal surgery, can be performed to correct the defect. This
surgical procedure involves the use of radial incisions to
alter the corneal curvature.
Refractive defects of the corneal surface do not permit
the formation of a sharp image. Nonuniform curvature of
1306
UNIT XIII
Special Sensory Function
the refractive medium (e.g., horizontal vs. vertical plane)
is called astigmatism. Astigmatism is usually the result of a
defect in the cornea, but it can result from defects in the
lens or the retina. Spherical aberration, another refractive
error, involves a cornea with nonspherical surfaces. Lens
correction is available for both refractive errors.
Disorders of Accommodation
Because the retina is at a fixed distance from the lens, adjustability in the refractive power of the lens is needed so
that a clear image is maintained as gaze is shifted from afar
to a near object. The process by which the refractory power
of the lens is increased and the diverging light rays are bent
more sharply is called accommodation. Accommodation requires convergence of the eyes, pupillary constriction, and
thickening of the lens through contraction of the ciliary
muscle. Contraction of the ciliary muscles is controlled
mainly by the parasympathetic fibers of the oculomotor
cranial nerve (CN III). In near vision, pupillary constriction
(i.e., miosis) improves the clarity of the retinal image. This
must be balanced against the resultant decrease in light intensity reaching the retina. During changes from near to
far vision, pupillary dilation partially compensates for the
reduced size of the retinal image by increasing the light
entering the pupil. A third component of accommodation
involves the reflex narrowing of the palpebral opening during near vision and widening during far vision.
Paralysis of the ciliary muscle, with loss of accommodation, is called cycloplegia. Pharmacologic cycloplegia is
sometimes necessary to aid ophthalmoscopic examination
of the fundus of the eye, especially in small children who
are unable to hold a steady fixation during the examination. Lens shape is totally controlled by the pretectal region
and the parasympathetic pathways through the oculomotor nerve to the ciliary muscle. Accommodation is lost
with destruction of this pathway.
The term presbyopia refers to changes in vision that
occur because of aging. The lens consists of transparent
fibers arranged in concentric layers, of which the external
layers are the newest and softest. No loss of lens fibers occurs with aging; instead, additional fibers are added to the
outermost portion of the lens. As the lens ages, it thickens,
and its fibers become less elastic, so that the range of focus
or accommodation is diminished to the point at which
reading glasses become necessary for near vision.
CATARACTS
A cataract is a lens opacity that interferes with the transmission of light to the retina. It has been estimated that
13 million persons in the United States 40 years of age or
older are visually disabled because of cataracts.1 Cataracts
are the most common cause of age-related visual loss in
the world; they are found in approximately 50% of those
between 65 and 74 years of age and in 70% of those older
than 75 years.1 Cataract surgery is the most common surgical procedure covered by Medicare, with more than 1 million procedures performed annually. More than 90% of
persons undergoing cataract surgery experience visual improvement if there is no ocular comorbidity.19
Causes and Types of Cataracts
The cause of cataract development is thought to be multifactorial, with different factors being associated with different types of opacities. The pathogenesis of cataracts is
not completely understood. Several risk factors have been
proposed, including the effects of aging, genetic influences,
environmental and metabolic influences, drugs, and injury.20 Metabolically induced cataracts are caused by disorders of carbohydrate metabolism (diabetes) or inborn errors
of metabolism.21 In a condition called Lowe’s syndrome, abnormal synthesis of lens proteins leads to opacification.
Long-term exposure to sunlight (ultraviolet B radiation)
and heavy smoking has been associated with increased risk
for cataract formation.20 Occasionally, cataracts occur as a
developmental defect (i.e., congenital cataracts) or secondary to trauma or diseases.
Cataracts can result from several drugs. Corticosteroid
drugs have been implicated as causative agents in cataract formation. Both systemic and inhaled corticosteroids have been cited as risk factors.22,23 Other drugs
associated with cataracts include the phenothiazines,
amiodarone, and strong miotic ophthalmic drugs such as
phospholine iodine.20 Frequent examination of lens transparency should accompany the use of these and any other
medications with potential cataract-forming effects.
Traumatic Cataract. Traumatic cataracts are most often
caused by foreign body injury to the lens or blunt trauma
to the eye. Foreign body injury that interrupts the lens
capsule allows aqueous and vitreous humor to enter the
lens and initiate cataract formation. Other causes of traumatic cataract are overexposure to heat (e.g., glassblower’s
cataract) or to ionizing radiation. The radiation dose necessary to cause a cataract varies with the amount and type
of energy; younger lenses are most vulnerable.
Congenital Cataract. A congenital cataract is one
that is present at birth. Among the causes of congenital
cataracts are genetic defects, toxic environmental agents,
and viruses such as rubella.8 A maternal rubella infection
during the first trimester of pregnancy can cause congenital cataract. Cataracts and other developmental defects of
the ocular apparatus depend on the total dose and the embryonic stage at the time of exposure. During the last
trimester of pregnancy, genetically or environmentally influenced malformation of the superficial lens fibers can
occur. Congenital lens opacities may occur in children of
diabetic mothers.
Most congenital cataracts are not progressive and are
not dense enough to cause significant visual impairment.
However, if the cataracts are bilateral and the opacity is
significant, lens extraction should be done on one eye by
the age of 2 months to permit the development of vision
and prevent nystagmus. If the surgery is successful, the
contralateral lens should be removed soon after.
Senile Cataract. Cataract is the most common
cause of age-related vision loss in the world.24 With normal aging, the nucleus and the cortex of the lens enlarge
CHAPTER 54
as new fibers are formed in the cortical zones of the lens.
In the nucleus, the old fibers become more compressed
and dehydrated. Metabolic changes also occur. Lens
proteins become more insoluble, and concentrations of
calcium, sodium, potassium, and phosphate increase.
During the early stages of cataract formation, a yellow
pigment and vacuoles accumulate in the lens fibers. The
unfolding of protein molecules, cross-linking of sulfhydryl groups, and conversion of soluble to insoluble proteins lead to the loss of lens transparency. The onset is
usually gradual, and the only symptoms are increasingly
blurred vision and visual distortion.
Manifestations
The manifestations of cataract depend on the extent of
opacity and whether the defect is bilateral or unilateral.
With the exception of traumatic or congenital cataract,
most cataracts are bilateral. Age-related cataracts, which are
the most common type, are characterized by increasingly
blurred vision and visual distortion. Vision for far and near
objects decreases. Dilation of the pupil in dim light improves vision. With nuclear cataracts (those involving the
lens nucleus), the refractive power of the anterior segment often increases to produce an acquired myopia. Persons with hyperopia may experience a “second sight” or
improved reading acuity until increasing opacity reduces
acuity. Central lens opacities may divide the visual axis and
cause an optical defect in which two or more blurred images are seen. Posterior subcapsular cataracts are located in
the posterior cortical layer and usually involve the central
visual axis. In addition to decreased visual acuity, cataracts
tend to cause light entering the eye to be scattered, thereby
producing glare or the abnormal presence of light in the
visual field.
Diagnosis and Treatment
Diagnosis of cataract is based on ophthalmoscopic examination and the degree of visual impairment on the
Snellen vision test. On ophthalmoscopic examination,
cataracts may appear as a gross opacity filling the pupillary
aperture or as an opacity silhouetted against the red background of the fundus. A Snellen test acuity of 20/50 is a
common requirement for drivers of motor vehicles. Other
tests of potential vision (e.g., the ability to see well after
surgery), such as electrophysiologic testing in which the
response to visual stimuli is measured electronically, may
be done.
There is no effective medical treatment for cataract. Use
of strong bifocals, magnification, appropriate lighting, and
visual aids may be used as the cataract progresses. Surgery is
the only treatment for correcting cataract-related vision
loss. Surgery usually involves lens extraction and intraocular lens implantation. It is commonly performed on an
outpatient basis with the use of local anesthesia. The use of
extracapsular surgery, which leaves the posterior capsule
of the lens intact, has significantly improved the outcomes
of cataract surgery. The cataract lens is usually removed
using phacoemulsification techniques.20,23 Phacoemulsification involves ultrasonic fragmentation of the lens into
fine pieces, which then are aspirated from the eye.
Disorders of Visual Function
1307
One of the greatest advances in cataract surgery has
been the development of reliable intraocular implants.
Until recently, only monofocal intraocular lenses that correct for distance vision were available, and eyeglasses were
needed for near vision. This has been remedied by the introduction of multifocal lenses.
In summary, the lens is a biconvex, avascular, colorless, and
almost transparent structure suspended behind the iris. The
shape of the lens is controlled by the ciliary muscle, which contracts and relaxes the zonule fibers, thus changing the tension
on the lens capsule and altering the focus of the lens. Refraction, which refers to the ability to focus an object on the retina,
depends on the size and shape of the eyeball and the cornea
and on the focusing ability of the lens. Errors in refraction
occur when the visual image is not focused on the retina because of individual differences in the size or shape of the eyeball or cornea. In hyperopia, or farsightedness, the image falls
behind the retina. In myopia, or nearsightedness, the image
falls in front of the retina. Accommodation is the process by
which a clear image is maintained as the gaze is shifted from
afar to a near object. It requires convergence of the eyes, pupillary constriction, and thickening of the lens through contraction of the ciliary muscle. Presbyopia is a change in the lens
that occurs because of aging such that the lens becomes
thicker and less able to change shape and accommodate for
near vision.
A cataract is a lens opacity. It can occur as the result of
congenital influences, metabolic disturbances, infection, injury,
and aging. The most common type of cataract is the senile
cataract that occurs with aging. Treatment of a totally opaque
or mature cataract is surgical extraction. An intraocular lens implant may be inserted during the surgical procedure to replace
the removed lens; otherwise, thick convex lenses or contact
lenses are used to compensate for the loss of lens function.
Disorders of the Vitreous and Retina
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ Relate the phagocytic function of the retinal pigment
epithelium to the development of retinitis pigmentosa
✦ Cite the manifestations and long-term visual effects of
papilledema and central artery and central venous
occlusions
✦ Describe the pathogenesis of background and proliferative diabetic retinopathies and their mechanisms of
visual impairment
✦ Discuss the cause of retinal detachment
✦ Explain the pathology and visual changes associated
with macular degeneration
The posterior segment, comprising five sixths of the
eyeball, contains the transparent vitreous humor and the
neural retina. The posterior aspect of the retina, the fundus,
is visualized through the pupil with an ophthalmoscope.
1308
UNIT XIII
Special Sensory Function
DISORDERS OF THE VITREOUS
Vitreous humor (i.e., vitreous body) is a colorless, amorphous biologic gel that fills the posterior cavity of the eye.
It consists of approximately 99% water, some salts, glycoproteins, proteoglycans, and dispersed collagen fibrils. The
vitreous is attached to the ciliary body and the peripheral
retina in the region of the ora serrata and to the periphery
of the optic disk.
Disease, aging, and injury can disturb the factors that
maintain the water of the vitreous humor in suspension,
causing liquefaction of the gel to occur. With the loss of
gel structure, fine fibers, membranes, and cellular debris
develop. When this occurs, floaters (images) can often be
noticed as these substances move within the vitreous cavity during head movement. Blood vessels may grow from
the surface of the retina or optic nerve into the posterior
surface of the vitreous, and cause bleeding into the vitreous cavity.
In a procedure called a vitrectomy, the removal and replacement of the vitreous with a balanced saline solution
can restore sight in some persons with vitreous opacities resulting from hemorrhage or vitreoretinal membrane formations that cause legal blindness. Using this procedure, a
small probe with a cutting tip is used to remove the opaque
vitreous and membranes. The procedure is difficult and
requires complex instrumentation. It is of no value if the
retina is not functional.
DISORDERS OF THE RETINA
The function of the retina is to receive visual images, partially analyze them, and transmit this modified information to the brain.11 It is composed of two layers: the outer,
melanin-containing layer and the inner neural layer. The
Direction
of light
light-sensitive neural retina covers the inner aspect of the
eyeball. A non–light-sensitive portion of the retina, along
with the retinal pigment epithelium, continues anteriorly
to form the posterior surface of the iris. A wavy border
called the ora serrata exists at the junction between the
light-sensitive and the non–light-sensitive retinas. Separating the vascular portion of the choroid from the single
layer of pigmented cells is a thin layer of elastic tissue,
Bruch’s membrane, which contains collagen fibrils in its
superficial and deep portions. Cells of the pigmented layer
receive their nourishment by diffusion from the choroid
vessels. Tight junctions between the endothelial cells of
the retinal blood vessels combine to form a blood–retina
barrier.
Disorders of the retina and its function include derangements of the pigment epithelium (e.g., retinitis pigmentosa); ischemic conditions caused by disorders of the
retinal blood supply; disorders of the retinal vessels such
as retinopathies that cause hemorrhage and the development of opacities; separation of the pigment and sensory
layers of the retina (i.e., retinal detachment); retinopathy
of prematurity; and abnormalities of Bruch’s membrane
and choroid (e.g., macular degeneration). Because the retina
has no pain fibers, most diseases of the retina are painless.
The Neural Retina
The neural retina is composed of three layers of neurons:
a posterior layer of photoreceptors, a middle layer of bipolar cells, and an inner layer of ganglion cells that communicate with the photoreceptors (Fig. 54-9). A pattern of
light on the retina falls on a massive array of photoreceptors. These photoreceptors synapse with bipolar and other
interneurons before action potentials in ganglion cells
relay the message to specific regions of the brain and the
brain stem associated with vision. For rods, this micro-
Axons of retinal
ganglion cells
Optic disk
Ganglion
cell layer
Bipolar
cell layer
Sclera
Optic
nerve
Layer of rods
and cones
Pigment
epithelium
FIGURE 54-9 Organization of the human retina. The visual pathway begins with photoreceptors (rods and cones) in the retina. The
responses of the photoreceptors are transmitted by the bipolar cells to the ganglion cell
layer of the retina.
CHAPTER 54
circuitry involves the convergence of signals from many
rods on a single ganglion cell. This arrangement maximizes
spatial summation and the detection of stimulated (light
vs. dark) receptors. The interneurons, composed of horizontal and amacrine cells, have cell bodies in the bipolar
layer, and they play an important role in modulating retinal function. A superficial marginal layer contains the
axons of the ganglion cells as they collect and leave the
eye by way of the optic nerve (see Fig. 54-9). These fibers
lie beside the vitreous humor. Light must pass through the
transparent inner layers of the sensory retina before it
reaches the photoreceptors.
Photoreceptors
Two types of photoreceptors are present in the retina:
rods, capable of black–white discrimination, and cones,
capable of color discrimination.11 Both types of photoreceptors are thin, elongated, mitochondria-filled cells with
a single, highly modified cilium (Fig. 54-10). The cilium
has a short base, or inner segment, and a highly modified
outer segment. The plasma membrane of the outer segment is tightly folded to form membranous disks (rods) or
conical shapes (cones) containing visual pigment. These
disks are continuously synthesized at the base of the outer
segment and shed at the distal end. Discarded membranes
Synaptic body
Nucleus
Disorders of Visual Function
1309
are phagocytized by the retinal pigment cells. If this phagocytosis is disrupted, as in retinitis pigmentosa, the sensory
retina degenerates.
Rods. Photoreception involves the transduction of light
energy into an altered ionic membrane potential of the
rod cell. Light passing through the eye penetrates the
nearly transparent neural elements to produce decomposition of the photochemical substance (visual pigment)
called rhodopsin in the outer segment of the rod. Light that
is not trapped by a rhodopsin molecule is absorbed by the
retinal pigment melanin or the more superficial choroid
melanin. Rhodopsin consists of a protein called opsin and
a vitamin A–derived pigment called retinal. During light
stimulation, rhodopsin is broken down into its component parts, opsin and retinal; retinal subsequently is converted into vitamin A. The reconstitution of rhodopsin
occurs during total darkness; vitamin A is transformed
into retinal, and then opsin and retinal combine to form
rhodopsin. Considerable stores of vitamin A are present in
the retinal pigment cells and in the liver; therefore, a vitamin A deficiency must be present for weeks or months to
affect the photoreceptive process. Reduced sensitivity to
light, a symptom of vitamin A deficiency, initially affects
night vision; however, this is quickly reversed by injection
or ingestion of the vitamin.
Rod-based vision is particularly sensitive to detecting
light, especially moving light stimuli, at the expense of clear
pattern discrimination. Rod vision is particularly adapted
for night and low-level illumination. Dark adaptation is
the process by which rod sensitivity increases to the optimum level. This requires approximately 4 hours in total or
near-total darkness and involves only rods or scotopic
vision (night vision). During daylight or high-intensity
bombardment, the concentration of vitamin A increases,
and the concentration of the photopigment retinal decreases. During dark adaptation, increased synthesis of
retinal from vitamin A results in a higher concentration of
rhodopsin for capture of light energy.
Cones and Color Sensitivity. Cone receptors that are selec-
Inner
segment
Connecting
structure
Outer
segment
FIGURE 54-10 Retinal rod, showing its component parts and the distribution of its organelles. Its outer segment contains the disks
(rods). The connecting structure joins the outer and inner segments.
The inner segment contains the mitochondria, the ribosomal endoplasmic reticulum, the free ribosomes, and the Golgi saccules.
The synaptic body forms the site where the photo receptors
synapse with other nerve cells.
tively sensitive to different wavelengths of light provide
the basis for color vision. Three types of cones, or conecolor systems, respond to the blue, green, and red portions
of the visible electromagnetic spectrum. This selectivity reflects the presence of one of three color-sensitive molecules
to which the photochemical substance (visual pigment) is
bound. The decomposition and reconstitution processes
of the cone visual pigments are believed to be similar to
that of the rods. The color a person perceives depends on
which set of cones or combination of sets of cones is stimulated in a given image.
Cones do not have the dark adaptation of rods. Consequently, the dark-adapted eye is a rod receptor eye with
only black-gray-white experience (scotopic or night vision).
The light-adapted eye ( photopic vision) adds the capacity
for color discrimination. Rhodopsin has its maximum sensitivity in the blue-green region of the electromagnetic
spectrum. If red lenses are worn in daylight, the red cones
(and green cones to some extent) are in use, whereas the
1310
UNIT XIII
Special Sensory Function
rods and blue cones are essentially in the dark, and therefore dark adaptation proceeds. This method is used by military and night-duty airport control tower personnel to
allow adaptation to take place before they go on duty in
the dark.
Macula and Fovea. An area approximately 1.5 mm in diameter near the center of the retina, called the macula lutea
(i.e., “yellow spot”), is especially adapted for acute and detailed vision.11 This area is composed entirely of cones. In
the central portion of the macula, the fovea centralis (foveola), blood vessels, and innermost layers are displaced to
one side instead of resting on top of the cones (Fig. 54-11).
This allows light to pass unimpeded to the cones without
passing through several layers of the retina. Of the approximately 6.4 million cones found in the retina, 200,000 cones
are located in the fovea. The density of cones drops off
rapidly away from the fovea. Rods are not present in the
fovea, but their numbers increase as the cones decrease in
density toward the periphery of the retina. An estimated
110 to 125 million rods are found in the retina.
Many cones are connected one-to-one with ganglion
cells. Retinal microcircuitry for cones emphasizes the detection of edges. This type of circuitry favors high acuity.
A concentration of acuity-favoring cones at the fovea supports the use of this part of the retina for fine analysis of
focused central vision. It is not until the age of 4 years that
the fovea is fully developed.
Color Blindness. Color blindness is a misnomer for a condition in which persons appear to confuse or mismatch
colors or experience reduced acuity for color discrimination. Such persons are often unaware of their defect until
they attempt to discriminate between red and green traf-
fic lights or show difficulty matching colors. Most often
the result of genetic factors, the deficit can result from the
defective function of one or more of the three color-cone
mechanisms. The deficiency is usually partial but can be
complete. Rarely are two of the color mechanisms missing;
when this occurs, usually red and green are missing. Persons
with no color mechanisms are rare. For them, the world is
experienced entirely as black, gray, and white.
The genetically color-blind person has never experienced the full range of normal color vision and is unaware
of what he or she is missing. Color discrimination is necessary for everyday living, and color-blind persons, knowingly or unknowingly, make color discriminations based
on other criteria, such as brightness or position. For example, the red light of a traffic signal is always the upper
light, and the green is the lower light. Color-blind persons
experience difficulties when brightness differences are
small and discrimination must be based on hue and saturation qualities.
The genes responsible for color blindness affect receptor mechanisms rather than central acuity. The gene for
the red and green mechanisms is sex linked (i.e., on the
X chromosomes), resulting in a much higher incidence
among males of red, green, or red-green color blindness;
however, the gene affecting the blue mechanism is autosomal. Acquired color defects are more complex but follow
a general rule: disease of the more peripheral retina affects
blue discrimination, and disease of the more central retina
affects red and green discrimination because blue cones
are not present in the central fovea.
Retinitis Pigmentosa. Retinitis pigmentosa (RP) is a group
of hereditary diseases that cause slow degenerative changes
in the retinal photoreceptors.25,26 There are several modes
FIGURE 54-11 Location of fovea in the retina. (Kandel E.R., Schwartz J.H., Jessell T.M. [1991]. Principles of
neural science [3rd ed.]. New York: Elsevier)
CHAPTER 54
of inheritance, including autosomal dominant, autosomal
recessive, sex linked, and sporadic. With the advances of
gene technology, there are now 36 known or predicted RP
genes.25 The most common group of these mutated genes
are those encoding proteins in the visual cascade of the
photoreceptor outer segment.
In typical cases, known as rod-cone RP, the rods are
the predominantly affected photoreceptor cells. This generally produces a number of characteristic clinical symptoms including night blindness, which is usually an early
symptom, and bilateral symmetric loss of mid-peripheral
fields. Although there is relative preservation of macular
vision, the visual field defects gradually increase both
centrally and peripherally. With progression, cone photoreceptor cells are also affected, and day vision and central
visual acuity are compromised. The rate of visual failure is
variable. Some people are severely visually handicapped
before the age of 20, whereas others experience few symptomatic visual deficits even after age 70.26
Disorders of Retinal Blood Supply
The blood supply for the retina is derived from two sources:
the choriocapillaris of the choroid and the branches of the
central retinal artery (Fig. 54-12). Oxygen and other nutritional substances needed by the retina and its component
parts (pigment cells, rods and cones) are supplied by diffusion from blood vessels in the choroid. Because the
choriocapillary layer provides the only blood supply for
the fovea centralis (i.e., foveola), detachment of this part
of the sensory retina from the pigment epithelium causes
irreparable visual loss.
The bipolar, horizontal, amacrine, and ganglion cells,
and the ganglion cell axons that gather at the optic disk,
are supplied by branches of the central retinal artery.27 The
1311
Disorders of Visual Function
DISORDERS OF THE RETINAL
BLOOD SUPPLY
➤ The blood supply for the retina is derived from the central
retinal artery, which supplies blood flow for the entire inside of the retina, and from vessels in the choroid, which
supply the rods and cones.
➤ Central retinal occlusion interrupts blood flow to the inner
retina and results in unilateral blindness.
➤ The retinopathies, which are disorders of the retinal vessels,
interrupt blood flow to the visual receptors, leading to visual
impairment.
➤ Retinal detachment separates the visual receptors from the
choroid, which provides their major blood supply.
central artery of the retina is a branch of the ophthalmic
artery. It enters the globe through the optic disk. Branches
of this artery radiate over the entire retina, except the central fovea, which is surrounded by, but is not crossed by,
arterial branches. The central retinal artery is an end artery
meaning that it does not anastomose with other arteries.
This is critical because an infarct in this artery will totally
deprive distal structures of their vascular supply. Retinal
veins follow a distribution parallel to the arterial branches
and carry venous blood to the central vein of the retina,
which exits the back of the eye through the optic disk.
Funduscopic examination of the eye with an ophthalmoscope provides an opportunity to examine the retinal
blood vessels and other aspects of the retina (Fig. 54-13).
Because the retina is an embryonic outgrowth of the brain
Vitreous chamber
(contains vitreous humor)
Sclera
Choroid
Retina
Fovea centralis
Macula lutea
Central retinal vein
Central retinal
artery
Optic nerve
Posterior ciliary artery
Optic disk (blind spot)
Retinal blood vessels
FIGURE 54-12 Retinal circulation.
1312
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Special Sensory Function
FIGURE 54-13 Fundus of the eye as seen in retinal examination with an ophthalmoscope: (left) normal
fundus; (middle) diabetic retinopathy—combination of microaneurysms, deep hemorrhages, and hard
exudates of background retinopathy; (right) hypertensive retinopathy with purulent exudates. Some exudates are scattered, while others radiate from the fovea to form a macular star. (Bates B. [1995].
A guide to physical examination and history taking. (pp. 208, 210). Philadelphia: J.B. Lippincott)
and the blood vessels are to a considerable extent representative of brain blood vessels, the ophthalmoscopic examination of the fundus of the eye permits the study and
diagnosis of metabolic and vascular diseases of the brain
and of pathologic processes that are specific to the retina.
Functioning of the retina, like that of other cellular portions of the central nervous system (CNS), depends on an
oxygen supply from the vascular system. One of the earliest
signs of decreased perfusion pressure in the head region is a
graying-out or blackout of vision, which usually precedes
loss of consciousness. This can occur during large increases
in intrathoracic pressure, which interfere with the return of
venous blood to the heart, as occurs with Valsalva’s maneuver; with systemic hypotension; and during sudden postural movements (e.g., postural hypotension).
Ischemia of the retina occurs during general circulatory collapse. If a person survives cardiopulmonary arrest,
for instance, permanently decreased visual acuity can occur
as a result of edema and the ischemic death of retinal neurons. This is followed by primary optic nerve atrophy proportional to the extent of ganglionic cell death. The
ophthalmic artery, the source of the central retinal artery,
takes its origin from the internal carotid artery.27
Intermittent retinal ischemia can accompany internal
carotid or common carotid stenosis. Amaurosis fugax is
characterized by transient episodes of monocular visual
loss lasting 5 to 10 minutes.27 Persons with the disorder
often describe a curtain coming down from above or
across their vision, usually with complete return of vision
within seconds or minutes. Besides the vision loss, contralateral hemiplegia or sensory deficits may accompany the
episodes. The condition, commonly due to emboli, is most
often the result of carotid artery disease.27
Papilledema. The central retinal artery enters the eye
through the optic papilla in the center of the optic nerve.
An accompanying vein exits the eye along the same path.
The entrance and exit of the central artery and vein of the
retina through the tough scleral tissue at the optic papilla
can be compromised by any condition causing persistent
increased intracranial pressure. The most common of these
conditions are cerebral tumors, subdural hematomas,
hydrocephalus, and malignant hypertension.
Usually, the thin-walled, low-pressure veins are the
first to collapse, with the consequent backup and slowing
of arterial blood flow. Under these conditions, capillary
permeability increases, and leakage of fluid results in
edema of the optic papilla, called papilledema. The interior
surface of the papilla is normally cup shaped and can be
evaluated through an ophthalmoscope. With papilledema,
sometimes called choked disk, the optic cup is distorted by
protrusion into the interior of the eye (Fig. 54-14). Because
this sign does not occur until the intracranial pressure is
significantly elevated, compression damage to the optic
nerve fibers passing through the lamina cribrosa may have
FIGURE 54-14 Chronic papilledema. The optic nerve head is congested and protrudes anteriorly toward the interior of the eye.
It has blurred margins, and vessels within it are poorly seen.
(Rubin E., Farber J.L. [1999]. Pathology [3rd ed., p. 1558]. Philadelphia:
Lippincott-Raven)
CHAPTER 54
Disorders of Visual Function
1313
begun. As a warning sign, papilledema occurs quite late.
Unresolved papilledema results in the destruction of the
optic nerve axons and blindness.
Microhemorrhages in the layer of ganglionic cell axon
bundles result in the appearance of cotton-wool spots on
the fundus.
Central Retinal Artery Occlusion. Complete occlusion of
Retinopathies
the central artery of the retina results in sudden unilateral
blindness. This is an uncommon disorder of older persons
and most often is caused by embolism or atherosclerosis.
Because the retina has a dual blood supply, the survival of
retinal structures is possible if blood flow can be reestablished within approximately 90 minutes.27 If blood flow is
not restored, the infarcted retina swells and opacifies. Because the receptors of the central fovea are supplied with
blood from the choroid, they survive (i.e., macular sparing). A cherry-red spot, indicating a healthy fovea, is surrounded by the pale white, opacified retina.31 Although
the nerve fibers of the optic disk are adequately supplied
by the choroid, the disk becomes pale after death of the
ganglion cells and their axonal processes (i.e., optic nerve
fibers), resulting in optic nerve atrophy.
Occlusions of branches of the central artery, called
branch arterial occlusions, are essentially retinal strokes.
These occur mainly from emboli and local infarction in
the neural retina. The opacification that follows is often
slowly resolved, and retinal transparency is restored. Local
blind spots or scotomas may occur after destruction of
local elements of the retina. Loss of the axons of destroyed
ganglion cells results in some optic nerve atrophy.
Central Retinal Vein Occlusion. Occlusion of the central
retinal vein results in venous dilation, stasis, and reduced
flow through the retinal veins. It is usually monocular and
causes painless rapid deterioration of visual acuity because
of an accompanying increase in capillary wall fragility and
a lack of arterial inflow.27 Superficial and deep hemorrhages
may occur throughout the retina.
Among the causes of central retinal vein obstruction
are hypertension, diabetes mellitus, and conditions such as
sickle cell anemia that slow venous blood flow. The reduction in blood flow results in neovascularization with fibrovascular invasion of the space between the retina and
the vitreous humor. Besides obstructing normal visual function, the new vessels are fragile and prone to hemorrhage.
Escaped blood may fill the space between the retina and vitreous, producing the appearance of a sudden veil over the
visual field. The blood can find its way into the aqueous
humor (i.e., hemorrhagic glaucoma). Photocoagulation of
the spreading new blood vessels with high-intensity light
or laser beam is used to prevent blindness and eye pain. As
the hemorrhage is resolved, degenerating blood products
can produce contraction of the vitreous and formation of
fibrous tissue within it, causing tears and detachment of
the retina.
Much more common are local vein occlusions with
regional and focal capillary microhemorrhages that produce the same but more restricted pathologic effects.
These microhemorrhages result in the formation of rings
of yellow exudate composed of lipid and lipoprotein blood
breakdown products. Microhemorrhages deep in the neural retina are somewhat restricted by the vertical organization of the neural elements and result in dot hemorrhages.
Disorders of the retinal vessels result in microaneurysms,
neovascularization, hemorrhage, and formation of retinal
opacities. Microaneurysms are outpouchings of the retinal
vasculature. On ophthalmoscopic examination, they appear as minute, unchanging red dots associated with blood
vessels. These microaneurysms tend to leak plasma, resulting in localized edema that gives the retina a hazy appearance. Microaneurysms can be identified with certainty
using fluorescein angiography; the fluorescein dye is injected intravenously, and the retinal vessels subsequently
are photographed using a special ophthalmoscope and
fundus camera. The microaneurysms may bleed, but areas
of hemorrhage and edema tend to clear spontaneously.
However, they reduce visual acuity if they encroach on the
macula and cause degeneration before they are absorbed.
Neovascularization involves the formation of new
blood vessels. They can develop from the choriocapillaris,
extending between the pigment layer and the sensory
layer, or from the retinal veins, extending between the
sensory retina and the vitreous cavity and sometimes into
the vitreous. These new blood vessels are fragile, leak protein, and are likely to bleed. Neovascularization occurs in
many conditions that impair retinal circulation, including
stasis because of hyperviscosity of blood or decreased flow,
vascular occlusion, sickle cell disease, sarcoidosis, diabetes
mellitus, and retinopathy of prematurity. Although the
cause of neovascularization is uncertain, research links the
process with a vascular endothelial growth factor (VEGF)
produced by the lining of blood vessels.28,29 The vitreous
humor is thought to contain a substance that normally
inhibits neovascularization, and this factor apparently is
suppressed under conditions in which the new blood vessels invade the vitreous cavity. It is likely that other growth
factors and signaling systems are also involved.
Hemorrhage can be preretinal, intraretinal, or subretinal. Preretinal hemorrhages occur between the retina
and the vitreous. These hemorrhages are usually large because the blood vessels are only loosely restricted; they
may be associated with a subarachnoid or subdural hemorrhage and are usually regarded as a serious manifestation
of the disorder. They usually reabsorb without complications unless they penetrate into the vitreous. Intraretinal
hemorrhages occur because of abnormalities of the retinal
vessels, diseases of the blood, increased pressure in the retinal vessels, or vitreous traction on the vessels. Systemic
causes include diabetes mellitus, hypertension, and blood
dyscrasias. Subretinal hemorrhages are those that develop
between the choroid and pigment layer of the retina. A
common cause of subretinal hemorrhage is neovascularization. Photocoagulation may be used to treat microaneurysms and neovascularization.
Light normally passes through the transparent inner
portions of the sensory retina before reaching the photoreceptors. Opacities such as hemorrhages, exudate, cottonwool patches, and edema can produce a localized loss of
1314
UNIT XIII
Special Sensory Function
transparency observable with an ophthalmoscope. Exudates are opacities resulting from inflammatory processes.
The development of exudates often results in the destruction of the underlying retinal pigment and choroid
layer. Deposits are localized opacities consisting of lipidladen macrophages or accumulated cellular debris. Cotton-wool patches are retinal opacities with hazy, irregular
outlines. They occur in the nerve fiber layer and contain
cell organelles. Cotton-wool patches are associated with
retinal trauma, severe anemia, papilledema, and diabetic
retinopathy.
Diabetic Retinopathy. Diabetic retinopathy is the third
leading cause of blindness for all ages in the United States.
It ranks first as the cause of newly reported cases of blindness in persons between the ages of 20 and 74 years. During the first two decades of the disease, nearly all persons
with type 1 diabetes and more than 60% of persons with
type 2 diabetes have retinopathy.30
Diabetic retinopathy can be divided into two types:
nonproliferative (i.e., background) and proliferative.31 Back-
ground or nonproliferative retinopathy is confined to the
retina. It involves thickening of the retinal capillary walls
and microaneurysm formation (Fig. 54-15). Ruptured capillaries cause small intraretinal hemorrhages, and microinfarcts may cause cotton-wool exudates. A sensation of glare
(because of the scattering of light) is a common complaint.
The most common cause of decreased vision in persons
with background retinopathy is macular edema.28,29 It represents fluid accumulation in the retina stemming from a
breakdown in the blood–retina barrier.
Proliferative diabetic retinopathy represents a more severe retinal change than background retinopathy. It is
characterized by formation of fragile blood vessels (i.e., neovascularization) at the disk and elsewhere in the retina.
These vessels grow in front of the retina along the posterior surface of the vitreous or into the vitreous. They
threaten vision in two ways. First, because they are abnormal, they often bleed easily, leaking blood into the vitreous cavity and decreasing visual acuity. Second, the blood
vessels attach firmly to the retinal surface and posterior
surface of the vitreous, such that normal movement of the
A
B
C
D
FIGURE 54-15 Diabetic retinopathy. (A) View of the ocular fundus in a patient with background diabetic
retinopathy. Several yellowish “hard” exudates, which are rich in lipids, are evident, together with several
relatively small retinal hemorrhages. (B) A vascular frond has extended anterior to the retina in the eye
with proliferative retinopathy. (C) Numerous microaneurysms are present in this flat preparation of a diabetic retina. (D) This flat preparation from a diabetic is stained with periodic acid–Schiff (PAS) after the
retinal vessels had been perfused with India ink. Microaneurysms (arrows) and an exudate (arrowhead) are
evident in a region of retinal nonperfusion. (Rubin E., Farber J.L. [1999]. Pathology [3rd ed., p. 1553].
Philadelphia: Lippincott-Raven)
CHAPTER 54
vitreous may exert a pull on the retina, causing retinal detachment and progressive blindness. Because early proliferative diabetic retinopathy is likely to be asymptomatic,
it must be identified early, before bleeding occurs and obscures light transmission or leads to fibrosis and retinal
detachment.
The cause of diabetic retinopathy is uncertain. Many
studies have demonstrated that hyperglycemia, hypertension, and hyperlipidemia contribute to the pathogenesis
of the disorder.30 Either directly or indirectly, these conditions are thought to contribute to the growth of abnormal
retinal blood vessels secondary to ischemia. These blood
vessels grow in an attempt to supply oxygenated blood to
hypoxic retinal. It has been suggested that conditions such
as leukostasis may play a role. Leukocytes have a large cell
volume, high cytoplasmic rigidity, a natural tendency to
adhere to the vascular endothelium, and a capacity to generate toxic superoxide radicals and proteolytic enzymes. In
diabetics, the leukocytes tend to be less deformable, and a
greater number are activated, indicating that they may be
involved in retinal capillary underperfusion, endothelial
cell damage, and vascular leakage.28,29 As a result of the occluded capillaries, retinal ischemia stimulates a pathologic
neovascularization mediated by angiogenic factors such as
VEGF. Whereas leukocytes have been implicated in the
pathogenesis of diabetic retinopathy, other blood components such as platelets and red blood cells are probably
also involved.
The American Diabetes Association, American College
of Physicians, and American Academy of Ophthalmology
have developed screening guidelines for diabetic retinopathy.30 These guidelines recommend that persons with
type 1 diabetes should have an initial dilated and comprehensive eye examination by an ophthalmologist or optometrist within 3 to 5 years after the onset of diabetes.
Usually, screening is not indicated before the start of puberty. Persons with type 2 diabetes should have an initial
comprehensive examination shortly after diagnosis. Subsequent examinations for persons with either type 1 or
type 2 diabetes should be repeated annually. The ocular
examination by the ophthalmologist should include acuity measurements, slit-lamp biomicroscopy, and direct and
indirect ophthalmoscopy of the retina through fully dilated pupils. When indicated, color fundus photographs
and fluorescein angiograms should be done. When planning pregnancy, women with preexisting diabetes should
be counseled about the risk for developing retinopathy or
progression of existing retinopathy. Women who become
pregnant should have a comprehensive eye examination
just before or soon after conception and at least every
3 months throughout pregnancy.
Preventing diabetic retinopathy from developing or
progressing is considered the best approach to preserving
vision. Growing evidence suggests that careful control of
blood glucose levels in persons with diabetes mellitus may
retard the onset and progression of retinopathy. The Diabetes Control and Complications Trial Research Group
demonstrated that intensive management of persons with
type 1 diabetes to maintain blood glucose levels at nearnormal levels reduced the risk for retinopathy by 76% in
Disorders of Visual Function
1315
persons with no retinopathy and slowed the progress by
54% in persons with early disease.33 There also is need for
intensive management of hypertension34 and hyperlipidemia, both which have been shown to increase the risk
for diabetic retinopathy.30
Photocoagulation using an argon laser provides the
major direct treatment modality for diabetic retinopathy.31,32
Treatment strategies include laser photocoagulation applied directly to leaking microaneurysms and grid photocoagulation with a checkerboard pattern of laser burns
applied to diffuse areas of leakage and thickening.31 Because laser photocoagulation destroys the proliferating
vessels and the ischemic retina, it reduces the stimulus
for further neovascularization. However, photocoagulation of neovascularization near the disk is not recommended.31 Vitrectomy has proved effective in removing
vitreous hemorrhage and severing vitreoretinal membranes
that develop.
Because of the limitations of current treatment measures, new pharmacologic therapies are being developed,
targeting the underlying biochemical mechanisms that
cause diabetic retinopathy. Among the pharmacologic therapies being explored are those related to the control of the
growth factors and signaling systems that participate in
the abnormal proliferation of retinal vessels.28,29
Hypertensive Retinopathy. Long-standing systemic hypertension results in the compensatory thickening of arteriolar
walls, which effectively reduces capillary perfusion pressure.
Ordinarily, a retinal blood vessel is transparent and seen as
a red line; in venules, the red cells resemble a string of boxcars. On ophthalmoscopy, arteries in persons with longstanding hypertension appear paler than veins because
they have thicker walls. The thickened arterioles in chronic
hypertension become opaque and have a copper-wiring
appearance. Edema, microaneurysms, intraretinal hemorrhages, exudates, and cotton-wool spots all are observed
(see Fig. 54-12). Malignant hypertension involves swelling
of the optic disk because of the local edema produced by
escaped fluid. If the condition is permitted to progress long
enough, serious visual deficits result.
Protective thickening of arteriolar walls cannot occur
with sudden increases in blood pressure. Therefore, hemorrhage is likely to occur. Trauma to the optic globe or the
head, sudden high blood pressure in preeclampsia, and
some types of renal disease may be accompanied by edema
of the retina and optic disk and by an increased likelihood
of hemorrhage.
Atherosclerosis of Retinal Vessels. In atherosclerosis, the
lumen of the arterioles becomes narrowed. As a result, the
retinal arteries become tortuous and narrowed. At sites
where the arteries cross and compress veins, the red cell
column of the vein appears distended. Exudate accumulates on arteriolar walls as “fluffy” white plaques (cottonwool patches). These patches are damaged axons that on
cross-section resemble crystalloid bodies. Deep and superficial hemorrhages are common. Atheromatous plaques of
the central artery are associated with increased danger of
stasis, thrombi of the central veins, and occlusion.
1316
UNIT XIII
Special Sensory Function
Retinal Detachment
Retinal detachment involves the separation of the sensory
retina from the pigment epithelium (Fig. 54-16). It occurs
when traction on the inner sensory layer or a tear in this
layer allows fluid, usually vitreous, to accumulate between
the two layers.31 Retinal detachment that results from
breaks in the sensory layer of the retina is called rhegmatogenous detachment (rhegma in Greek, meaning “rent” or
“hole”). The vitreous normally adheres to the retina at the
optic disk, macula, and periphery of the retina. When the
vitreous shrinks, it separates from the retina at the posterior pole of the eye (posterior vitreous detachment). However, at the periphery, the vitreous pulls on the attached
retina, which can lead to tearing of the retina. Vitreous
fluid can enter the tear and contribute to further separation of the retina from its overlying pigment layer.
Persons with high grades of myopia may have abnormalities in the peripheral retina that predispose to sudden
detachment. Intraocular surgery such as cataract extraction
may produce traction on the peripheral retina that causes
eventual detachment months or even years after surgery.
Detachment may result from exudates that separate the two
retinal layers. Exudative retinal detachment may be caused
by intraocular inflammation, intraocular tumors, or certain
systemic diseases. Inflammatory processes include posterior
scleritis, uveitis, or parasitic invasion. Retinal detachment
also can follow trauma immediately or at some later time.
Detachment of the neural retina from the retinal pigment layer separates the receptors from their major blood
supply, the choroid. If retinal detachment continues for
some time, permanent destruction and blindness of that part
of the retina occur. The bipolar and ganglion cells may survive because their blood supply, by way of the retinal arteries, remains intact. Without receptors, however, there is no
visual function. The primary symptom of retinal detachment is loss of vision. Sometimes, flashing lights or sparks,
followed by small floaters or spots in the field of vision, occur
as the retina pulls away from the posterior pole of the eye.
No pain is associated with this retinal detachment. As de-
Detached retina
Retinal tear
FIGURE 54-16 Detached retina.
tachment progresses, the person perceives a dark curtain
progressing across the visual field. Because the process begins
in the periphery and spreads circumferentially and posteriorly, initial visual disturbances may involve only one quadrant of the visual field. Large peripheral detachments may
occur without involvement of the macula, so that visual acuity remains unaffected. The tendency, however, is for detachments to enlarge until the entire retina is detached.
Diagnosis is based on the ophthalmoscopic appearance of the retina. Treatment is aimed at closing retinal
tears and reattaching the retina. Rhegmatogenous detachment usually requires surgical treatment. Scleral buckling
or pneumatic retinopexy is the most commonly used surgical technique.31 Scleral buckling is the primary surgical
procedure performed to reattach the retina. The procedure
requires careful location of the retinal break and treatment
with diathermy, cryotherapy, or laser to produce chorioretinal adhesions that seal the retinal tears so that the vitreous can no longer leak into the subretinal space. With
scleral buckling, a piece of silicone (i.e., the buckle) is sutured and infolded into the sclera, physically indenting
the sclera so that it contacts the separated pigment and
retinal layers. Pneumatic retinopexy involves the intraocular injection of an expandable gas instead of a piece of
silicone to form the indentation. An overall reattachment
rate of 90% is reported, but the visual results depend on
the preoperative status of the macula.31 The most common
cause of failure after surgical treatment for detached retina
is the development of membranes on the retina.
Macular Degeneration
Macular degeneration is characterized as loss of central vision due to destructive changes of the yellow-pigmented
area surrounding the central fovea. Age-related macular
degeneration is the most common cause of reduced vision
in the United States. It is the leading cause of blindness
among persons older than 75 years and of newly reported
cases of blindness among those older than 65 years of age.1
The cause of age-related macular degeneration is poorly
understood. In addition to older age, identifiable risk factors include female sex, white race, cigarette smoking, and
low dietary intake of carotenoids. Increasing evidence
suggests that genetic factors may also play a role.35
There are two types of age-related macular degeneration: an atrophic nonexudative or “dry” form and an exudative or “wet” form. Both types are progressive, but differ
in manifestations, prognosis, and management. Although
most people with age-related macular degeneration manifest nonproliferative changes only, most people who experience severe vision loss do so from the development of the
exudative form of the disease.31
Nonexudative age-related macular degeneration is
characterized by various degrees of atrophy and degeneration of the outer retina, Bruch’s membrane, and the choriocapillaris. It does not involve leakage of blood or serum;
hence, it is called dry age-related macular degeneration. On
ophthalmoscopic examination, there are visible changes
in the retinal pigmentary epithelium and pale yellow spots,
called drusen, that may occur individually or in groups
CHAPTER 54
throughout the macula. Histopathologically, most drusen
contain remnants of materials representative of focal detachment of the pigment epithelium. With time, the drusen
enlarge, coalesce, and increase in number. The level of
associated visual impairment is variable and may be minimal. Most people with macular drusen do not experience
significant loss of central vision; and the atrophic changes
may stabilize or progress slowly. However, people with the
nonexudative form of age-related macular degeneration
need to be followed closely because the exudative stage
may develop suddenly at any time.
The exudative form of macular degeneration is characterized by the formation of a choroidal neovascular
membrane that separates the pigmented epithelium from
the neuroretina. These new blood vessels have weaker
walls than normal and are prone to leakage; therefore, this
condition is called wet age-related macular degeneration. The
leakage of serous or hemorrhagic fluid into the subretinal
space causes separation of the pigmented epithelium from
the neurosensory retina.
Even though some subretinal neovascular membranes
may regress spontaneously, the natural course of exudative macular degeneration is toward irreversible loss of
central vision. Persons with late-stage disease often find it
difficult to see at long distances (e.g., in driving), do close
work (e.g., reading), see faces clearly, or distinguish colors.
However, they may not be severely incapacitated because
the peripheral retinal function usually remains intact. With
the help of low-vision aids, many of them are able to continue many of their normal activities. The early stages of
subretinal neovascularization may be difficult to detect
ophthalmoscopically. Therefore, there is need to be alert
for recent or sudden changes in central vision, blurred vision, or scotoma in persons with evidence of age-related
macular degeneration.
Although there is no treatment for the nonexudative
form of macular degeneration, argon laser photocoagulation may be useful in treating the neovascularization that
occurs with the exudative form.36,37 Another method used
to halt neovascularization is photodynamic therapy. It is a
nonthermal process leading to localized production of reactive oxygen species that mediate cellular, vascular, and
immunologic injury and destruction of new blood vessels.
There has been recent interest in the effect of supplemental
antioxidants (high-dose vitamin C, vitamin E, and betacarotene) and zinc for persons at risk for developing macular degeneration.38
While rare, macular degeneration can occur as a hereditary condition in young persons and sometimes in adults.
The genetic form of macular dystrophy, Stargardt’s disease,
becomes manifest in the middle of the first to second
decade of life and is inherited as a classic recessive trait, requiring mutations in both alleles of a single disease-related
gene.39 The gene codes for an ABCR (ATP-binding cassette
transporter—retina) transporter molecule located in the
retina that functions in the transport of retinal lipids and
peptides. It is thought that mutations in this gene allow
degraded material to accumulate and interfere with retinal
function.
Disorders of Visual Function
1317
In summary, the retina covers the inner aspect of the posterior two thirds of the eyeball and is continuous with the optic
nerve. It contains the neural receptors for vision, and it is here
that light energy of different frequencies and intensities is converted to graded local potentials, which then are converted to
action potentials and transmitted to visual centers in the brain.
The photoreceptors normally shed portions of their outer segments. These segments are phagocytized by cells in the pigment epithelium. Failure of phagocytosis, as occurs in one
form of retinitis pigmentosa, results in degeneration of the pigment layer and blindness.
The retina receives its blood from two sources: the choriocapillaris, which supplies the pigment layer and the outer portion of the sensory retina adjacent to the choroid, and the
branches of the retinal artery, which supply the inner half of the
retina. Retinal blood vessels are normally apparent through
the ophthalmoscope. Disorders of retinal vessels can result
from many local and systemic disorders, including diabetes
mellitus and hypertension. They cause vision loss through
changes that result in hemorrhage, production of opacities, and
separation of the pigment epithelium and sensory retina. Retinal detachment involves separation of the sensory receptors
from their blood supply; it causes blindness unless reattachment is accomplished promptly. Macular degeneration is characterized as loss of central vision due to destructive changes of
the yellow-pigmented area surrounding the central fovea. Agerelated macular degeneration is the most common cause of
reduced vision in the United States.
Disorders of Neural Pathways
and Cortical Centers
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ Characterize what is meant by a visual field defect
✦ Explain the use of perimetry in the diagnosis of a visual
field defect
✦ Define the terms hemianopia, quadrantanopia,
heteronymous hemianopia, and homonymous hemianopia
and relate to disorders of the optic pathways
✦ Describe visual defects associated with disorders of the
visual cortex and visual association areas
Full visual function requires the normally developed
brain-related functions of photoreception and the pupillary reflex. These functions depend on the integrity of all
visual pathways, including retinal circuitry and the pathway from the optic nerve to the visual cortex and other
visual regions of the brain and brain stem.
OPTIC PATHWAYS
Visual information is carried to the brain by axons of the
retinal ganglion cells, which form the optic nerve. Sur-
1318
UNIT XIII
Special Sensory Function
Left visual
field
Temporal
Right visual
field
Nasal
Temporal
Whitened field
no vision
Right
eye
Left
eye
Left
Right
Lesion 1
Optic
nerve
Optic
tract
1
Right optic nerve
2
3
Lesion 2
Optic chiasm
Lesion 3
Optic
radiation
Right optic tract
rounded by pia mater, cerebrospinal fluid (CSF), arachnoid,
and the dura mater, the optic nerve represents an outgrowth of the brain rather than a peripheral nerve. The
optic nerve extends from the back of the optic globe
through the orbit and the optic foramen, into the middle
fossa, and on to the optic chiasm at the base of the brain11
(Fig. 54-17). Axons from the nasal portion of the retina remain medial, and those from the temporal retina remain
lateral in the optic nerve.
The two optic nerves meet and fuse in the optic chiasm, beyond which they are continued as the optic tracts.
In the optic chiasm, axons from the nasal retina of each
eye cross to the opposite side and join with the axons of
the temporal retina of the contralateral eye to form the
optic tracts. Thus, one optic tract contains fibers from both
eyes that transmit information from the same visual field.
VISUAL CORTEX
The primary visual cortex (area 17) surrounds the calcarine
fissure, which lies in the occipital lobe. It is at this level
that visual sensation is first experienced (Fig. 54-18). Immediately surrounding area 17 are the visual association
cortices (areas 18 and 19) and several other association cortices.11 These association cortices, with their thalamic nuclei, must be functional for added meaningfulness of
visual perception. This higher-order aspect of the visual
experience depends on previous learning.
Circuitry in the primary visual cortex and the visual
association areas is extremely discrete with respect to the
location of retinal stimulation. For example, specific neurons respond to the particular orientation of a moving
edge, specific colors, or familiar shapes. This elaborate organization of the visual cortex, with its functionally separate and multiple representations of the same visual field,
FIGURE 54-17 Diagram of optic pathways. The red lines
indicate the right visual field and the blue lines the left
visual field. Note the crossing of fibers from the medial half of each retina at the optic chiasm. Lesion 1
(right optic nerve) produces unilateral blindness. Lesion 2 (optic chiasm) may involve only those fibers
that originate in the nasal half of each retina and
cross to the opposite side in the optic chiasm; visual
loss involves the temporal half of each field (bitemporal hemianopia). Lesion 3 (right optic tract) interrupts fibers (and vision) originating on the same side
of both eyes (homonymous) with loss of vision from
half of each field (hemianopia).
provides the major basis for visual sensation and perception. Because of this discrete circuitry, lesions of the visual
cortex must be large to be detected clinically.
VISUAL FIELDS
The visual field refers to the area that is visible during fixation of vision in one direction. Because visual system
deficits are often expressed as visual field deficits rather
than as direct measures of neural function, the terminology for normal and abnormal visual characteristics usually
is based on visual field orientation.
Most of the visual field is binocular, or seen by both
eyes. This binocular field is subdivided into central and pe-
Frontal eye field
(part of 8)
Somatosensory
(3, 1, 2)
Somatosensory
association (5, 7)
Visual
(17)
(19) (18)
Second
somatosensory
Auditory (41)
Auditory
association (42, 22)
Visual
association
FIGURE 54-18 Lateral view of the cortex illustrating the location of
the visual, visual association, auditory, and auditory association
areas.
CHAPTER 54
ripheral portions. Central portions of the retina provide
high visual acuity and correspond to the field focused on
the central fovea; the peripheral and surrounding portion
provides the capacity to detect objects, particularly moving objects. Beyond the visual field shared by both eyes,
the left lateral periphery of the visual field is seen exclusively by the left nasal retina, and the right peripheral field
is seen by the right nasal retina.
As with a camera, the simple lens system of the eye inverts the image of the external world on each retina. In
addition, the right and left sides of the visual field also are
reversed. The right binocular visual field is seen by the left
retinal halves of each eye: the nasal half of the right eye
and the temporal half of the left eye.
Once the level of the retina is reached, the nervous system plays a consistent role. The upper half of the visual
field is received by the lower half of the retinas of both eyes.
Representations of this upper half of the field are carried in
the lower half of each optic nerve: they synapse in the
lower half of the lateral geniculate nucleus (LGN) of each
side of the brain. Neurons in this part of the LGN send their
axons through the inferior half of the optic radiation, looping into the temporal lobe to terminate in the lower half of
the primary visual cortex on each side of the brain.
Because of the lateral separation of the two eyes, each
eye contributes a different image of the world to the visual
field. This is called binocular disparity. Disparity between
the laterally displaced images seen by the two eyes provides a powerful source of three-dimensional depth perception for objects within a distance of 30 m. Beyond that
distance, binocular disparity becomes insignificant: depth
perception is based on other cues (e.g., the superimposition
of the image of near objects over that of far objects, and the
faster movement of near objects than of far objects).
Visual Field Defects
Visual field defects result from damage to the visual pathways or the visual cortex. Perimetry or visual field testing,
in which the visual field of each eye is measured and plotted in an arc, is used to identify defects and determine the
location of lesions.
Retinal Defects. All of us possess a hole, or scotoma, in our
visual field, of which we are unaware. Because the optic
disk, where the optic nerve fibers exit the retina, does not
contain photoreceptors, the corresponding location in the
visual field constitutes a blind spot. Local retinal damage
caused by small vascular lesions (i.e., retinal stroke) and
other localized pathologies can produce additional blind
spots. As with the normal blind spot, persons are usually
not aware of the existence of scotomata in their visual
fields unless they encounter problems seeing objects in
certain restricted parts of the visual field.
Absences near or in the center of the bilateral visual
field can be annoying and even disastrous. Although the
hole is not recognized as such, the person finds that a part
of a printed page appears or disappears, depending on
where the fixation point is held. Most persons learn to position their eyes to use the remaining central foveal vision
for high-acuity tasks. Defects in the peripheral visual field,
including the monocular peripheral fields, are less annoy-
Disorders of Visual Function
1319
ing but potentially more dangerous. The person who is
unaware of the defect, when walking or driving an automobile, does not see cars or bicyclists until their image
reaches the functional visual field—sometimes too late to
avert an accident. With careful education, a person can
learn to shift the gaze constantly to obtain visual coverage
of important parts of the visual field. If the damage is at
the retinal or optic nerve level, only the monocular field
of the damaged eye becomes a problem. A lesion affecting
the central foveal vision of one eye can result in complaints of eyestrain during reading and other close work
because only one eye really is being used. Localized damage to the optic tracts, LGN, optic radiation, or primary visual cortex affects corresponding parts of the visual fields
of both eyes.
Disorders of the Optic Pathways
The visual pathway extends from the front to the back of
the head. It is much like a telephone line between distant
points in that damage at any point along the pathway results in functional defects (see Fig. 54-17). Among the disorders that can interrupt the visual pathway are vascular
lesions, trauma, and tumors. For example, normal visual
system function depends on vascular adequacy in the ophthalmic artery and its branches; the central artery of the
retina; the anterior and middle cerebral arteries, which
supply the intracranial optic nerve, chiasm, and optic
tracts; and the posterior cerebral artery, which supplies the
LGN, optic radiation, and visual cortex. The adequacy of
posterior cerebral artery function depends on that of the
vertebral and basilar arteries that supply the brain stem.
Vascular insufficiency in any one of these arterial systems
can seriously affect vision. Examination of visual system
function is of particular diagnostic use because lesions at
various points along the pathway have characteristic
symptoms that assist in the localization of the pathology.
Visual field defects of each eye and of the two eyes together are useful in localizing lesions affecting the system.
Blindness in one eye is called anopia. If half of the visual
field for one eye is lost, the defect is called hemianopia; loss
of a quarter field is called quadrantanopia. Enlarging pituitary tumors can produce longitudinal damage through
the optic chiasm with loss of the medial fibers of the optic
nerve representing both nasal retinas and both temporal
visual half-fields. Loss of the temporal or peripheral visual
fields on both sides results in a narrow binocular field,
commonly called tunnel vision. The loss of different halffields in the two eyes is called a heteronymous loss, and the
abnormality is called heteronymous hemianopia. Destruction of one or both lateral halves of the chiasm is common
with multiple aneurysms of the circle of Willis. In this condition, the function of one or both temporal retinas is lost,
and the nasal fields of one or both eyes are lost. The loss of
the temporal fields (nasal retina) of both eyes is called
bitemporal heteronymous anopia. With both eyes open, the
person with bilateral defects still has the full binocular visual field.
Loss of the optic tract, LGN, full optic radiation, or complete visual cortex on one side results in loss of the corresponding visual half-fields in each eye. Homonymous means
1320
UNIT XIII
Special Sensory Function
“the same” for both eyes. In left-side lesions, the right visual
field is lost for each eye and is called complete right homonymous hemianopia. Partial injury to the left optic tract, LGN,
or optic radiation can result in the loss of one fourth of the
visual field in both eyes. This is called homonymous quadrantanopia, and depending on the lesion, it can involve the
upper (superior) or lower (inferior) fields. Because the optic
radiation fibers for the superior fourth of the visual field traverse the temporal lobe, superior quadrantanopia is more
common. The LGN, optic radiation, and visual cortex all receive their major blood supply from the posterior cerebral
artery; unilateral occlusion of this artery results in complete
loss of the opposite field (i.e., homonymous hemianopia).
Bilateral occlusion of these arteries results in total cortical
blindness.
Disorders of the Visual Cortex
Discrete damage to the binocular portion of the primary
visual cortex also can result in scotomata in the corresponding visual fields. If the visual loss is in the central
high-acuity part of the field, severe loss of visual acuity
and pattern discrimination occurs. The central high-acuity
portion of the visual field is located at the occipital pole.
This region can be momentarily compressed against the
occipital bone (i.e., contrecoup) after severe trauma to the
frontal part of the cranium. Mechanical trauma to the cortex results in firing of neurons, experienced as flashes of
light or “seeing stars.” Destruction of the polar visual cortex
causes severe loss of visual acuity and pattern discrimination. Such damage is permanent and cannot be corrected
with lenses.
The bilateral loss of the entire primary visual cortex,
called cortical blindness, eliminates all visual experience.
Crude analysis of visual stimulation at reflex levels, such
as eye-orienting and head-orienting responses to bright
moving lights, pupillary reflexes, and blinking at sudden
bright lights, may be retained even though vision has been
lost. Extensive damage to the visual association cortex
(areas 18 and 19) that surrounds an intact primary visual
cortex results in a loss of the learned meaningfulness of
visual images (i.e., visual agnosia). The patient can see the
patterns of color, shapes, and movement, but no longer
can recognize formerly meaningful stimuli. Familiar objects can be described but not named or reacted to meaningfully. However, if other sensory modalities, such as
hearing and touch, can be applied, full recognition occurs.
This disorder represents a problem of recognition rather
than intellect.
Testing of Visual Fields
Crude testing of the binocular visual field and the visual
field of each individual eye (i.e., monocular vision) can be
accomplished without specialized equipment. In the confrontation method, the examiner stands or sits 2 to 3 feet
in front of the person to be tested and instructs the person
to focus on an object such as a penlight with one eye
closed. The object is moved from the center toward the periphery of the person’s visual field and from the periphery
toward the center, and the person is instructed to report
the presence or absence of the object. By moving the ob-
ject through the vertical, horizontal, and oblique aspects
of the visual field, a crude estimate can be made of the visual field. Large field defects can be estimated by the confrontation method, and it may be the only way for testing
young children and uncooperative adults.
Accurate determination of the presence, size, and
shape of smaller holes, or scotomata, in the visual field of
a particular eye can be demonstrated only by perimetry.
This is done by having the person look with one eye toward a central spot directly in front of the eye while the
head is stabilized by a chin rest or bite board. A small dot
of light or a colored object is moved back and forth in all
areas of the visual field. The person reports whether the
stimulus is visible and, if a colored stimulus is used, what
the perceived color is. A hemispheric support is used to
control and standardize the movement of the test object,
and a plot of radial coordinates of the visual field is made.
Perimetry provides a means of determining alterations
from normal and, with repeated testing, a way of following the progress of the disease or treatment.
In summary, visual information is carried to the brain by
axons of the retinal ganglion cells that form the optic nerve.
The two optic nerves meet and fuse in the optic chiasm. The
axons of each nasal retina cross in the chiasm and join the uncrossed fibers of the temporal retina of the opposite eye in the
optic tract to form the optic tracts. The fibers of each optic tract
then synapse in the LGN, and from there, travel by way of the
optic radiations to the primary visual cortex in the calcarine
area of the occipital lobe. Damage to the visual pathways or
visual cortex leads to visual field defects that can be identified
through visual field testing or perimetry and used to determine
the lesion’s location. Damage to the visual association cortex
can result in the phenomenon of seeing an object, but with
loss of learned recognition (i.e., visual agnosia).
Disorders of Eye Movement
After completing this section of the chapter, you should be able to
meet the following objectives:
✦ Describe the function and innervation of the extraocular
muscles
✦ Recognize the use of smooth pursuit, saccadic, and
vergence conjugate gaze movements in self or others
✦ Cite the function of optic tremor in terms of visual function
✦ Explain the difference between paralytic and
nonparalytic strabismus
✦ Define amblyopia and explain its pathogenesis
✦ Explain the need for early diagnosis and treatment of
eye movement disorders in children
Normal vision depends on the coordinated action of
the entire visual system and a number of central control
systems. It is through these mechanisms that an object is
simultaneously imaged on the fovea of both eyes and perceived as a single image. Strabismus and amblyopia are
CHAPTER 54
two disorders that affect this highly integrated system.
Although strabismus may develop in later life, it is seen
most commonly in children, among whom its incidence
is approximately 4%.40
EXTRAOCULAR EYE MUSCLES
AND THEIR INNERVATION
Each eyeball can rotate around its vertical axis (lateral or
medial rotation in which the pupil moves away from or toward the nose), its horizontal left-to-right axis (vertical elevation or depression, in which the pupil moves up or
down), and longitudinal horizontal axis in which the top
of the pupil moves toward or away from the nose.
Three pairs of extraocular muscles—the superior and
inferior recti, the medial and lateral recti, and the superior
and inferior obliques—control the movement of each eye
(Fig. 54-19). The four rectus muscles are named according
to where they insert into the sclera on the medial, lateral,
inferior, and superior surfaces of each eye. The two oblique
muscles insert on the lateral posterior quadrant of the eyeball—the superior oblique on the upper surface and the inferior oblique on the lower. Each of the three sets of
muscles in each eye is reciprocally innervated so that one
muscle relaxes when the other contracts. Reciprocal contraction of the medial and lateral recti moves the eye from
side to side (adduction and abduction); the superior and
inferior recti move the eye up and down (elevation and depression). The oblique muscles rotate (intorsion and extorsion) the eye around its optic axis. A seventh muscle,
the levator palpebrae superioris, elevates the upper lid.
The extraocular muscles are innervated by three cranial nerves. The abducens nerve (CN VI) innervates the
lateral rectus, the trochlear nerve (CN IV) innervates the
superior oblique, and the oculomotor nerve (CN III) innervates the remaining four muscles. The extraocular muscles are among the most rapidly contracting and precisely
controlled skeletal muscles in the entire body. This reflects
the high nerve-to-muscle ratio, with one lower motoneu-
Disorders of Visual Function
ron (LMN) innervating 2 to 10 muscle fibers. Table 54-1
describes the function and innervation of the extraocular
muscles.
The CN VI (abducens) nucleus, in the caudal pons, innervates the lateral rectus muscle, which rotates the ipsilateral (same side) eye laterally (abduction). The long
pathway of CN VI along the floor of the cranial cavity from
the pons to the orbit makes it vulnerable to damage from
severe injury or fracture of the cranial base. Partial or complete damage to this nerve results in weakness or complete
paralysis of the muscle. Medial gaze is normal, but the affected eye fails to rotate laterally with an attempted gaze toward the affected side, a condition called medial strabismus.
The CN IV (trochlear) nucleus, at the junction of the
pons and midbrain, innervates the contralateral or opposite
side superior oblique muscle, which rotates the top of the
globe inward toward the nose, a movement called intorsion.
In combination with other muscles, it also contributes
strength to movement of the innervated eye downward
and inward. The superior oblique muscle neurons cross
over the roof of the midbrain, drop vertically through the
roof of the cavernous sinus, and then enter the orbit. This
short pathway is rarely damaged, and signs of dysfunction
are usually the result of a small brain stem stroke affecting
the CN IV nucleus or from damage to the midbrain-level
vertical gaze networks.
The CN III (oculomotor) nucleus, which extends
through a considerable part of the midbrain, contains clusters of LMNs for each of the five eye muscles it innervates:
inferior rectus, ipsilateral superior rectus, inferior oblique,
medial rectus, and levator palpebrae superioris. The medial rectus, superior rectus, and inferior rectus rotate the
eye in the directions shown in Table 54-1. The action of
the inferior rectus is antagonistic to the superior rectus. Because of its plane of attachment to the globe, the inferior
oblique rotates the eye in the frontal plane (i.e., torsion),
pulling the top of the eye laterally (i.e., extorsion). The levator palpebrae superioris elevates the upper lid and is involved only in vertical gaze eye movements. As the eyes
rotate upward, the upper lid is reflexively retracted, and in
Inferior oblique
Medial
rectus
Temporalis muscle
Lateral rectus
Superior
oblique
Levator palpebrae
superioris
Superior
rectus
Medial rectus
Superior oblique
Inferior rectus
Superior rectus
Stump of
levator palpebrae
Optic nerve
FIGURE 54-19 Extraocular eye muscles of the right eye.
1321
Inferior
rectus
Lateral
rectus
Inferior
oblique
1322
UNIT XIII
Special Sensory Function
3–IO
SR–3
6–LR
TABLE 54-1
MR
MR
3
3
IR–3
4–SO
3–SR
3–IR
IO–3
LR–6
SO–4
Eye in Primary Position: Extrinsic Ocular Muscle Actions
Muscle*
Innervation
Primary
Secondary
Tertiary
MR: medial rectus
LR: lateral rectus
SR: superior rectus
IR: inferior rectus
SO: superior oblique
IO: inferior oblique
III
VI
III
III
IV
III
Adduction
Abduction
Elevation
Depression
Intorsion
Extorsion
Intorsion
Extorsion
Depression
Elevation
Adduction
Adduction
Abduction
Abduction
*In the schema of the functional roles of the six extraocular muscles, the major directional force applied by
each muscle is indicated on the top. These muscles are arranged in functionally opposing pairs per eye and in
parallel opposing pairs for conjugate movements of the two eyes. The numbers associated with each muscle
indicate the cranial nerve innervation: 3, oculomotor (III) cranial nerve; 4, trochlear (IV) cranial nerve; 6,
abducens (VI) cranial nerve.
the downward gaze, the upper lid is lowered, restricting
the exposure of the conjunctiva to air and reducing the
effects of drying.
Eye movements include lateral gaze, vertical gaze,
oblique gaze, and torsional gaze. Lateral gaze movements
are in the horizontal plane and are controlled by lateral
gaze centers. These gaze centers consist of interneurons in
the abducens (CN VI) nucleus on the side of abduction.
Vertical gaze upward or downward is controlled by a vertical gaze center of interneurons in and near the oculomotor
(CN III) nucleus. Oblique gaze involves a combination of
lateral and vertical gaze controls. Torsional gaze movements involve ocular rotation around the optical axis of the
eyes, and although they occur frequently, they are more
difficult to observe. The torsional gaze center involves interneurons in the bilaterally located trochlear (CN IV) nucleus on both sides. Each of these control centers maintains
a moderate level of tonic activity (i.e., low level of “spontaneous” action potentials) in each opposing pair of muscles.
This maintains a neutral position of eye posture. More
motor units are automatically recruited when an eye deviates from this neutral position. When released from a directional signal, the eyes automatically return to the neutral
position.
Communication between the eye muscle nuclei of
each side occurs primarily through the posterior commissure at the rostral end of the midbrain. Longitudinal communication among the three nuclei occurs along a fiber
tract called the medial longitudinal fasciculus (MLF), which
extends from the midbrain to the upper part of the spinal
cord. Each pair of eye muscles is reciprocally innervated,
by way of the MLF or other associated pathways, so that as
one muscle contracts, the other relaxes. For example, the
LMNs of CN VI produce lateral rotation (i.e., abduction) of
the left eye. Simultaneously, interneurons of the left CN VI,
which communicate with the right CN III by way of the
MLF, move the right eye medially (adduction). These MLFlinked communication paths are vulnerable to damage in
the caudal midbrain and pons. Damage to the pontine
MLF on one side results in a loss of this linkage such that
lateral deviation in the ipsilateral eye no longer is linked
to adduction on the contralateral side (i.e., internuclear
ophthalmoplegia). If the MLF is damaged bilaterally, the
linkage is lost for lateral gaze in either direction.
Eye Movements and Gaze
Conjugate movements are those in which the optical axes of
the two eyes are kept parallel, sharing the same visual field.
Gaze refers to the act of looking steadily in one direction.
Eye movements can be categorized into five classes of
movements: smooth pursuit movements, saccadic movements, optic tremor, vergence movement, and nystagmus.
Although the conjugate reflexes are essential to efficient
visual function during head movement or target movement, their circuitry is so deeply embedded in CNS function that they are present and can be elicited when the
eyes are closed, during sleep, and in deep coma, and they
function normally and accurately in congenitally blind
persons.
Smooth Pursuit Movements. Smooth pursuit movements
are tracking movements that serve to maintain an object
at a fixed point in the center of the visual fields of both
eyes. The object may be moving and the eyes following, or
the object may be stationary and the head of the observer
moving. Normal eye posture is a conjugate gaze directed
straight forward with the head held in a forward-looking
posture. Smooth pursuit movements normally begin from
this position. In fact, holding a strongly deviated gaze becomes tiring after about 30 seconds, and most people will
CHAPTER 54
make head and body rotation adjustments to bring the
eyes to a central position within that time.
Voluntary pursuit movements are tested by asking the
person to follow a finger or another object as it is moved
slowly through the visual field. Successful conjugate following requires a functional optic system communicating
to the superior colliculus and to the primary visual cortex.
Saccadic Eye Movements. Saccadic eye movements are
sudden, jerky conjugate movements that quickly change
the fixation point. During reading, the fixation pattern involves a focus on a word or short series of words and then
a sudden jump of the eyes to a new fixation point on the
next word or phrase. These shifts in fixation points are saccadic movements. During the saccade, a person does not
experience the blur of the rapidly moving visual field. The
mechanism by which visual experience is momentarily
shut off is not understood. Saccadic movements are automatic reflex movements that, in most situations, operate
at the brain stem level.
Saccadic shifts of a gaze toward the source of a sudden,
unexpected visual, auditory, tactile, or painful stimulus
are a component of the startle pattern. This is functionally
a visual grasping reflex, redirecting conjugate gaze in the
direction of the startle stimulus. This reflex occurs in the
absence of the cortical portion of the visual system (cortical blindness). The auditory startle reaction (startle reflex)
involves rapid saccadic movements in the direction of a
auditory stimulus. It is present in the neonate and in persons with impaired cortical auditory apparatus.
The frontal eye fields of the premotor cortex are important for voluntary saccadic movements such as reading.
If this frontal premotor area is not functional, a person can
describe objects in the visual field but cannot voluntarily
search the visual environment.
Disorders of Visual Function
1323
STRABISMUS
Strabismus, or squint, refers to any abnormality of eye coordination or alignment that results in loss of binocular
vision (Fig. 54-20). When images from the same spots in
visual space do not fall on corresponding points of the two
retinas, diplopia, or double vision, occurs.
In standard terminology, the disorders of eye movement are described according to the direction of movement. Esotropia refers to medial deviation, exotropia refers
to lateral deviation, hypertropia refers to upward deviation, hypotropia refers to downward deviation, and cyclotropia refers to torsional deviation. The term concomitance
refers to equal deviation in all directions of gaze. A nonconcomitant strabismus is one that varies with the direction of gaze. Strabismus may be divided into paralytic
(nonconcomitant) forms, in which there is weakness or
paralysis of one or more of the extraocular muscles, and
nonparalytic (concomitant) forms, in which there is no
primary muscle impairment. Strabismus is called intermittent, or periodic, when there are periods in which the eyes
are parallel. It is monocular when the same eye always deviates and the fellow eye fixates. Figure 54-21 illustrates abnormalities in eye movement associated with esotropia
and exotropia.
Strabismus affects approximately 4% of children
younger than 6 years of age.40,41 Because 30% to 50% of
these children sustain permanent secondary loss of vision,
or amblyopia, if the condition is left untreated, early diagnosis and treatment are essential.42,43
Eye Tremor. An eye tremor refers to involuntary, rhythmic, oscillatory eye movements, occurring approximately
10 times per second. Small-range optical tremors are a normal and useful independent function of each eye. One function of the fine optic tremor is to constantly move a bright
image onto a new bank of cones, permitting previously
stimulated receptors quickly to recover from adaptation.
Vergence Eye Movements. Vergence movements are those
that move the eyes in opposite directions to keep the image
of an object precisely positioned on the fovea of each eye.
Convergence and divergence, which assist in maintaining
a binocularly fixed image in near vision, have a major role
in accurate depth perception. The vergence system is driven
by retinal disparity (i.e., differential placement of an object’s image on each retina). A nearby target (<30 feet)
moving in the same dimension as the optical axis elicits a
reflex mechanism that provides redirection of the optical
axes of each eye away from parallel (i.e., in opposite directions) in the horizontal plane (i.e., vergence gaze). This
process permits a continued binocular focus on the near
target. Perception of depth is a higher-order function of
the cortical visual system and is based on one or more of
several classes of stimuli, such as superimposition and relative movement.
FIGURE 54-20 Photograph of a child with intermittent exotropia
squinting in the sunlight. (Vaughn D.G., Asbury T., Riordon-Eva P.
[1995]. General ophthalmology [p. 239]. Stamford, CT: Appleton &
Lange)
1324
UNIT XIII
Special Sensory Function
A
Left gaze: no deviation
B
D
Right hypertropia
Primary position: right esotropia
C Right gaze: left esotropia
E
Paralytic Strabismus
Paralytic strabismus results from paresis (i.e., weakness) or
plegia (i.e., paralysis) of one or more of the extraocular
muscles. When the normal eye fixates, the affected eye is
in the position of primary deviation. A manifestation of
esotropia is a weakness of one of the lateral rectus muscles,
usually the result of weakness of the abducens nerve (CN
VI). When the affected eye fixates, the unaffected eye is in
a position of secondary deviation. The secondary deviation of the unaffected eye is greater than the primary deviation of the affected eye. This is because the affected eye
requires an excess of innervational impulse to maintain
fixation; the excess impulses also are distributed to the unaffected eye, causing overaction of its muscles.40
Paralytic strabismus is uncommon in children but accounts for nearly all cases of adult strabismus; it can be
caused by many conditions. Paralytic strabismus is seen
most commonly in adults who have had cerebral vascular
accidents and may occur as the first sign of a tumor or inflammatory condition involving the CNS. One type of
muscular dystrophy exerts its effects on the extraocular
muscles. Initially, eye movements in all directions are
weak, with later progression to bilateral optic immobility.
Weakness of eye movement and lid elevation is often the
first evidence of myasthenia gravis. The pathway of the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI)
nerves through the cavernous sinus and the back of the
Right exotropia
FIGURE 54-21 Paralytic strabismus associated
with paralysis of the right lateral rectus muscle: (A) primary position (looking straight
ahead) of the eyes; (B) left gaze with no deviation; and (C) right gaze with left esotropia.
(D) Primary position of the eyes with weakness of the right inferior rectus and right hypertropia; and (E) primary position of the
eyes with weakness of the right medial rectus and right exotropia.
orbit make them vulnerable to basal skull fracture and
tumors of the cavernous sinus (e.g., cavernous sinus syndrome) or orbit (e.g., orbital syndrome).44 In infants, paralytic strabismus can be caused by birth injuries affecting
the extraocular muscles or the cranial nerves supplying
these muscles. It also can result from congenital anomalies
of the muscles. In general, paralytic strabismus in an adult
with previously normal binocular vision causes diplopia.
This does not occur in persons who have never developed
binocular vision.
Nonparalytic Strabismus
In nonparalytic strabismus, there is no extraocular muscle
weakness or paralysis, and the angle of deviation is always
the same in all fields of gaze. With persistent deviation,
secondary abnormalities may develop because of overaction or underaction of the muscles in some fields of
gaze. Nonparalytic esotropia is the most common type of
strabismus. The disorder may be accommodative, nonaccommodative, or a combination of the two. Accommodative strabismus is caused by disorders such as uncorrected
hyperopia, in which the esotropia occurs with accommodation. Onset of this type of esotropia characteristically
occurs between 18 months and 4 years of age because accommodation is not well developed until that time. The
disorder most often is monocular but may be alternating.
Approximately 50% of the cases of esotropia are accom-
CHAPTER 54
Disorders of Visual Function
1325
modative. The causes of nonaccommodative strabismus are
obscure. This disorder may be related to faulty muscle insertion, fascial abnormalities, or faulty innervation. Research
suggests that idiopathic strabismus may have a genetic
basis; siblings often present with similar disorders.
cal procedures may be used to strengthen or weaken an extraocular muscle by altering its length or attachment site.
Diagnosis and Treatment
Amblyopia describes a condition of diminished vision
(uncorrectable by lenses) in which no detectable organic
lesion of the eye is present.40,47,48 This condition is sometimes called lazy eye. Types of amblyopia include deprivation occlusion, strabismus, and refractive and organic
amblyopia. It is caused by visual deprivation (e.g., cataracts,
severe ptosis) or abnormal binocular interactions (e.g., strabismus, anisometropia) during visual immaturity. Normal
development of the thalamic and cortical circuitry necessary for binocular visual perception requires simultaneous
binocular use of each fovea during a critical period early in
life (0 to 5 years). In infants with unilateral cataracts that
are dense, central, and larger than 2 mm in diameter, this
time is before 2 months of age.40 In conditions causing abnormal binocular interactions, one image is suppressed to
provide clearer vision. In esotropia, vision of the deviated
eye is suppressed to prevent diplopia. A similar situation
exists in anisometropia, in which the refractive indexes of
the two eyes are different. Although the eyes are correctly
aligned, they are unable to focus together, and the image
of one eye is suppressed. In animal experiments, monocular deprivation results in reduced synaptic density in the
LGN and the primary visual cortical areas that process information from the affected eye or eyes.47
The reversibility of amblyopia depends on the maturity of the visual system at the time of onset and the duration of the abnormal experience. If esotropia is involved,
some persons alternate eyes and do not experience diplopia. With late adolescent or adult onset, this habit pattern
must be unlearned after correction.
Peripheral vision is less affected than central foveal
vision in amblyopia. Suppression becomes more evident
with high illumination and high contrast. It is as if the affected eye did not possess central vision and the person
learns to fixate with the nonfoveal retina. If bilateral congenital blindness or near blindness (e.g., from cataracts)
occurs and remains uncorrected during infancy and early
childhood, the person remains without pattern vision and
has only overall field brightness and color discrimination.
This is essentially bilateral amblyopia.
The treatment of children with the potential for development of amblyopia must be instituted well before the
age of 6 years to avoid the suppression phenomenon.
Surgery for congenital cataracts and ptosis should be done
early. Severe refractive errors should be corrected. In strabismus, alternately blocking vision in one eye and then
the other forces the child to use both eyes for form discrimination. The duration of occlusion of vision in the
good eye must be short (2 to 5 hours per day) and closely
monitored, or deprivation amblyopia can develop in the
good eye as well. Although amblyopia is not likely to occur
after 8 or 9 years of age, some plasticity in central circuitry
is evident even in adulthood. For example, after refractive
correction for long-standing astigmatism in adults, visual
All infants and children should be examined for visual
alignment. Alignment of the visual axis occurs in the first
3 months of life.45 All infants should have consistent, synchronized eye movement by 5 to 6 months of age. Infants
who have reached this age and whose eyes are not aligned
at all times during waking hours should be examined by a
qualified practitioner.
Rapid assessment of extraocular muscle function is accomplished by three methods. The first occurs in a darkened room with the child staring straight ahead. A penlight
is then pointed at the midpoint between the two eyes, and
a bright dot of reflected light can be seen on the cornea of
each eye. With normal eye alignment, the reflected light
should appear at the same spot on the cornea of each eye.
Nonparallelism of the two eyes indicates muscle imbalance
because of weakness or paralysis of the deviant eye. In a second method, the child is asked to follow the movement of
a small object (e.g., a pencil point, lighted penlight) as it is
moved through the extremes of what are called the six cardinal positions of gaze. In extreme lateral gaze, normal subjects can show a few quick beats of a jerky or nystagmoid
movement. Nystagmoid movement is abnormal if it is prolonged or present in any other eye posture. The third method, called the cover–uncover test, eliminates binocular
fusion as a factor in maintaining parallelism between the
eyes and is used to determine which eye is used for fixation. The child’s attention is directed toward a fixed object
such as a small picture or tongue blade. A light should not
be used because it may not stimulate accommodation. If a
mild weakness is present, the eye with blocked vision drifts
into a resting position, the extent of which depends on the
relative strength of the muscles. The eye should snap back
when the card is removed. This test is always done for near
and far fixation. Visual acuity is evaluated to obtain a comparison of the two eyes. A tumbling E chart (or similar test
chart) can be used for young children.46
Treatment of strabismus is directed toward the development of normal visual acuity, correction of the deviation, and superimposition of the retinal images to provide
binocular vision. Nonsurgical and surgical methods can be
used. In children, early treatment is important; the ideal
age to begin is 6 months. Nonsurgical treatment includes
occlusive patching, pleoptics (i.e., eye exercises), and prism
glasses. Because prolonged occlusive patching leads to loss
of useful vision in the covered eye, patching is alternated
between the affected and unaffected eye. This improves
the vision in the affected eye without sacrificing vision in
the unaffected eye. Prism glasses compensate for an abnormal alignment of an optic globe. Occasionally, longacting miotics in weak strengths (e.g., echothiophate iodide
solution [Phospholine Iodide] or pilocarpine [Pilocarpine
HS]) are used to cause pharmacologic accommodation in
place of or in combination with corrective lenses.41 Surgi-
AMBLYOPIA
1326
UNIT XIII
Special Sensory Function
acuity improves slowly, requiring several months to reach
normal levels.
red, swollen, and watering. She takes him to the pediatrician in the morning and is told that he has “pink eye.”
She is told that the infection should go away by itself.
In summary, binocular vision depends on three pairs of
A. What part of the eye is involved?
extraocular muscles–the medial and lateral recti, which move
the eye from side to side; the superior and inferior recti, which
move the eye up and down; and the superior and inferior
obliques, which rotate the eye around its optical axis. The
extraocular muscles are innervated by three pairs of cranial
nerves: (1) the trochlear nerve (CN IV), which innervates the
superior oblique and turns the eye downward and laterally;
(2) the abducens nerve (CN VI), which innervates the lateral
rectus and moves the eye laterally; and (3) the oculomotor
nerve (CN III), which innervates the medial rectus and turns the
eye medially, the superior rectus, which elevates the eye and
rolls it upward, the inferior rectus, which depresses the eye and
rolls it downward, and the inferior oblique, which elevates the
eye and turns it laterally.
For full visual function, it is necessary that the two eyes
point toward the same fixation point and the two images become fused. Binocular fusion is controlled by ocular reflex
mechanisms that adjust the orientation of each eye to produce
a single image. The term conjugate gaze refers to the use of both
eyes to look steadily in one direction. During conjugate eye
movements, the optical axes of the two eyes are maintained
parallel with each other as the eyes rotate upward, downward,
or from side to side in their sockets. Smooth pursuit movements are tracking movements that serve to maintain an object at a fixed point in the center of the visual fields of both
eyes. Saccadic eye movement consists of small jumping movements that represent rapid shifts in conjugate gaze orientation.
Vergence movements (convergence and divergence) are necessary for maintaining the focus on a near image.
Strabismus refers to abnormalities in the coordination of
eye movements with loss of binocular eye alignment. This inability to focus a visual image on corresponding parts of the
two retinas results in diplopia. Esotropia refers to medial deviation, exotropia refers to lateral deviation, hypertropia refers
to upward deviation, hypotropia refers to downward deviation, and cyclotropia refers to torsional deviation. Paralytic
strabismus is caused by weakness or paralysis of the extraocular muscles. Nonparalytic strabismus results from the inappropriate length or insertion of the extraocular muscles or
from accommodation disorders. Amblyopia (i.e., lazy eye) is a
condition of diminished vision that cannot be corrected by
lenses and in which no detectable organic lesion in the eye
can be observed. It results from inadequately developed CNS
circuitry because of visual deprivation (e.g., cataracts) or abnormal binocular interactions (e.g., strabismus, anisometropia)
during visual immaturity.
B. What type of conjunctivitis do you think this child
has: bacterial, viral, or allergic?
REVIEW EXERCISES
The mother of a 3-year-old boy notices that his left eye
is red and watering when she picks him up from day
care. He keeps rubbing his eye as though it itches. The
next morning, however, she notices that both eyes are
C. Why didn’t the pediatrician order an antibiotic?
D. Is the condition contagious? What measures should
the mother take to prevent its spread?
During a routine eye exam to get new glasses because
she had been having difficulty with her distant vision,
a 75-year-old woman is told that she is developing
cataracts.
A. What type of visual changes occur as the result of a
cataract?
B. What can the woman do to prevent the cataracts
from getting worse?
C. What treatment may she eventually need?
A 50-year-old woman is told by her eye doctor that her
intraocular pressure is slightly elevated and that although
there is no evidence of damage to her eyes at this time,
she is at risk for developing glaucoma and should have
regular eye exams.
A. Describe the physiologic mechanisms involved in
the regulation of intraocular pressure.
B. What are the risk factors for development of glaucoma?
C. Explain how an increase in intraocular pressure produces its damaging effects.
The parents of a newborn infant have been told that
their son has congenital cataracts in both eyes and will
require cataract surgery so that he does not lose his sight.
A. Explain why the infant is at risk for losing his sight
if the cataracts are not removed.
B. When should this procedure be done to prevent loss
of vision?
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