Download Astigmatism

You must know!!!
Write these on a sheet of paper to
take notes on.
• What causes cataracts & glaucoma.
• Definition of
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Emmetropia,
Myopia,
Hypermetropia,
Astigmatism,
diplopia,
Amblyopia
• Equilibrium tests
• Definition of Deafness
• Hearing tests
Problems with the Special
Senses
Clinical Note: Cataracts
• Occurs as the lens fibers darken with age, fluid filled
bubbles and clefts appear between the lens fibers.
• When the lens loses its transparency
• Can result from drug reactions, injuries, or radiation,
viruses, and other causes.
• Causes vision to appear milky or as if one were looking
from behind a waterfall.
• Senile cataracts: most common form; over time, the lens
becomes yellowish and cloudy
• Can be treated by replacing the natural lens with a
plastic one.
Clinical Note: Glaucoma
• Group of eye diseases in which the optic nerve at the
back of the eye is slowly destroyed, slowly destroying
vision
• Various causes:
– Increased pressure inside the eye - a result of blockage of the
circulation of aqueous, or its drainage.
– Poor blood supply to the vital optic nerve fibers
– A weakness in the structure of the nerve, and/or a problem in
the health of the nerve fibers themselves.
• Symptoms include illusory flashes of light, dimness of
vision, narrowed visual field, and colored halos around
artificial lights.
• Vision cannot be restored
Cataract (opaque lens)
Other Conditions of the Eye
• Hyperopia: far-sightedness
• Myopia: near-sightedness
Process of Vision
• Convergence – Many rays
focusing into one spot – Convex
lens
• Divergence – going from one
point to many points – Concave
lens
• The cornea acts as a converging
surface – convex lens
• Helps to focus light on the back
of your eye
Normal Eye
• Emmetropia is the
optical term for
normal vision. The
light from an object
is focused directly
onto the retina
forming a perfect
image.
• An emmetropic
person can see
distance and close
objects clearly.
Astigmatism
• Astigmatism is the
optical term for more
than one point of
focus.
It occurs when the
surface of the cornea
or crystalline lens is
not spherical.
Light from an object
may focus on the
retina in one direction
but not the other.
An astigmatic eye has curves that
are steeper in one direction than the
other, like a rugby ball.
A normal, spherical eye has curves
that are the same in all directions,
like a football.
An astigmatic person will have
distorted vision.
Hypermetropia – Farsighted
• The image on the right
shows a hypermetropic
eye.
• Hypermetropia means
long sight and is where
the image is formed
behind the retina. This
could be because the
eye is too short, or the
cornea or crystalline
lens does not refract
the light enough.
Myopia – Near
Sighted
• This image on the left
shows a myopic eye.
Myopia means short
sight and is where light
from an object forms an
image before it reaches
the retina. This could be
because the eye is too
long, or the cornea or
crystalline lens refracts
(bends) the light too
steeply.
A myopic person has clear vision
when looking at objects close to
them, and blurred vision when
looking at objects in the distance.
• Double vision: diplopia (what the patient experiences)
– Eyes do not look at the same point in the visual field
• Misalignment: strabismus (what is observed when shine a
light: not reflected in the same place on both eyes) – can be a
cause of diplopia
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Cross eyed
Gaze & movements not conjugate (together)
Medial or lateral, fixed or not
Many causes
• Weakness or paralysis of extrinsic muscle of eye
– Surgical correction necessary
• Oculomotor nerve problem, other problems
• Lazy eye: amblyopia
– Cover/uncover test at 5 yo
– If don’t patch good eye by 6, brain ignores lazy eye and visual pathway
degenerates: eye functionally blind
NOTE: some neurological development and connections have a
window of time - need stimuli to develop, or ability lost
Links
• Near & distant vision
Problems with Hearing
Equilibrium
• Lesions of the inner ear, vestibular
apparatus of the inner ear, the
vestibulocochlear nerve, cerebellum, and
other parts of the nervous system can
affect a person’s ability to maintain
balance or execute smooth movement.
• Romberg test- put feet together and close
your eyes.
– If defective, you should lose your balance and
fall to one side.
Deafness
• Any partial or complete loss of hearing.
• Conductive deafness- inability to transfer
vibrations through the auditory canal and
middle ear ossicles to inner ear.
• Sensorineural deafness (nerve)- results
from defects in the cochlea,
vestibulocochlear nerve, or CNS.
– Most common cause is cochlear damage
resulting from exposure to loud noise.
Hearing tests
• Rhine test- tuning fork against mastoid
process, behind the ear.
– When the patient no longer hears it, the
examiner holds it next to the auditory meatus.
– Normal hearing or partial nerve deafness hear
the hum, because air conduction is more
efficient than bone conduction.
– With conductive deafness, the airborne sound
disappears more quickly than the bone
conducted sound.
Hearing test
• Audiometry
– Patient listens to audiometer that plays pure
tones of specific frequencies and volumes.
– Shows degree of hearing loss.
– Auditory brainstem responses can be used to
test hearing in patients who cannot or will not
report whether they hear a sound.
• (comatose patients, infants, people feigning
deafness)
Cochlear Implants
• Implants that lead to the cochlear nerve.
• Detects sound and randomly stimulates
cochlear nerve fibers.
• Patients learn to associate the frequency
responses they hear with the origin and
relevance of the sound.
• Aids in distinguishing the rhythm of
speech, when words begin & end, car
horns, & alarms.