Download Strabismus Terminology

Strabismus Terminology
The term strabismus is derived from the Greek
word strabismos, "to squint, to look obliquely
or askance:' Strabismus means ocular
misalignment, whether caused by
Abnormalities in binocular vision or by
anomalies of neuromuscular control of ocular
motility.
Many terms are employed in discussions of
strabismus, and unless these terms are used
correctly and uniformly, confusion and
misunderstanding can occur.
Strabismus Terminology
Orthophoria is the ideal condition of ocular
Alignment under binocular conditions. In
reality, orthophoria is seldom encountered;
a small heterophoria can be Documented in
most persons. Some ophthalmologists
therefore prefer orthotropia to Mean
correct direction or position of the eyes.
Both terms are commonly used to describe
eyes without strabismus. Heterophoria is an
ocular deviation kept latent by the fusional
mechanism (latent strabismus).
Strabismus Terminology
Heterotropia is a deviation that is
Manifest and not kept under control by
the fusional mechanism (manifest
strabismus). It is important to identify
the deviating eye, especially when
seeking to call attention to the
"offending" eye as causing the deviation.
This usage is particularly helpful when
dealing with vertical deviations,
restrictive or paretic strabismus, or
amblyopia in a preverbal child.
Prefixes
Eso- this is also known as
convergent strabismus.
Exo- this is also known as
divergent strabismus.
Hyper- This is also known as
vertical strabismus.
Hypo- This is also known as
vertical strabismus.
Incyclo- This is also known as
intorsional strabismus.
Excyclo- This is also known as
extorsional strabismus.
Suffixes
Phoria: A latent deviation (e.g.,
esophoria, exophoria, right
hyperphoria) that is controlled
by the fusional mechanism so that
The eyes remain aligned under
normal binocular vision
Tropia: A manifest deviation (eg,
esotropia, exotropia, right
hypertropia, excyclotropia) that
exceeds the control of the fusional
mechanism so that the eyes are not
aligned under binocular conditions
Strabismus Classification Terms
No classification is perfect
or all-inclusive, and
several methods of
classifying eye alignment
and motility disorders are
used. Following are
terms used in these
classifications.
Fusional Status
Phoria: A latent deviation in
which fusional control is
always present.
Intermittent tropia: A
deviation in which fusional
control is present part of
the time.
Tropia: A manifest deviation
in which fusional control is
not present.
Variation Deviation With Gaze Position
Comitant : (concomitant) The
size of the deviation does not
vary by more than a few prism
diopters with direction of gaze
or with the eye used for fixating.
Incomitant : (noncomitant) The
deviation varies in size with the
direction of gaze or with the
eye used for fixating. Most
incomitant strabismus is
paralytic or restrictive.
Fixation
Alternating: Spontaneous
alternation of fixation
from one eye to the other.
Monocular: Definite
preference for fixation
with one eye.
Age of onset
Congenital: A deviation
documented before age 6
months, presumably related
to a defect present at birth;
the term infantile might be
more appropriate.
Acquired: A deviation with
later onset, after a period of
apparently normal visual
development.
Type of Deviation
Horizontal: Esodeviation
or Exodeviation
Vertical: Hyperdeviation
or Hypodeviation
Torsional: Incyclodeviation
or Excyclodeviation
Combined: Horizontal,
vertical, Torsional, or any
type combination .
Abbreviated of Strabismus Types
The addition of a prime (') to any of the following
indicates near fixation (e.g., E' indicates esophoria at near).
E, X, RH, LH Esophoria, exophoria, right hyperphoria, left
Hyperphoria at distance fixation, respectively
ET, XT RHT, LHT Constant esotropia, exotropia, right
hypertropia, left hypertropia at distance fixation.
E(T), X(T), RH(T), LH(T) Intermittent esotropia, exotropia,
Right hypertropia, left hypertropia at distance fixation .
RHol LHoT Right hypotropia, left hypotropia at distance
fixation, respectively
0, EX = 0 Orthophoria (orthotropia)
Important Axes of the Eye
There is occasionally confusion regarding terms
used to describe the axes of the eye. Following are
Some important definitions:
Optical axis: A line that best approximates the line
passing through the optical centers of the cornea,
lens, and fovea.
Pupillary axis: The imaginary line perpendicular to
The corneal surface and passing through the
midpoint of the entrance pupil.
Visual axis: The imaginary line connecting fixation
Point and the fovea
Anatomy of the Extra ocular
Muscles and Their Fascia
Origin, Course, Insertion, Innervations,
and Action of the EOM
There are 7 EOM: the 4 rectus muscles, the 2 oblique
muscles, and the lps muscle. Cranial nerve (CN) VI
innervates the LR; CN IV the SO; and CN III lps, SR, MR,
IR, and IO. CN III has an upper and a lower division.
The upper division supplies the LPS and SR.
The lower division supplies the MR, IR, and IO
muscles.
The parasympathetic innervations of the sphincter
pupillae and ciliary muscle travels with the branch of
The lower division of CN III that supplies the inferior
oblique muscle.
Eye position
When the eye is directed straight ahead and the head is
also straight, the eye is said to be in primary position.
The primary action of a muscle is its major effect on the
position of the eye when the muscle contracts while the
eye is in primary position. The secondary and tertiary
Actions of a muscle are the additional effects on the
position of the eye in Primary position .The globe usually
can be moved about 50° in each direction from primary
position. Under normal viewing, the eyes move
only about 15°- 20° from primary position before head
Movement occurs.
Horizontal Rectus Muscles
The horizontal rectus muscles are the
MR and LR. Both arise From the
annulus of Zinn. The MR courses along
the medial orbital wall. The proximity of
the MR to the medial orbital wall means
the MR can be injured during ethmoid
sinus surgery. The LR courses along the
lateral orbital wall. In primary position,
the MR is an adductor, and the LR is an
abductor.
Horizontal Rectus Muscles
The MR muscle is the only
rectus muscle that does not
have an oblique muscle running
tangential to it. This makes
surgery on the MR less
complicated, but means that
there is neither a point of
reference if the surgeon
becomes disoriented nor a point
Of attachment if the muscle is
lost.
Vertical Rectus Muscles
The vertical rectus muscles are the
SR and IR. The SR originates from
the annulus of Zinn and courses
anteriorly, upward over the eyeball,
and laterally, forming an angle of
23° with the visual axis of the eye in
primary position .In primary
position, this muscle's primary
action is elevation, secondary
action is intorsion ,and tertiary
action is adduction.
Vertical Rectus Muscles
The IR also arises from the annulus
of Zinn, and it then courses
anteriorly, downward, and laterally
along the floor of the orbit, forming
an angle of 23° with the visual axis
of the eye in primary position . In
primary position, the IR primary
action is depression, secondary
action is extorsion, and tertiary
action is adduction.
Oblique Muscles
The SO originates from the orbital apex above the
annulus of Zinn and passes anteriorly and Upward
along the superomedial wall of the orbit. The muscle
becomes tendinous before passing through the
trochlea, a cartilaginous saddle attached to the frontal
bone in the superior nasal orbit. A bursa-like cleft
separates the trochlea from the loose fibro vascular
sheath surrounding the tendon. The discrete fibers of
the tendon telescope as they move through the
trochlea, the central fibers moving farther than the
peripheral ones .
Oblique Muscles
The function of the trochlea is to redirect
the tendon inferiorly, posteriorly, and laterally,
forming an angle of 510 with the visual axis of
the eye in primary position .The tendon
penetrates the Tenon capsule 2 mm nasally and 5
mm posteriorly to the nasal insertion of the SR.
Passing under the SR, the tendon inserts in the
posterosuperior quadrant of the eyeball, almost
or entirely laterally to the midvertical plane or
center of rotation. In primary position, the
primary action of the SO is intorsion , secondary
action is depression, and tertiary action is
abduction.
Oblique Muscles
The IO originates from the periosteum of
the maxillary bone, just posterior to the
orbital rim and lateral to the orifice of the
Lacrimal fossa. It passes laterally,
superiorly, and posteriorly, going inferior to
the IR and inserting under the LR in the
posterolateral portion of the globe, in the
area of the macula.
Oblique Muscles
The IO forms an angle of 510
with the visual axis of the
eye in primary position . In
primary position, the
muscle's primary action is
extorsion , secondary action
is elevation, and tertiary
action is abduction.
LPS Muscle
The LPS arises at the apex of the orbit from
The Lesser wing of the sphenoid bone just
Superior to the annulus of Zinn. The origin
of this muscle blends with the SR inferiorly
and with the SO medially. The LPS passes
anteriorly, lying just above the SR; the fascial
sheaths of these 2 muscles are connected.
The LPS muscle becomes an aponeurosis in
the region of the superior fornix.This muscle
has both a cutaneous and a tarsal insertion
Insertion Relationships of the Rectus Muscles
Starting at the MR and proceeding to IR,
LR, and SR muscles, the rectus muscle
tendons insert progressively farther from
the limbus. A continuous curve drawn
through these insertions yields a spiral,
known as the spiral of Tillaux . The
temporal side of the vertical rectus muscle
insertion is farther from the limbus (ie,
more posterior) than is the nasal side.
Blood Supply of the EOM
Arterial System
The muscular branches of the
ophthalmic artery provide the most
Important Blood Supply for the EOM.
The lateral muscular branch supplies
the LR, SR, SO, and LPS muscles; the
medial muscular branch, the larger of
the 2, supplies the IR, MR, and IO
muscles.
The LR is partially supplied by the
lacrimal artery; the infraorbital
artery partially supplies the IO and IR
muscles.
Blood Supply of the EOM
The muscular branches give rise
to the anterior ciliary arteries
accompanying the rectus
muscles; each rectus muscle has
1 to 3 anterior ciliary arteries.These
pass to the episclera of the globe
and then supply blood to the
anterior segment. The superior and
inferior rectus muscles carry the
bulk of the blood supply.
Venous System
The venous system parallels
the arterial system, emptying
into the superior and inferior
orbital veins. Generally, 4
vortex veins are located
posterior to the equator; these
Are Usually found near the
nasal and temporal margins of
the SR and IR muscles.
Structure of the EOM
The important functional characteristics of
muscle fibers are contraction speed and
Fatigue resistance. The eye muscles participate
in motor acts that are among the fastest
(saccadic eye movements) in the body and in
those that are among the most sustained (gaze
fixation and vergence movements). Like skeletal
muscle, EOM is Voluntary striated muscle.
However, developmentally, biochemically,
structurally, and functionally, it is different from
typical skeletal muscle.
Structure of the EOM
The EOM are innervated at a ratio of nerve
fiber to muscle fiber up to 10 times that of
skeletal muscle.
This difference may allow for more accurate
eye movements controlled by an array of
systems ranging from the primitive vestibulo
ocular reflex to highly evolved vergence
movements.
The EOM exhibit a distinct 2-layer organization:
an outer orbital layer, which acts only
on connective tissue pulleys , and an inner
global layer, whose tendon inserts on the
sclera to move the globe.
Structure of the EOM
The muscle fibers comprising the orbital and global layers can be
Either singly or multiply innervated.
Singly innervated fibers are fast-twitch generating and resistant to
fatigue. Eighty percent of the fibers comprising the orbital layer
Muscle are singly innervated. Ninety percent of the fibers making
up the global layer muscle are singly innervated, and they can be
subdivided into 3 groups (red, intermediate, and white), based on
mitochondrial content, with the red fibers being the most fatigue
resistant and the white fibers, the least. The orbital singly
innervated fibers are considered the major contributor to sustained
EOM force in primary and deviated positions, and, of all muscle
fiber types, this type is the most affected by denervation from
damage to the motor nerves or from damage to the end plates,
occurring after botulinum toxin injection.
Structure of the EOM
The function of the multiply innervated fibers
of the orbital and global layers is not clear.
These fibers are not seen in the LPS, and it is
thought that they are involved in the finer
control of fixation and in the smooth and
finely graded eye movements, particularly
vergence control.
These novel properties of eye muscles lead to
differential responses to pharmaceuticals
such as botulinum toxin, channel blockers, or
local anesthetics, as well as disease processes
such as myasthenia gravis and muscular
dystrophy.
Orbital and Fascial Relationships
Within the orbit, a complex musculo fibroelastic
Structure suspends the globe, supports the EOM, and
compartmentalizes the fat pads .In the past, the
Distinctness of these layers has been overstated. The
extent and complexity of the interconnectedness of
the orbital tissues has recently come to light and is
still being investigated.
Clinically, the consequences of tissue entrapment in
Blowout fractures and post-retro bulbar hemorrhage
fibrosis of delicate fibrous septa illustrate the intense
fibrous connections throughout the orbit.
Adipose Tissue
The eye is supported and cushioned
in the orbit by a large amount of
fatty tissue. External to the muscle
cone, fatty tissue comes forward
with the rectus muscles, stopping
About 10 mm from the limbus. Fatty
tissue is also present inside the
muscle cone, kept away from the
sclera by the Tenon capsule .
Muscle Cone
The muscle cone lies
posterior to the equator. It
consists of the EOM, the
EOM sheaths, and the
Intermuscular membrane.
The muscle cone extends
posteriorly to the annulus
of Zinn at the orbital apex.
Muscle Capsule
Each rectus muscle has a
surrounding fascial capsule
That extends with the muscle
from its origin to its insertion.
These capsules are thin
posteriorly, but near the
equator they thicken as they
pass through the sleeve of the
Tenon capsule, continuing
anteriorly with the muscles to
their insertions.
Muscle Capsule
Anterior to the equator
between the undersurface of
the muscle and the sclera
there is almost no fascia, only
connective tissue footplates
that connect the muscle to the
globe. The smooth avascular
surface of the muscle capsule
allows the muscles to slide
smoothly over the globe.
The Tenon Capsule
The Tenon capsule is the
principal orbital fascia and
Forms the envelope within
which the eyeball moves. The
Tenon capsule fuses posteriorly
with the optic nerve sheath
and fuses anteriorly with the
intermuscular septum at a
position 3 mm from the limbus
.
The Tenon Capsule
The posterior portion of the
Tenon capsule is thin and
flexible, allowing for free
movement of the optic
nerve, ciliary nerves, and
ciliary vessels as the globe
rotates, while separating
the orbital fat inside the
muscle cone from the sclera.
The Tenon Capsule
At and just posterior to the equator, the Tenon
capsule is thick, suspending the globe like a
Trampoline by means of connections to the
periorbital tissues. The global layer of the 4
Rectus muscles penetrates this thick
Musculofibroelastic tissue approximately 10mm
posterior to their insertions. The oblique
muscles penetrate the Tenon capsule anterior
to the equator. The tenon capsule continues
forward over these 6 extra ocular muscles
and separates them from the orbital fat and
structures lying outside the muscle cone.
Pulley System
The 4 rectus muscles are surrounded by distinct
fibroelastic pulleys. Though not as distinct as the
trochlea of the SO, these pulleys maintain the position
of the EOM relative to the orbit. They consist of
collagen, elastin, and smooth muscle, which allows them
to contract and relax. As the orbital muscle layer
contracts, this pulley must be pulled back so that the
distance between the location of the pulley and the
insertion of the muscle on the globe remains
approximately constant Just as the trochlea acts as the
functional origin of the SO, these pulleys act
mechanically as the Rectus muscle origins. The pulleys
consist of discrete rings of dense collagen, which
encircle the EOM, transitioning into less substantial but
broader collagen sleeves both posteriorly and anteriorly.
Pulley System
These sleeves stabilize the muscle path,
preventing side slipping or movement
perpendicular to the muscle axis.
Anteriorly, the sleeves thin to form slings
between the rectus muscles (the
intermuscular septum), which fuse with the
conjunctiva 3 mm posterior to the limbus.
The posterior section of the intermuscular
Septum separates the intraconal fat pads
from the extraconal fat pads. Numerous
extensions from all of the extraocular
muscle sheaths attach to the orbit and
help support the globe.
Pulley System
The inferior oblique muscle originates anteriorly from the
Infero nasal orbital rim adjacent to the anterior lacrimal
crest, continuing laterally to enter its connective tissue
Pulley inferior to the IR at a site where the IO penetrates
the Tenon capsule. The IO pulley and IR pulley are
coupled to form the Lockwood ligament . The orbital
layer of the IO muscle inserts partly on the conjoined IO
IR pulleys, partly on the IO sheath temporally, and partly
on the inferior aspect of the LR pulley, forming a
connective tissue "hammock" across the inferior orbit.
The smooth muscle retractors of the lower eyelid (the
Muller inferior tarsal muscle) and connective tissues
extending to the inferior tarsal plate are also coupled to
the conjoined IR/ I O