Islamic University of Gaza
Faculty of science
Optometry department
2nd level
The Extraocular Muscles
Done By :
*Dana
*Staaar
Under supervision of:
Dr. Farouk El Baz
Second term
2007-2008
Extra Eye Muscles
Introduction:
THE EXTRAOCULAR MUSCLES (EOMs) are a group of unique skeletal
muscles that are required to locate and accurately track objects by
the visual system. The EOMs faithfully, rapidly, and accurately
effectuate a variety of reflex and voluntary eye movements.
The functional requirements for EOMs are wide ranging and include
1) provision of the relatively slow vestibuloocular and optokinetic eye
movement reflexes for baseline ocular stability and 2) effectuating
pursuit and vergence eye movements to maintain fixation on slowly
moving targets, as well as 3) providing rapid saccadic eye movements
to quickly reorient the visual system to new targets. In humans,
coordinate functioning of EOMs is critical for provision of binocular
vision. Furthermore, the EOMs need to function on demand with
high fidelity, for relatively long periods of time
The muscles involved in the movements of eye and its
adnexa may be divided into three broad groups:
- extrinsic muscles of the eyeyball
-muscles of the lids
-nonstriated muscles of the orbit
Ocular muscles are 2 types:-
A-Extraocular muscles:
1-superior rectus muscle
2-inferior rectus muscle
3-lateral rectus muscle
4-medial rectus muscle
5-superior oblique muscle
6-inferior oblique muscle
Annulas of Zinn is the origin for the extraocular eye
muscles except the inferior oblique muscle.
B-Intraocular muscles:
1-sphincter pupillae muscle in the iris.
2-dilator pupillae muscle in the iris.
3-ciliary muscle in the ciliary body.(accommodation)
The Extraocular Muscles
The six tiny muscles that surround the eye and control its
movements are known as the extraocular muscles (EOMs).
The primary function of the four rectus muscles is to control
the eye's movements from left to right and up and down. The
two oblique muscles move the eye rotate the eyes inward and
outward.
All six muscles work in unison to move the eye. As
one contracts, the opposing muscle relaxes,
creating smooth movements. In addition to the
muscles of one eye working together in a
coordinated effort, the muscles of both eyes work in
unison so that the eyes are always aligned.
The extraocular muscles, considering their relatively small size, are
incredibly strong and efficient. There are six extraocular muscles
which act to turn or rotate an eye about its vertical, horizontal, and
antero-posterior axes:
medial rectus (MR)
superior rectus (SR)
superior oblique (SO)
lateral rectus (LR)
inferior rectus (IR)
inferior oblique (IO)
Here is a schematic of a left eye, showing how its extraocular muscles
insert into the eye:
Muscles of orbit
There are seven muscles of the orbit; one controls the
movement of the upper eyelid, and six others control the
movement of the eye.
Paths
View of right eye from the right
Annulus tendineus communis1 =
Superior rectus muscle2 =
Inferior rectus muscle3 =
4=Medial rectus muscle
5 =Lateral rectus muscle
6=Superior oblique muscle
7=Trochlea of superior oblique
8=Inferior oblique muscle
Levator palpebrae superioris muscle9=
10=Eyelid
11=Eyeball
12 =Optic nerve
Five with paths from annulus of zinn
Five of the extraocular muscles have their origin in the back of the
orbit in a fibrous ring called the annulus of Zinn.
Four of these then course forward through the orbit and insert onto
the globe on its anterior half (i.e., in front of the eye's equator). These
muscles are named after their straight paths, and are called the four
rectus muscles, or four recti.




superior rectus - inserts on the globe at 2
inferior rectus - inserts on the globe at 3
medial rectus - inserts on the globe at 4
lateral rectus - inserts on the globe at 5
(Note that lateral and medial are relative to the subject, with lateral
toward the side and medial toward the midline, thus the medial
rectus is the muscle closest to the nose
List of muscles
Muscle Innervation Origin Insertion
Primary Secondary Tertiary
function function function
Superior
Superior branch of
rectus
oculomotor
nerve
eye
Annulus (anterior,
Elevation
of Zinn superior
surface)
Intorsion
Adduction
Inferior
Inferior branch of
rectus
oculomotor
nerve
eye
Annulus (anterior,
Depression Extorsion
of Zinn inferior
surface)
Adduction
Lateral
rectus
Abducens
nerve
eye
Annulus (anterior,
Abduction
of Zinn lateral
surface)
Medial
rectus
Inferior
branch of
oculomotor
nerve
eye
Annulus (anterior,
Adduction
of Zinn medial
surface)
Superior Trochlear
oblique nerve
eye
(posterior,
Annulus
superior, Intorsion
of Zinn
lateral
surface)
Inferior
Inferior branch of
oblique oculomotor
nerve
eye
(posterior,
Lacrimal
inferior, Extorsion Elevation
bone
lateral
surface)
Depression Abduction
Abduction
EOM actions and innervations
Muscle
Medial Rectus
Inferior Rectus
Lateral Rectus
Actions
Primary CN Innervation
& Secondary
Adduction CN III
Depression
Extortion
CN III
Adduction
Abduction
Elevation
Superior Rectus Intorsion
Adduction
Depression
Superior Oblique Intorsion
Abduction
Elevation
Inferior Oblique Extorsion
Abduction
CN VI
CN III
CN IV
CN III
Actions
Note that intorsion and extorsion are not included in the following
table; their actions are accounted for via summation of other actions.
Medial (towards nose) Lateral (towards temple)
Elevation, adduction:
Superior rectus
Elevation, abduction:
inferior oblique
Adduction:
Medial rectus
Abduction:
Lateral rectus
Depression, adduction:
Inferior rectus
Depression, abduction:
Superior oblique
In an eye examination, the inability of the patient to move the
eye in the specified direction can indicate a problem with the
associated muscle, and the nerve associated with that
muscle.
Ocular Movements
 ABDUCTION: eye
moves temporally
 ADDUCTION: eye
moves nasally
 ELEVATION: Eye
moves up
 DEPRESSION: Eye
moves down
 INTORSION: Top of cornea
rotates toward nose
 EXTORSION: Top of cornea
rotates away from nose
 CONVERGENCE: Both eyes
move nasally at the same time
 DIVERGENCE: Both eyes move
temporally at the same time
Primary Fields of Action in Directed Gaze Positions
Extorsion
Intorsion
Intorsion
Elevation
Abduction
Extorsion
Elevation
Adduction
Adduction
Depression
Abduction
Depression
R.Sup.Rectus
R.Inf.Oblique
L.Inf.Oblique
L.Sup.Rectus
R.Lat.Rectus
R.Med.Rectus
L.Med.Rectus
L.Lat.Rectus
R.Inf.Rectus
R.Sup.Oblique
L.Sup.Oblique
L.Inf.Rectus
Planes of action and axes of rotation
The visual axis is a straight line that can be drawn from a
distant object of regard to the fovea. In the normal eye, the
visual axis passes through the apex of the cornea, the center
of the pupil, and the thickest anterior-posterior part of the
lens.
The horizontal plane is a primary plane of action. This is illustrated in
the animation below. The horizontal rod going through the cornea
represents the visual axis (also called the optical axis). The vertical rod
with the arrow at the top represents the vertical axis. As the eye turns
around the vertical axis, the visual axis sweeps along the horizontal
plane.
The vertical plane is the second of the primary planes of action. This is
illustrated in the animation below. The rod going through the cornea
represents the visual axis (also called the optical axis). The horizontal
rod with the arrow represents the horizontal axis. As the eye turns
around the horizontal axis, the visual axis sweeps along the vertical
plane.
The third plane of action can be represented as the plane of this screen
(or paper) as you view the drawings below. Intortion and extortion refer
to rotation around the visual axis, as illustrated below. Intortion refers
to a nasal rotation from the 12 o'clock position. Extortion refers to a
temporal rotation from the 12 o'clock position.
The movements of the eyes can be described by their actions
in one or more of these planes of action.
The MR and the LR each have only one "action". The action
of the MR is adduction and the action of the LR is abduction.
These actions occur only along the horizontal plane.
The other EOMs are called cyclovertical muscles. Each of these
muscles has more than one action. They act in the vertical plane as
well as the horizontal plane, and they also intort or extort the globe.
This will be illustrated for each of the cyclovertical muscles. These
muscles each have a primary action (1°), a secondary action (2°), and
a tertiary action (3°).
muscle movements
A given extraocular muscle moves an eye in a specific manner, as
follows:






medial rectus (MR)—
o moves the eye inward, toward the nose (adduction)
lateral rectus (LR)—
o moves the eye outward, away from the nose (abduction)
superior rectus (SR)—
o primarily moves the eye upward (elevation)
o secondarily rotates the top of the eye toward the nose
(intorsion)
o tertiarily moves the eye inward (adduction)
inferior rectus (IR)—
o primarily moves the eye downward (depression)
o secondarily rotates the top of the eye away from the nose
(extorsion)
o tertiarily moves the eye inward (adduction)
superior oblique (SO)—
o primarily rotates the top of the eye toward the nose
(intorsion)
o secondarily moves the eye downward (depression)
o tertiarily moves the eye outward (abduction)
inferior oblique (IO)—
o primarily rotates the top of the eye away from the nose
(extorsion)
o secondarily moves the eye upward (elevation)
o tertiarily moves the eye outward (abduction)
The primary muscle that moves an eye in a given direction is known
as the “agonist.” A muscle in the same eye that moves the eye in the
same direction as the agonist is known as a “synergist,” while the
muscle in the same eye that moves the eye in the opposite direction of
the agonist is the “antagonist.” According to “Sherrington’s Law,”
increased innervation to any agonist muscle is accompanied by a
corresponding decrease in innervation to its antagonist muscle(s).
Cardinal positions of gaze
The “cardinal positions” are six positions of gaze which allow
comparisons of the horizontal, vertical, and diagonal ocular
movements produced by the six extraocular muscles. These are the
six cardinal positions:






up/right
up/left
right
left
down/right
down/left
In each position of gaze, one muscle of each eye is the primary mover
of that eye and is yoked to the primary mover of the other eye.
Below, each of the six cardinal positions of gaze is shown, along with
upward gaze, downward gaze, and convergence
MR = Medial Rectus
SR = Superior Rectus
SO = Superior Oblique
LR = Lateral Rectus
IR = Inferior Rectus
IO = Inferior Oblique
muscle innervations
Each extraocular muscle is innervated by a specific cranial nerve
(C.N.):






medial rectus (MR)—cranial nerve III (Oculomotor)
lateral rectus (LR)—cranial nerve VI (Abducens)
superior rectus (SR)—cranial nerve III (Oculomotor)
inferior rectus (IR)—cranial nerve III (Oculomotor)
superior oblique (SO)—cranial nerve IV (Trochlear)
inferior oblique (IO)—cranial nerve III (Oculomotor)
The following can be used to remember the cranial nerve
innervations of the six extraocular muscles:
LR6(SO4)3.
That is, the lateral rectus (LR) is innervated by C.N. 6, the superior
oblique (SO) is innervated by C.N. 4, and the four remaining muscles
(MR, SR, IR, and IO) are innervated by C.N. 3.
Another way to remember which nerves innervate which muscles is
to understand the meaning behind all of the Latin words. The fourth
cranial nerve, the trochlear, is so named because the muscle it
innervates, the superior oblique, runs through a little fascial pulley
that changes its direction of pull. This pulley exists in the
superiomedial corner of each orbit, and "trochl-" is Latin for
"pulley." The sixth cranial nerve, the abducens, is so named because
it controls the lateral rectus, which abducts the eye (rotates it
laterally) upon contraction. All of the other muscles are controlled by
the third cranial nerve, the oculomotor, which is so named because it
is in charge of the movement (motor) of the eye (oculo-).
Innervation to agonist and antagonist muscles is described
by Sherrington's Law of reciprocal innervation.
Remember that the RMR and the RLR are antagonists of one
another. As one contracts, the other must relax.
Sherrington's Law states that for the amount of contraction
innervation given to the RMR, an equal amount of relaxation
innervation must be given to the RLR. That makes sense.
Otherwise, their actions would not be coordinated.
Innervation to yoke muscles is described by Hering's Law
of simultaneous innnervation. The RMR and the LLR are yoke
muscles because they contract simultaneously to move the
gaze to the left. Hering's Law states that the innervation to
the yoke muscle in the non-fixing eye must equal the
innervation to the corresponding agonist muscle in the fixing
eye.
Anatomical arrangement
All of the extraocular muscles, with the exception of the inferior
oblique, form a “cone” within the bony orbit. The apex of this cone
is located in the posterior aspect of the orbit, while the base of the
cone is the attachment of the muscles around the midline of the eye.
This conic structure is referred to as the “annulus of Zinn,” and
within the cone runs the optic nerve (cranial nerve II), and within
the optic nerve are contained the ophthalmic artery and the
ophthalmic vein.
The superior oblique muscle, although part of the cone-shaped
annulus of Zinn, differs from the recti muscles in that before it
attaches to the eye it passes through a ring-like tendon, the
“trochlea” (which acts as a pulley), in the nasal portion of the orbit.
The inferior oblique, which is not a member of the annulus of Zinn,
arises from the lacrimal fossa in the nasal portion of the bony orbit
and attaches to the inferior portion of the eye.
The Medial Rectus
The MR originates in the annulus of Zinn the back of the bony orbit,
along with all of the other EOMs, with the exception of the inferior
oblique (IO). The MR inserts into the globe about 5mm behind the
limbus on the medial side of the cornea.
The MR is the strongest of the EOMs. It has the most mass, and it has
the most anterior insertion into the globe (for greater leverage). It is
used often to converge the eyes into near (reading) gaze.
It is innervated by the third cranial nerve (CN III). When the MR
contracts, the eye rotates toward the nose (adduction). In the animation
below, the LMR is contracting and the left eye is adducting.
The Lateral Rectus
The lateral rectus (LR) originates in the annulus of Zinn and inserts
about 7mm behind the limbus on the temporal side of the globe. The LR
works only on the horizontal plane of action. When the LR contracts,
the eye rotates temporally (abduction). The LR is the only muscle
innervated by CN VI, the "abducens nerve". In the animation above, the
RLR is contracting and the right eye is abducting.
The Superior Rectus
The SR is innervated by CN III. The SR inserts superiorly on the globe
about 8mm behind the limbus. Notice that the tendon of the SO muscle
passes underneath the SR muscle (arrow).
The primary action of the SR is elevation of the globe. That is, as the SR
contracts, the cornea and the visual axis move upward as the globe
rotates about the horizontal axis and moves in the vertical plane. But
notice that the SR does not travel straight back from it's insertion on the
globe, it angles nasally when compared to the visual axis.
Thus, it's action is not purely along the vertical plane. When the globe is
in the primary position (as pictured above), contraction of the SR will
not only elevate the eye, but will also tend to rotate the eye nasally from
the 12 o'clock position (intortion). This is called the secondary action of
the SR. Contraction of the SR with the globe in the primary position will
also move the eye somewhat nasally along the horizontal plane
(adduction). This is the tertiary action of the SR.
The three actions of the SR are illustrated in the animation below:
Look at the illustration of the SR in the right eye below. In number 1 we
have the eye in the primary position of gaze with the optical axis
illustrated by the double headed arrow and the action of the SR
illustrated by the single headed arrow.
In number 2, look what happens when the eye is abducted 23 degrees.
Now the plane of action of the SR lines up with the visual axis. From
this position of abduction, the primary action of elevation is strongest
for the SR. As illustrated in number 3, in the position of adduction, the
elevating effect of the SR is reduced. The effect of the SR acting by
itself can be tested when the eye is elevated from a position of
abduction, as illustrated with drawing number 2.
The Inferior Rectus
The inferior rectus (IR) is very similar to the SR, except that it inserts
underneath the globe instead of on top. It originates in the annulus of
Zinn and it also travels at a 23 degree angle to the primary position
visual axis to it's insertion about 6mm behind the limbus. From this
position, the primary action of the IR is depression of the globe.
In the top photo below we see a view of the SR and LR on cut away
model of the right eye. In the bottom photo, the SR and LR have been
removed to reveal a view of the IR.
The secondary action of the IR is extortion, and the tertiary action is
adduction. Just like the SR, the primary action (depression) increases
in abduction and decreases in adduction. To test the action of the IR by
itself, have the patient abduct the eye slightly (23 degrees to be exact)
and look down.
Note that the SR and IR both are adductors in their tertiary action, so
that they help each other in that regard, but they work opposite to one
another with regard to the secondary tortional action (SR-intortion, IRextortion).
The Oblique Muscles
The oblique muscles have two primary functions. The first is intortion
or extortion of the globe to keep the eyeballs level as the head tilts.
Notice the dots on the corneas at 12o'clock in the animation below. As
the head tilts to the right, the right eye intorts and the left eye extorts to
keep the eyeballs level.
The other major function is to create a counterbalancing force to that of
the rectus muscles. The rectus muscles are pulling the globe inward
toward the back of the bony orbit. The oblique muscles pull outwardly
to keep the globe "floating" in the orbital cavity.
The Superior Oblique
The SO is the longest of the EOMs at about 60mm. The other muscles
are about 40mm in length. The SO has to be longer because it passes
through a "pully" called the trochlea, which redirects the action of this
muscle. Look at the picture of the right eye model below, in which the
the SR and the MR have been removed.
Your can see the SO as it originates in the annulus of Zinn and passes
along the medial wall of the orbit and threads through the trochlea.
Notice that the tendon of the SO inserts into the globe underneath the
SR. The arrows on the muscle indicate the direction of action as the SO
contracts. From this angle it is apparent that rotation around the visual
axis would result in intortion as the SO contracts (right eye model). This
is the primary action of the SO.
The next two photos show the model from the front and from above.
Since the SO inserts near the top of the globe, and there is a posterior to
anterior deflection of the SO tendon, as the SO contracts, the back of
the globe moves upward and the front of the globe moves downward,
thus there is also some depression of the globe around the horizontal
axis. This is the secondary action of the SO.
The photo below is from above the orbit, showing the posterior to
anterior bias to the direction of the SO tendon. Thus, as the SO
contracts there is a rotation about the vertical axis that results in some
abductive movement. Abduction is the tertiary action of the SO.
The SO is innervated by the trochlear nerve (CN IV). The animation
below demonstrates the three actions of the SO.
The Inferior Oblique
You may remember that all of the EOMs originate in the annulus of Zinn,
except for one. You guessed it, that would be the inferior oblique. The
IO originates in the inferior nasal orbital rim and travels slightly
posteriorly to the insertion point underneath the globe. Look at the
model photos below. The top photo shows the model of the right eye
with the SR and LR muscles removed. You can see the insertion of the
IO just below the LR The bottom photo shows the model with the SR,
LR, and the globe removed so that we can see the IO.
Notice that the IO passes underneath the IR. As the IO contracts, I think
it is fairly obvious that the eye is going to extort as it turns on the visual
axis. This is the primary action of the IO.
It may be difficult to tell from the photo, but the IO travels slightly
posterior to anterior from insertion to origin. Since the insertion of the
IO is on the bottom half of the globe, contraction will also result in the
bottom of the globe rotating forward and upward along the horizontal
axis. Thus, the secondary action of the IO is elevation.
Also note that the insertion of the IO is posterior to the equator and on
the temporal half of the globe. When the IO contracts, the back of the
globe is pulled nasally, resulting in abductive rotation of the eye around
the vertical axis. The tertiary action of the IO is abduction. Below is an
animation of the three actions of the IO.
The movements of the eye
Ductions
When considering each eye separately, any movement is called a
“duction.” Describing movement around a vertical axis, “abduction”
is a horizontal movement away from the nose caused by a contraction
of the LR muscle with an equal relaxation of the MR muscle.
Conversely, “adduction” is a horizontal movement toward the nose
caused by a contraction of the MR muscle with an equal relaxation of
the LR muscle.
Describing movement around a horizontal axis, “supraduction”
(elevation) is a vertical movement upward caused by the contraction
of the SR and IO muscles with an equal relaxation of the of the IR
and SO muscles. Conversely, “infraduction” (depression) is a
vertical movement downward caused by the contraction of the IR
and SO muscles with an equal relaxation of the SR and IO muscles.
Describing movement around an antero-posterior axis,
“incycloduction” (intorsion) is a nasal or inward rotation (of the top
of the eye) caused by the contraction of the SR and SO muscles with
an equal relaxation of the IR and IO muscles. Conversely,
“excycloduction” (extorsion) is a temporal or outward rotation (of
the top of the eye) caused by the contraction of the IR and IO
muscles with an equal relaxation of the SR and SO muscles.
Versions
When considering the eyes’ working together, a “version” or
“conjugate” movement involves simultaneous movement of both eyes
in the same direction. Agonist muscles in both eyes which work
together to move the eyes in the same direction are said to be
“yoked” together. According to “Hering’s Law,” yoked muscles
receive equal and simultaneous innervation. There are six principle
versional movements where both eyes look or move together in the
same direction, simultaneously:






dextroversion (looking right)—
o right lateral rectus
o left medial rectus
levoversion (looking left)—
o left lateral rectus
o right medial rectus
supraversion or sursumversion (looking straight up)—
o right & left superior recti
o right & left inferior obliques
infraversion or deorsumversion (looking straight down)—
o right & left inferior recti
o right & left superior obliques
dextroelevation (looking right and up)—
o right superior rectus
o left inferior oblique
dextrodepression (looking right and down)—
o right inferior rectus
o left superior oblique




levoelevation (looking left and up)—
o right inferior oblique
o left superior rectus
levodepression (looking left and down)—
o right superior oblique
o left inferior rectus
dextrocycloversion (rotation to the right)—
o right inferior rectus & inferior oblique
o left superior rectus & superior oblique
levocycloversion (rotation to the left)—
o left inferior rectus & inferior oblique
o right superior rectus & superior oblique
Here is a cross diagram that shows which muscles move the
eyes into the positions of gaze. There is not one muscle
responsible for just elevation or just depression, so there is
no single muscle labeled for these positions. So, according
to the diagram, which muscles are responsible for elevation?
The SR and the IO. Which muscles are responsible for
depression? This is a very handy diagram for test taking
purposes.
Vergences
A “vergence” or “disconjugate” movement involves simultaneous
movement of both eyes in opposite directions. There are two
principle vergence movements:


convergence—both eyes moving nasally or inward
divergence—both eyes moving temporally or outward
If one eye constantly is turned inward (“crossed-eye”) or outward
(“wall-eye”), this is referred as a “strabismus” or “heterotropia,”
discussed later.
Usually, a vergence is performed relative to a point of fixation. For
instance, one could be looking at TV across the room (at a far
distance) and, when a commercial comes on, converge both eyes to
read a book (at a near distance). Then, after the commercial is over,
one could diverge both eyes to look at the TV again.
Yoked muscles
Both eyes actually cannot, at the same time, diverge outward from
looking straight ahead. That is, the two lateral recti muscles
cannot pull the eyes outward, simultaneously and voluntarily,
while one is viewing something far away. However, if one is
falling asleep with one’s eyes still open, it is possible for the eyes to
diverge, momentarily and involuntarily, causing temporary
diplopia (double visionYoke muscles are the primary muscles in
each eye that accomplish a given version (eg, for right gaze, the
right lateral rectus and left medial rectus muscles are yoke
muscles).
Each extraocular muscle has a yoke muscle in the opposite eye to
accomplish versions into each gaze position.
•
By the Hering law, yoke muscles receive equal and
simultaneous innervation (stimulation) the magnitude of which
is determined by the fixating eye movement
Agonist and antagonist
In the same eye (Ductions)
The primary muscle that moves an eye in a given direction
is known as the agonist.
A muscle in the same eye that moves the eye in the same
direction as the agonist is known as the synergist.
while a muscle in the same eye that moves the eye in the
opposite direction of the agonist is the antagonist
Synergists and antagonists
For example, in abduction of the right eye, the right •
lateral rectus muscle is the agonist; the right superior
and inferior oblique muscles are the synergists;
(abductors) and the right medial, superior, and inferior
recti are the antagonists. (adductors) , and vise versa .
By the Sherrington law, increased ( stimulation ) •
innervation to any muscle (agonist) is accompanied by a
corresponding decrease (inhibition ) in innervation to its
antagonists
In both eyes together
Binocular eye movements are either conjugate
(versions) or disconjugate (vergences).
•
1- Conjugate versions
are movements of both eyes in the same direction
•
A-Dextroversion is movement of both eyes to the right.
B- levoversion is movement of both eyes to the left.
C-Sur/sumversion or (supraversion )is elevation of both eyes .
D-Deor/sumversion or (infraversion ) is depression of both eyes
2- Disconjugate vergence
contains each of convergence and divergence.
a_ convergence in which the 2 eyes move inwards (nasally) at the same
time “crossed-eyes’’.
b_ divergence in which the 2 eyes move outwards (temporally) at the
same time. (wall-eyes
Strabismus (Heterotropia)
Normally, when viewing an object, the “lines of sight” of both eyes
intersect at the object; that is, both eyes point directly at the object
being viewed. An image of the object is focused upon the macula of
each eye, and the brain merges the two retinal images into one.
Sometimes, however, due to some type of extraocular muscle
imbalance, one eye is not aligned with the other eye, resulting in a
“strabismus,” also called a “heterotropia” or simply “tropia.”
(Occasionally, this ocular deviation is referred to as a “squint,”
although this term is not very descriptive and no longer is commonly
used.)
With strabismus, while one eye is fixating upon a particular object,
the other eye is turned in another direction, relative to the first eye,
whether inward (“cross-eyed”), outward (“wall-eyed”), upward, or
downward. As a result, the person either experiences “diplopia”
(double vision), since two different objects are imaged onto the
maculas of both eyes, or else the person’s brain learns to “suppress”
(turn off) the image of the strabismic eye to maintain single vision.
If the strabismus occurs sometimes, but not all the time, it is said to
be “intermittent.” If the strabismus occurs all the time, it is said to
be “constant.” Occasionally, whether the strabismus is intermittent
or constant, one eye will be the deviating eye at certain times, while
the opposite eye will be the deviating eye at other times; this is
referred to as “alternating” strabismus.
The misalignment of a strabismic eye occurs in about 2% of children
and may be in any direction: inward (“esotropia” or “crossed-eye”),
outward (“exotropia” or “wall-eye”), upward (“hypertropia”),
downward (“hypotropia”), or any combination of these. Strabismus
also can occur due to a nerve paralysis or paresis, retinal disease,
injury, or the presence of a very different refractive error (usually
much higher) in the strabismic eye compared to the other eye.
The angle of deviation of the strabismus is measured in “prism
diopters.” If the angle of deviation remains the same in all cardinal
positions of gaze (see the previous section), the strabismus is
classified as “concomitant” (or “nonparalytic”). If the angle of
deviation is not the same in all cardinal positions of gaze, the
strabismus is classified as “nonconcomitant” (or “paralytic”).
Below, views of the two most common types of strabismus—esotropia
and exotropia—are displayed:
OD (Right Eye) Esotropia
OD (Right Eye) Exotropia
Esotropia can be congenital (a muscle imbalance present from
birth), and usually the angle of deviation is large. Management
involves surgical correction (at age six months or earlier). Some
cases of low-angle esotropia respond successfully to visual therapy,
especially in a child or an adult for which the esotropia is of recent
onset and for which there is no macular damage (that is, the
strabismic eye is capable of good visual acuity).
The esotropia also can be accommodative, usually due to a high
amount of uncorrected hyperopia (farsightedness), causing a great
deal of accommodation to be required to focus retinal images,
resulting in a subsequent over-convergence and esotropia. The usual
treatment for accommodative esotropia is eyeglasses or contact
lenses, which compensate for the hyperopia and allow the deviating
eye to straighten.
Exotropia also can be congenital, although this is very unusual.
More commonly, exotropia develops in infancy or in early childhood,
often beginning as an intermittant (occasional) strabismus and
sometimes leading to a constant strabismus. A carefully planned
regimen of visual therapy can be used to treat exotropia (especially in
cases where complete suppression of the strabismic eye has not yet
occurred and the eye is capable of good visual acuity). However, in
cases where visual therapy is unsuccessful, surgical correction must
be used to provide a cosmetically improved appearance to the
deviating eye, although this does not necessarily ensure that
binocular vision will result.
Amblyopia and eccentric fixation
If the vision in a strabismic (deviating or turning) eye is suppressed
(turned off) for too long, that eye very well may develop “amblyopia”
or a “lazy eye” condition. This means that the visual acuity in that
eye no longer is as good as the visual acuity in the other eye, which is
used all the time.
In this case, when the normal eye is covered, thus forcing the
strabismic eye to take over, the strabismic eye usually does not point
exactly straight at the object being fixated, so the image of the object
being viewed does not fall directly upon the macula, as it should.
Rather, the image falls upon some eccentric point away from the
macula, where the acuity is not a good. Thus, this is referred to as
“eccentric fixation.”
An eye is not a “lazy eye” simply because it turns and does not align
with the other eye. Amblyopia (“lazy eye”) simply refers to
decreased visual acuity in one eye, compared to the other eye. That
is, an eye is referred to as “lazy” because it does not see as clearly as
the other eye. The most common reason for amblyopia is the
presence of eccentric fixation in a strabismic eye.
Acquired muscle palsy
Damage to cranial nerve III, IV, or VI often will cause a “palsy”
(paralysis or paresis) of the extraocular muscle(s) innervated by that
nerve. The cause of the palsy usually is acquired (due to a lesion, a
stroke, or other trauma), although occasionally it can be congenital.
When the oculomotor nerve (cranial nerve III) is damaged, a palsy in
the medial rectus, superior rectus, inferior rectus, and/or inferior
oblique muscle(s) may occur. If all of these muscles are affected, the
effected eye will be turned outward and downward (due to
unopposed action of the lateral rectus and superior oblique muscles).
The affected eye cannot turn inward past the midline, nor can it turn
upward past the midline. In a complete cranial nerve III paralysis,
the upper eyelid also will be nearly closed from ptosis, and the pupil
might be dilated and unreactive.
When the trochlear nerve (cranial nerve IV) is damaged, a palsy of
the superior oblique (SO) muscle may occur, resulting in a
hypertropia of the affected eye. People with this condition will
experience both a vertical and a torsional diplopia (double vision),
and they will compensate for this by tilting the head toward the
shoulder of the unaffected eye. When utilizing the Bielschowsky
head-tilt test, if the person is told to tilt his/her head toward the
shoulder of the affected eye, an overaction of the inferior oblique (IO)
and elevation of the affected eye (and marked diplopia) will result.
When the abducens nerve (cranial nerve VI) is damaged, a palsy of
the lateral rectus (LR) muscle may occur, resulting in an esotropia of
the affected eye. That eye generally will not be able to look outward
past the midline, and it will be somewhat turned inward when the
other eye is fixating straight ahead. Diplopia will be observed when
the person gazes to the side with the palsied muscle, and the person
will compensate for this by turning the face toward the side of the
palsied eye.