Module 27

Extra-ocular muscle (EOM) function is about maintaining our visual orientation to the world. As discussed in Module 28, normal binocular vision gives us fantastic spatial orientation. The EOMs are the tracking mechanisms that lock on to track the target, maintaining foveal fixation and binocular vision. In order to understand abnormalities of any system, we must first gain an understanding of how the system normally functions.

EOM function is complicated, and the study of EOM function is full of specialized terminology. Taking time to learn the terminology and what each term describes will help you break down a complicated subject into more simple, understandable concepts.

The nine diagnostic positions of gaze

Notice that "left" and "right" are the patient's left and right. I have also seen the positions labeled with the observer's left and right. "Superior" and "inferior" are sometimes used instead of "up" and "down". "Straight" or "center" are sometimes used instead of "primary". The term "elevation" is sometimes used for up-gaze, and "depression" for down-gaze.

A basic test of EOM function is to observe and describe the movement of the eyes in all positions of gaze.

Terminology

The movement of one eye by itself is called a duction.

The movement of the two eyes in the same direction is termed a version.

If the eye looks toward the nose (nasally), it is called adduction.

If the eye toward the ear (temporally), it is called abduction.

When both eyes look to the right, the movement is called dextroversion. Left gaze is called levoversion. Both eyes in upgaze is termed supraversion. Downgaze is called infraversion.

In the animation below, the right eye is adducting, the left eye is abducting, and both eyes together are performing levoversion.

If both eyes look toward the nose, then the eyes are said to be converging (convergence). The eyes converge when reading. The animation below demonstrates near convergence.

What else do you notice in the animation above? The pupils are getting smaller as the eyes converge. This is part of the synkinetic near response. When you look at something up close, three actions happen simultaneously, 1) the eyes converge, 2) the pupils get smaller, and 3) the lenses accommodate. The synkinetic near response triggers accommodative esotropia (see Module 5 for a discussion of esotropia) in some far-sighted children. Here is how it works:

Take, for example, a five year old with a cycloplegic refraction of +5.00 D in each eye. This child has to accommodate 5 D just to see in the distance. When looking at near, the child accommodates even more. This accommodative overload is neurologically tied to convergence (the synkinetic near response). The accommodative output stimulates the convergence response, and the child's eyes inappropriately converge, even when not looking at near. The result is esotropia. This child can be given the full cycloplegic refractive error in a glasses correction, the accommodative overload is relieved, and the esotropia disappears.

Back to the subject at hand:

If both eyes move toward the ears (temporally), then the eyes are said to be diverging (divergence). This action takes place when the eyes move from near gaze to distance gaze. The eyes don't normally diverge from the primary (distance) gaze. In the animation below, the eyes are diverging from the near position.

Each eye has six muscles attached to it that together can turn the eyes in almost any direction. They are the medial rectus (MR), lateral rectus (LR), superior rectus (SR), inferior rectus (IR), superior oblique (SO), and inferior oblique (IO).

For smooth eye movements, the actions of the EOMs must be coordinated. In terms of action, the rectus muscles are easiest to understand.

The rectus muscles attach (insert), with tendons, to the globe 5.5 (MR) to 7.7 mm (SR) behind the limbus. Each rectus muscles extends approximately 41mm to its origin at the back of the orbit at the Annulus of Zinn. Below is pictured a model of the right eye:

When the medial rectus (MR) contracts, the eye moves toward the nose (adduction). The MR is the agonist for adduction. When the lateral rectus (LR) contracts, the eye moves temporally (abduction). The LR is the agonist muscle for the action of abduction. The muscle actions in abduction are illustrated in the animation below:

When the LR muscle contacts, the MR muscle must relax (as demonstrated above), otherwise the muscles would be working against one another and the eye would not move. Therefore, the MR is the antagonist of the LR and the antagonist of the action of abduction.

The MR of the right eye and the LR of the left eye are called yoke muscles. These two muscles work at the same time and in the same direction to create levoversion (the movement of both eyes in left gaze) and dextroversion ( the movement of both eyes in right gaze). In pre-industrial agriculture, the yoke was a collar that attached two oxen together so that their efforts at pulling a plow would be coordinated.

The animation below illustrates dextroversion:

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°).

Innervation

Muscles work only when they are innervated. That is, they contract or relax after receiving a nerve impulse. You may remember that frog leg experiment in your first biology class. The frog leg jumps when you apply battery current to it with two wires. Cranial nerves III, IV, and VI are the motor nerves that control the extraocular muscles.

Nerve Function

III oculomotor MR, SR, IR, IO

levator muscles

iris sphinctor muscles

IV tochlear SO

VI abducens LR

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.

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. It also travels at a 23 degree angle to the primary position visual axis. It's insertion is 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, IR-extortion).

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.

Table and Diagram Summaries

The table and diagram below summarize the information given in the text above. For COT and COMT test taking purposes, you should know this information and understand the terminology. You should also be able to figure out the actions of the muscles from a mental picture of the anatomy of the muscle (hint: study the anatomy given in the discussions above).

Muscle	primary action	secondary action	tertiary action	testing position
LR	abduction	none	none	abduction
MR	adduction	none	none	adduction
SR	elevation	intortion	adduction	up and out
IR	depression	extortion	adduction	down and out
IO	extortion	elevation	abduction	up and in
SO	intortion	depression	abduction	down and in

Here is a cross diagram that shows which muscles move the eyes into the positions of gaze. There is not just one muscle responsible for just elevation or just depression, therefore there is no single muscle labeled for these positions. According to the diagram, which muscles are responsible for elevation? Answer: the SR and the IO. Which muscles are responsible for depression? This is a very handy diagram for test taking purposes.

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