Eye Theory

Theory of Eye Motion and Neurological Testing

The Eye Muscles

The image shows the extraocular muscles of the right eye viewed from above. The lateral and medial rectus muscles are the easiest to understand. The lateral rectus muscles abduct the eye (away from the nose) and the medial rectus muscles adduct the eye (towards the nose). The lateral and medial muscles move the eye only in the horizontal plane.

The vertical motion is a little more complex. Four muscles (superior rectus, inferior rectus, superior oblique and inferior oblique) control the vertical motion.

The easiest way to understand the action of these muscles is to isolate the superior/inferior rectus from the superior/inferior oblique. When the right eye is fully abducted (away from the nose), only the superior and inferior rectus muscles can elevate and depress the eye. This is purely a mechanical property due to the axis of the eye lining up perpendicular to the superior/inferior muscles.

When the right eye is fully adducted (towards the nose), only the inferior and superior oblique muscles can elevate and depress the eye. This is again due to mechanical properties of the attachment of these muscles. On the image of the eye, it is possible to picture how the superior oblique muscle will depress the eye when the eye is looking at the nose. The inferior oblique will elevate the eye.

What happens when the eye is neither fully abducted or adducted? All four of the muscles contribute a percentage of the vertical motion depending on the position of the eye. If the eye is looking straight forward, about 50% of vertical motion is due to the inferior/superior oblique muscle combination and 50% is due to the superior/inferior rectus muscle combination.

The Cranial Nerves

There are three right cranial nerves (III, IV, and VI) involved in innervating the right eye muscles and three left cranial nerves (III, IV, and VI) involved in innervating the left eye muscles. Cranial nerve III innervate four muscles (medial rectus muscle, superior rectus muscle, inferior rectus muscle, and inferior oblique muscle). Cranial nerve IV innervates the superior oblique muscle. Cranial nerve VI innervates the lateral rectus muscle.

Cranial Nerve III	medial rectus muscle, superior rectus muscle, inferior rectus muscle, inferior oblique muscle
Cranial Nerve IV	superior oblique muscle
Cranial Nerve VI	lateral rectus muscle

A damaged cranial nerve will produce the same symptoms that would occur if the associated eye muscle is damaged. For example if cranial nerve VI is damaged, the eye will have the same motion as when the lateral rectus muscle is damaged. If cranial nerve III is damaged, the symptoms will be the same as if the four eye muscles controlled by CN III is damaged.

A damaged cranial nerve III has an additional characteristic that affects the eye. The parasympathetic nerve innervating the pupillary sphincter travels with CN III. If CN III is damaged, the eye pupil will dilate.

Best Method to Test Eye Motion

The best method to test eye motion is ask the patient to follow your finger drawing a large H pattern in the air a few feet from their face. The two legs of the H will isolate and test the motion of the superior/inferior rectus pair and inferior/superior oblique pair. The center part of the H will test the medial and lateral muscles.