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 IVsuperior oblique muscle
Cranial Nerve VIlateral rectus muscle

A damaged cranial nerve will produce the same symptoms that would occur if the associated eye muscle or muscles are damaged. For example if cranial nerve VI is damaged, the eye will have similar motion as if 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 are damaged.

In addition, CNIII also innervates the levator palpebrae superioris muscle. This is one of the two muscles that raise the upper eyelid. When CNIII is damaged, the eyelid will demonstrate ptosis (upper eyelid droop). The eyelid is not completely closed because the other muscle partially raises the eyelid. Note: the other muscle involved with raising the eyelid is the superior tarsal muscle, which is innervated by the sympathetic nervous system. Cervical sympathetic trunk injury will paralyze the superior tarsal muscle and also present with ptosis (one of the signs of Horner Syndrome).

One additional sign that is often seen with damage to CNIII is pupillary dilation (mydriasis). The pupillary sphincter is NOT controlled by CNIII. It is innervated by the parasympathetic nervous system. The nerve that innervates the sphincter runs along the outside of CNIII. Most compressing lesion that damages CNIII (such as a brain tumor) usually also damages the parasympathetic nerve innervating the sphincter. If CNIII is damaged due to a non compressing lesion then the pupil size should not be affected.

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.