Theory of Eye Motion and Neurological Testing

The Eye Muscles






The Cranial Nerves

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
