Anatomy clinical correlates: Oculomotor (CN III), trochlear (CN IV) and abducens (CN VI) nerves

In order to move our eyes and see the world around us, like watching this osmosis video, we rely on the control of our extraocular muscles which are primarily innervated by three cranial nerves; cranial nerve III, or the oculomotor nerve, cranial nerve IV or the trochlear nerve, and cranial nerve VI or the abducens nerve. These cranial nerves allow us to move our eyes in various directions, and also help to govern certain reflexes of the eyes. Injury to these cranial nerves can lead to a number of problems involving the eye, so understanding their anatomy can help us better understand the clinical consequences of these conditions!

First up, let’s discuss damage to the oculomotor nerve, also called oculomotor nerve palsy. The oculomotor nerve carries both motor and parasympathetic fibers which can be injured either individually or together. When looking at the anatomy of the oculomotor nerve, the parasympathetic fibers are found in the peripheral or superficial portion of the nerve, and the motor fibers are found in the central or deep portion of the nerve. This is significant as more external compression may only affect the parasympathetic fibers, where lesions of central nerve fibers might only affect the motor fibers. Remember, Motor is Middle, and Parasympathetic is Peripheral.

Now, if the motor fibers of the oculomotor nerve are damaged, that results in ophthalmoplegia, meaning impaired function of the four extraocular muscles innervated by the oculomotor, which are the superior, medial and inferior rectus, and the inferior oblique muscles. When these four muscles are impaired, that leaves the two other extrinsic eye muscles unaffected, so their actions are now left unopposed. This means the lateral rectus pulls the eye laterally, and the superior oblique pulls it inferiorly and laterally. Individuals with this type of injury present with the ipsilateral eye in a characteristic ‘down and out’ position. When the individual tries to look in any other direction, they will complain of double vision, or diplopia. Furthermore, there is also paralysis of the levator palpebrae superioris muscle that leads to ptosis, or drooping of the upper eyelid.