Pupillary Pathways

The abducens nerve (6th cranial nerve) controls a single extraocular muscle; the lateral rectus. The lateral rectus is primarily responsible for abducting the eye. A palsy of the abducens nerve is the most common ocular motor paralysis; the affected eye turns inward toward the nose and is unable to abduct properly.

The deviation is constant and is typically greater at distance fixation than at near. The esotropia is also more noticeable when the patient is looking toward the affected side. Diagnosis of a 6th cranial nerve palsy may seem pretty straight forward when piecing together the patient’s history and examination findings; however, you should keep in mind that there are several conditions that may imitate isolated lateral rectus weakness.

We will review these 6 mimickers of a 6th nerve palsy for your review!

The fourth cranial nerve (trochlear nerve) controls the actions of a single extraocular muscle; the superior oblique. The trochlear nerve has several unique aspects; it is the smallest nerve in terms of the number of axons it contains, it has the longest course through the skull than any other cranial nerve, it is the only cranial nerve that exits through the dorsal aspect of the brainstem, and it innervates the superior oblique muscle on the contralateral side from its nucleus.

Diseases or injuries of the fourth cranial nerve can lead to paralysis or weakness of the superior oblique muscle which will cause certain signs and symptoms that you will see upon examination.