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Cranial Nerve Palsies

There are a total of twelve cranial nerves but neuro-ophthalmology really only deals with nerves II, III, IV and VI.

CN2

Optic Nerve

As one could guess, problems with the optic nerve cause vision trouble. Since we are just looking into palsies for now, we’ll come back to CN2 in a later post.

CN3

Oculomotor Nerve

Cranial Nerve III is a powerhouse. This nerve is in control of all but two EOMs. The oculomotor nerve innervates the superior, inferior and medial recti, the inferior oblique, the levitator palpebrae (which controls the upper eyelids), the constrictor pupillae (for pupil constriction) and the ciliary muscles. Since this nerve is in charge of the muscles that move the eye up, down, in and torsionally(in and up), a third nerve palsy could cause an ocular misalignment and diplopia. Palsies of CN3 also often result in pupillary defects (affected side can’t constrict) and ptosis (affected upper lid may not open or close properly).

CN4

Trochlear Nerve

The fourth cranial nerve is in charge if the superior oblique. When this nerve is damaged or palsied, the eye has trouble moving in toward the nose, side to side or downward (remember- the SO is one of those torsional muscles). Patients with a fourth nerve palsy usually experience double vision in downgaze and often complain that their diplopia is worse when reading, as people generally lower their sight to read. Creating a mnemonic for this was a bit rough so I just remember SO Fo’ (like Superior Oblique Four).

CN6

Abducens Nerve

The final nerve that we’re going to pay close attention to is the sixth nerve, or the Abducens Nerve. CN6 innervates the eyes’ lateral rectus muscles, which control the movement of the eye temporally, or towards the ear. To help me remember this, I think adduct adds toward the face so to abduct is to take away from the face (like abolish). Because the medial rectus has nothing to keep it in check, patients with a sixth nerve palsy present with esotropia and constant horizontal diplopia which is worse at distance. CN6 palsies are the most commonly seen nerve palsies seen in ophthalmology.

Just as a point of reference, the followngis a cheat sheet to the cranial nerves and their titles. Again, for neuro-ophthalmology, you don’t need to know all of them, but if you are a nerd like me, you may want to just know them in case your doctor starts rattling off jargon about them.

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