How is fourth cranial nerve palsy diagnosed in the setting of third cranial nerve palsy (oculomotor cranial nerve palsy)?

Updated: Oct 08, 2018
  • Author: James Goodwin, MD; Chief Editor: Andrew G Lee, MD  more...
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Fourth cranial nerve palsy is difficult to diagnose in the presence of third cranial nerve palsy because the small increment of depressor deficit (superior oblique muscle) cannot be discerned readily from the depressor palsy that results from weakness of the third nerve innervated depressor (inferior rectus muscle).

The superior oblique muscle depresses the globe most efficiently with the eye in adduction, a position that may not be achievable with medial rectus palsy from third cranial nerve involvement.

The best marker for fourth cranial nerve function in the presence of dense third cranial nerve palsy is intorsion of the globe on attempted down gaze. If no intorsion is present, one should suspect concomitant fourth cranial nerve palsy as part of a cavernous sinus syndrome. Torsion of the globe can be discerned by simultaneously watching landmarks, such as conjunctival vessels lateral and medial to the iris. With intorsion, the lateral vessels rise and the nasal vessels drop.

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