A 24-year-old woman reports vertical diplopia that worsens when walking downstairs. On examination there is weakness of depression of the adducted right eye, and she tilts her head to the left to compensate. The causative nerve is unique among cranial nerves because it:

• A Exits the brainstem ventrally between two other nerves
• B Exits the brainstem dorsally and decussates completely ✓
• C Runs in the wall of the cavernous sinus without decussating
• D Passes through the superior orbital fissure outside the tendinous ring

Explanation

The findings describe right superior oblique palsy from a trochlear (fourth) nerve lesion. The trochlear nerve is the only cranial nerve to emerge from the dorsal aspect of the brainstem, from the dorsal midbrain just below the inferior colliculus, and its fibres decussate completely in the superior medullary velum, so it innervates the contralateral superior oblique. Oculomotor exits ventrally, and both oculomotor and trochlear pass through the superior orbital fissure within the common tendinous ring.

Reference: Gray's Anatomy, 42nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.