A patient develops diplopia on downward gaze and difficulty descending stairs. Examination shows the right eye is slightly higher and extorted. The affected nerve courses around which structure at risk during posterior fossa surgery?

• A CN VI (abducens) courses through the cavernous sinus
• B CN IV (trochlear) exits the dorsal midbrain and wraps around the cerebral peduncle ✓
• C CN III (oculomotor) compresses against the posterior communicating artery
• D CN IV passes through the orbit via the inferior orbital fissure

Explanation

The trochlear nerve (CN IV) is the only cranial nerve to exit from the dorsal surface of the brainstem (posterior to the inferior colliculus), has the longest intracranial course, and wraps around the lateral midbrain (cerebral peduncle). Right CN IV palsy causes weakness of the right superior oblique (SO) — SO intorts and depresses the eye; without it, the right eye is relatively extorted and elevated, causing diplopia worst on downward gaze (stairs). CN IV is exquisitely vulnerable to trauma and during tentorial meningioma or posterior fossa surgery. CN IV enters the orbit via the superior orbital fissure (not inferior).

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.