A 32-year-old presents with sudden-onset headache, neck stiffness, and photophobia. LP shows xanthochromia. CT angiography reveals an aneurysm at the junction of the internal carotid artery and posterior communicating artery. Which CN is most at risk from this aneurysm?

• A CN III (oculomotor nerve) ✓
• B CN IV (trochlear nerve)
• C CN VI (abducens nerve)
• D CN II (optic nerve)

Explanation

A posterior communicating artery (PComm) aneurysm lies in close proximity to the oculomotor nerve (CN III) as the nerve courses anteriorly from the midbrain through the subarachnoid space toward the cavernous sinus. Expansion or rupture of a PComm aneurysm compresses the peripherally situated pupillomotor parasympathetic fibers of CN III first, producing a painful third nerve palsy with a dilated, unreactive ('blown') pupil — distinguishing it from a pupil-sparing diabetic CN III palsy.

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

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Written and medically reviewed by the StethoPrep medical team.