CT angiography of the brain in a 45-year-old with sudden severe headache shows an aneurysm at the junction of the internal carotid artery and posterior communicating artery (PComA). Which cranial nerve palsy is classically associated with this aneurysm's location?
- A Sixth cranial nerve (abducens) palsy
- B Fourth cranial nerve (trochlear) palsy
- C Second cranial nerve (optic) palsy
- D Third cranial nerve (oculomotor) palsy with pupil involvement ✓
Explanation
A posterior communicating artery (PComA) aneurysm expanding laterally compresses the third cranial nerve (oculomotor nerve) as it runs between the PComA and the tentorium cerebelli. The resulting compressive third nerve palsy characteristically involves the pupil (dilated, unreactive pupil — 'surgical' third nerve palsy) because parasympathetic fibres to the pupil travel on the outer surface of the nerve and are preferentially compressed. This is in contrast to 'medical' third nerve palsy (ischaemic, eg in diabetes) which typically spares the pupil. This is a neurosurgical emergency requiring aneurysm coiling/clipping.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.