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CN III Palsy — RACP Adult Medicine MCQ

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ModerateNeurologyCN III PalsyRACP Adult Medicine

A 40-year-old woman presents with sudden onset severe headache and unilateral third cranial nerve palsy with pupil involvement (dilated, unreactive pupil with ptosis and 'down and out' eye position). What is the most likely diagnosis and what must be urgently excluded?

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Correct answer: A — Posterior communicating artery aneurysm – urgent CT angiography

A painful third nerve palsy WITH pupil involvement (dilated pupil) is a posterior communicating artery aneurysm until proven otherwise. This is a neurosurgical emergency. The parasympathetic fibres travel on the surface of CN III and are compressed by an expanding aneurysm. Urgent CT angiography (or conventional angiography) is mandatory. Diabetic third nerve palsy classically SPARES the pupil. Rupture of a PComA aneurysm causes subarachnoid haemorrhage.

Reference: eTG – 2025 – Neurology