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CN III Palsy Aneurysm — RACP Adult Medicine MCQ

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ModerateNeurologyCN III Palsy AneurysmRACP Adult Medicine

A 40-year-old man presents with acute onset double vision. Examination shows right-sided ptosis with a dilated right pupil (6 mm, unreactive). The right eye is deviated laterally and slightly downward. Left eye is normal. There is no proptosis. What is the most urgent investigation?

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Correct answer: D — CT angiography of intracranial vessels

A painful CN III palsy with pupil involvement (dilated, unreactive pupil – parasympathetic fibres on the surface of CN III are compressed) is a posterior communicating artery aneurysm until proven otherwise. CT angiography (CTA) of the intracranial circulation is the most urgent investigation to identify the aneurysm before it ruptures causing subarachnoid haemorrhage. If CTA is negative, conventional catheter angiography should be considered. This is a neurosurgical emergency.

Reference: eTG – 2025 – Neurology; Stroke Foundation – 2024 – Clinical Guidelines