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Pupil-Sparing CN III Palsy – Microvascular — SCE Neurology MCQ

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ModerateNeuro-ophthalmologyPupil-Sparing CN III Palsy – MicrovascularSCE Neurology

A 60-year-old woman with type 2 diabetes presents with acute onset of right CN III palsy — right ptosis, eye deviated down and out — but the pupil is SPARED (normal size, reactive). She has right retro-orbital pain. What is the most likely cause?

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Correct answer: B — Microvascular (ischaemic) third nerve palsy — diabetes is the most common cause; pupil-sparing distinguishes it from compressive lesions (where the pupil is typically involved early due to the peripheral location of parasympathetic fibres)

Microvascular (ischaemic) CN III palsy from diabetes or hypertension affects the central vasa nervorum of the nerve, causing ischaemia to the core motor fibres while sparing the peripherally located parasympathetic pupillary fibres (which have an independent blood supply from surface vessels). This results in a pupil-SPARING CN III palsy. Recovery typically occurs within 2–3 months. In contrast, compressive lesions (PComA aneurysm) compress from outside, affecting the superficial parasympathetic fibres first — causing pupil-INVOLVING CN III palsy. Key: pupil-sparing = likely ischaemic; pupil-involving = urgent imaging for aneurysm. A: PComA aneurysm causes pupil-INVOLVING palsy. C: MG has fatigable ptosis without fixed deviation. D: Cavernous sinus would involve multiple nerves. E: Orbital tumour causes proptosis.

Reference: ABN Neuro-ophthalmology Guidelines