Pupil-Sparing Third Nerve Palsy — SCE Neurology MCQ

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EasyNeuro-ophthalmologyPupil-Sparing Third Nerve PalsySCE Neurology

A 35-year-old woman with type 1 diabetes presents with acute onset of painless diplopia. Examination reveals a right pupil-sparing third nerve palsy (ptosis, down-and-out eye, but normal pupil). What is the most likely aetiology?

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Correct answer: D — Microvascular (diabetic) third nerve palsy

A pupil-sparing third nerve palsy in a patient with diabetes is most likely due to microvascular ischaemia of the third nerve. The pupillomotor fibres run on the outside of the nerve and receive a separate blood supply; ischaemic lesions of the nerve core spare the pupil. However, a posterior communicating artery aneurysm must be excluded, particularly if the pupil is involved or if the palsy is partial/progressive. A: PComA aneurysm classically causes pupil-involving third nerve palsy (painful, pupil dilated). C: Brainstem stroke would cause additional signs (contralateral hemiplegia, etc.). D: Cavernous sinus lesions would affect multiple cranial nerves. E: MG causes fluctuating ptosis without fixed palsy.

Reference: ABN Neuro-ophthalmology Guidelines