Diabetic CN III Palsy — RACP Adult Medicine MCQ

Instant feedback + full explanation. One question, done properly.

EasyNeurologyDiabetic CN III PalsyRACP Adult Medicine

A 55-year-old woman with type 2 diabetes develops a unilateral third cranial nerve palsy. Her pupil is SPARED (reactive, normal size). She has pain behind the right eye. MRI and MRA are normal. What is the most likely diagnosis?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — Orbital tumour

A painful CN III palsy with pupil SPARING in a patient with diabetes is characteristic of diabetic microvascular ischaemic third nerve palsy. The pathophysiology involves ischaemia of the nerve's central fibres (which control extraocular muscles) while the peripherally located parasympathetic pupillary fibres (which run on the nerve surface) are spared due to their separate blood supply. In contrast, compressive lesions (PComA aneurysm) compress the surface first, causing pupil dilation. Recovery typically occurs spontaneously over 2–3 months. MRA/CTA is still performed to exclude aneurysm when there is any diagnostic uncertainty.

Reference: eTG – 2025 – Neurology