A 50-year-old diabetic presents with sudden right eye ptosis and divergent squint. On examination, the pupil is mid-dilated and non-reactive. Which cranial nerve is affected and what is the anatomical reason for pupil involvement in this case?
- A CN IV; superior oblique palsy sparing the pupil
- B CN III; extrinsic compression (e.g., posterior communicating artery aneurysm) affects peripheral pupillomotor fibres first ✓
- C CN III; ischaemic palsy in diabetics preferentially affects peripheral fibres
- D CN VI; convergence insufficiency
Explanation
A complete CN III palsy with pupil involvement (dilated, non-reactive pupil) in the context of acute onset and divergent squint suggests an extrinsic compressive lesion — classically a posterior communicating artery (PCoA) aneurysm. Parasympathetic pupillomotor fibres travel on the outer surface of CN III and are compressed first by external pressure, causing pupil dilation before or simultaneous with extra-ocular motility deficits. Diabetic ischaemic CN III palsy preferentially spares the pupil because ischaemia affects the central microvasculature, sparing superficially located pupillomotor fibres.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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