A 70-year-old man develops a complete right third nerve palsy with ptosis, a 'down and out' eye, and a dilated, non-reactive right pupil. CT and MRI angiography reveals a posterior communicating artery aneurysm on the right. Why does aneurysmal CN III palsy characteristically involve the pupil first?

• A Pupillomotor fibres run in the core of CN III surrounded by somatic motor fibres
• B The pupil is controlled by the ophthalmic division of CN V, not CN III
• C Pupillomotor (parasympathetic) fibres travel on the outer superficial surface of CN III and are first compressed by an enlarging aneurysm from outside the nerve ✓
• D Aneurysm compresses the sympathetic fibres on the carotid artery which synapse with CN III

Explanation

The parasympathetic pupillomotor fibres of CN III, originating from the Edinger-Westphal nucleus, travel on the outer surface (dorsomedial epineural layer) of the third nerve. An expanding posterior communicating artery aneurysm compresses CN III from the outside, affecting the superficially placed pupillomotor fibres first, producing a dilated non-reactive pupil—the 'surgical pupil.' In contrast, ischaemic third nerve palsy (from microvascular disease in diabetes) typically spares the pupil because ischaemia affects the inner core vascular fibres while the outer pupillomotor fibres are supplied by epineural vessels and preserved.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.