A patient with a posterior communicating artery aneurysm develops a complete third nerve palsy with a dilated, unreactive pupil. The pupil is affected because the aneurysm compresses which fibres of CN III FIRST?

• A Parasympathetic (Edinger-Westphal) fibres running on the outside of the nerve ✓
• B Central sympathetic fibres running in the core of the nerve
• C Somatic motor fibres to the superior rectus
• D Sympathetic fibres accompanying the ophthalmic artery

Explanation

The preganglionic parasympathetic fibres from the Edinger-Westphal nucleus travel on the periphery (dorsomedial outer surface) of the oculomotor nerve. External compressive lesions such as an expanding aneurysm preferentially damage these superficial fibres first, causing early pupillary dilation and loss of the light reflex. In contrast, medical (ischaemic) third nerve palsies — where the central vessels are occluded — damage the central motor fibres first and typically spare the pupil.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.