In a patient with a complete right CN III palsy, which of the following is MOST indicative of surgical (compressive) rather than medical (ischaemic) aetiology?

• A Ptosis of the right eye
• B Right eye deviated down and out
• C Pupil-involving CN III palsy (dilated, unreactive pupil) ✓
• D Diplopia on rightward gaze

Explanation

Pupil involvement (mydriasis — dilated, non-reactive pupil) in CN III palsy strongly suggests a compressive (surgical) aetiology such as a posterior communicating artery aneurysm. The pupillomotor fibres (from Edinger-Westphal nucleus) travel on the outer surface of the oculomotor nerve and are compressed early by external masses. Medical (ischaemic — diabetes, hypertension) CN III palsies characteristically spare the pupil because the ischaemic damage preferentially involves the central nerve fibres (supplying extraocular muscles) while sparing the peripheral parasympathetic fibres.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.