A 70-year-old presents with sudden severe 'thunderclap' headache. On examination, there is bilateral ptosis with pupils fixed and dilated. No meningismus. Which CN is compressed and where?

• A CN II compressed at the optic chiasma
• B CN VI compressed at Dorello's canal
• C CN IV compressed at the tentorial edge
• D CN III compressed at the posterior communicating artery aneurysm ✓

Explanation

A posterior communicating artery (PComm) aneurysm characteristically compresses CN III as it passes between the posterior communicating artery and the superior cerebellar artery, lateral to the posterior clinoid process. Because parasympathetic pupillomotor fibers travel on the outer surface of CN III, compressive lesions affect the pupil early (dilated, fixed pupil — 'surgical' CN III palsy), unlike ischemic CN III palsies which typically spare the pupil. Bilateral involvement here would suggest herniation or bilateral PComm aneurysms, but the classic presentation of PComm aneurysm includes this scenario.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.