A patient presents with unilateral ptosis, mydriasis, and the eye is directed 'down and out.' The pupillary light reflex is absent. MRI shows an aneurysm at the junction of the posterior communicating artery and the internal carotid artery. Which structural feature of CN III makes it susceptible to compressive (surgical) aneurysms?

• A Pupillomotor fibers are carried in the core of the nerve
• B Pupillomotor fibers travel in the peripheral part of the nerve and are first compressed by external mass effect ✓
• C Pupillomotor fibers are part of the ophthalmic division of CN V
• D Pupillomotor fibers only travel in the superior division of CN III

Explanation

The parasympathetic pupillomotor fibers of CN III travel in the superficial (peripheral) periphery of the nerve, specifically the superomedial aspect. Compressive lesions (aneurysms, herniation) affect these outer fibers first, causing a pupil-involving third nerve palsy with ptosis, ophthalmoplegia, and mydriasis. In contrast, ischemic lesions (diabetes, hypertension) affect the central vasa nervorum supply first, causing pupil-sparing third nerve palsy. This distinction is crucial clinically — a pupil-involving CN III palsy demands urgent imaging to exclude posterior communicating artery aneurysm.

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

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Written and medically reviewed by the StethoPrep medical team.