A patient presents with a right third nerve palsy (ptosis, mydriasis, adduction deficit). MRI brain shows a 7 mm aneurysm at the posterior communicating artery (PCoA)-ICA junction. The mydriasis in this case is best explained by:

• A Compression of sympathetic fibers traveling in the center of CN III by the aneurysm
• B Ischemia of the central core of CN III sparing outer pupillary fibers
• C Compression of parasympathetic pupillomotor fibers located in the outer epineurium of CN III by the expanding aneurysm ✓
• D Secondary Horner's syndrome from carotid dissection

Explanation

Parasympathetic fibers destined for the pupillary sphincter travel in the outer (epineural) surface of the oculomotor nerve, making them vulnerable to compression by aneurysms, herniation, or masses. External compression (surgical/compressive CN III palsy) therefore causes mydriasis early. In contrast, microvascular ischemic CN III palsy (diabetes, hypertension) affects the central vascular supply, sparing the peripheral pupillary fibers — resulting in pupil-sparing CN III palsy. Aneurysmal CN III palsy is a neurosurgical emergency due to subarachnoid hemorrhage risk.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.