What is the most likely etiology of a 'pupil sparing' CN III palsy?

• A. Vasculopathic
• B. Aneurysmal compression
• C. Traumatic
• D. Inflammatory

Answer: (A)

Pupil-sparing CN III palsy points to microvascular ischemia of the nerve’s central motor fibers. In vasculopathic conditions like diabetes or hypertension, the blood supply to the nerve (the vasa nervorum) tends to affect the central fibers first, causing eye movement weakness while the superficial parasympathetic fibers that control the pupil are preserved. That’s why the pupil remains reactive and the signs are consistent with a vasculopathic process.

In contrast, compressive lesions—such as an aneurysm pressing on the nerve—often hit the outer parasympathetic fibers first, leading to a dilated, nonreactive pupil with the palsy. Traumatic or inflammatory processes can cause CN III symptoms too, but the classic pupil-sparing pattern most strongly suggests a vasculopathic ischemic mechanism.