A patient has a right third cranial nerve palsy with a dilated, fixed pupil, ptosis, and 'down-and-out' deviation of the right eye. Which finding best distinguishes a compressive (surgical) cause from a microvascular (medical) cause?

• A Ptosis severity — more severe ptosis in medical cause
• B 'Down-and-out' eye position — present only in medical cause
• C Pupil involvement (dilated fixed pupil) — present in compressive, typically spared in microvascular ✓
• D Pain — present only in medical cause, absent in compressive

Explanation

The pupillary constrictor fibers of CN III travel on the outer surface (periphery) of the nerve; compressive lesions (posterior communicating artery aneurysm, uncal herniation, tumors) stretch and compress these peripheral fibers first, producing a dilated fixed pupil. Microvascular lesions (diabetic, hypertensive) affect the central vascular core of the nerve, sparing the outer pupillomotor fibers — producing pupil-sparing palsy. Pain may occur in both. The 'down-and-out' position occurs in complete CN III palsy regardless of cause.

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

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Written and medically reviewed by the StethoPrep medical team.