A 50-year-old man develops ptosis and diplopia. Examination reveals complete ptosis of the right eye, the globe is deviated down and out, the pupil is dilated and fixed at 6 mm, and there is no light reflex. Which structure is most likely compressed?
- A Abducens (VI) nerve
- B Oculomotor (III) nerve with pupil involvement ✓
- C Trochlear (IV) nerve
- D Sympathetic pathway to the pupil
Correct answer: B. Oculomotor (III) nerve with pupil involvement
Explanation
Third cranial nerve (CN III) palsy with pupil involvement (surgical CN III palsy) indicates external compression, most commonly by a posterior communicating artery (PCoA) aneurysm or transtentorial herniation. The parasympathetic pupilloconstrictor fibres travel on the outer surface of CN III and are compressed first by external pressure, causing pupil dilation. A fixed, dilated pupil in CN III palsy mandates urgent neuroimaging (CT angiography or MR angiography) to exclude an aneurysm.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.