A 24-year-old man develops sudden headache, a fixed dilated right pupil, and then coma after head trauma. CT shows an acute extradural haematoma displacing the right uncus medially over the tentorial edge. Which structure compressed against the midbrain best explains the pupillary finding?

• A Right trochlear nerve
• B Left oculomotor nerve
• C Right oculomotor nerve ✓
• D Right abducens nerve

Explanation

Uncal herniation forces the uncus of the temporal lobe over the free edge of the tentorium cerebelli, compressing the ipsilateral oculomotor nerve against the midbrain. Parasympathetic fibres run peripherally in the nerve, so the earliest sign is ipsilateral pupillary dilation with loss of the light reflex before complete ophthalmoplegia appears. The trochlear nerve wraps around the midbrain dorsally and the abducens nerve runs in the cavernous sinus, neither of which is compressed by uncal descent.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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