The falx cerebri and tentorium cerebelli are folds of dura mater. At the tentorial notch (incisura), uncal herniation from supratentorial mass lesions first compresses which cranial nerve, producing a fixed dilated pupil?

• A CN IV (trochlear nerve)
• B CN VI (abducens nerve)
• C CN II (optic nerve at chiasm)
• D CN III (oculomotor nerve) ✓

Explanation

In transtentorial (uncal) herniation, the medial temporal lobe (uncus) herniates through the tentorial notch and compresses CN III (oculomotor nerve) against the petroclinoid ligament or posterior cerebral artery. The parasympathetic pupillomotor fibres travel on the outer surface of CN III and are compressed first, producing ipsilateral pupil dilation (fixed dilated pupil — 'surgical third nerve palsy'). CN IV runs in the free margin of tentorium and may also be affected but is less critical early. CN VI palsy ('false localising sign') occurs due to brainstem shift stretching the long intracranial course of CN VI.

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

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Written and medically reviewed by the StethoPrep medical team.