A 55-year-old woman with a cerebellopontine angle tumour develops a right abducens palsy. Imaging shows no lesion along the course of the sixth nerve within the cavernous sinus or orbit. The raised intracranial pressure responsible for her diplopia acts by:

• A Traction on the nerve at its dural entry point into the cavernous sinus
• B Direct compression of the abducens nucleus in the pons
• C Ischaemia of the nerve within Dorello's canal
• D Downward displacement and stretching of the nerve over the petrous apex ✓

Explanation

The sixth nerve has the longest intracranial course of all cranial nerves and runs upward over the sharp ridge of the petrous temporal bone before entering Dorello's canal. Raised intracranial pressure displaces the brainstem downward, stretching the nerve over the petrous apex, so an abducens palsy can be a false localising sign unrelated to the actual pathology. Direct nuclear compression would produce additional pontine signs, which are absent here.

Reference: Snell's Clinical Neuroanatomy, 8th ed.

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