A 45-year-old woman with headache, vomiting and bilateral papilloedema develops horizontal diplopia. She cannot abduct either eye beyond the midline, but MRI shows no mass, hydrocephalus or cavernous sinus disease. The mechanism of her diplopia is best explained by:

• A Direct compression of the sixth nerves by the swollen pituitary gland
• B Demyelination of the abducens fascicles in the basis pontis
• C Ischaemia of the abducens nuclei from venous congestion
• D Stretching of the abducens nerves over the petrous apex by raised intracranial pressure ✓

Explanation

Sixth nerve palsy from raised intracranial pressure is the classic false localising sign. The long intracranial course of the abducens nerve makes it vulnerable to stretching against the petrous tip of the temporal bone and at the Dorello canal as the brainstem is displaced downward. It may be unilateral or bilateral and carries no localising value regarding the site of the pathology. Treatment is directed at lowering intracranial pressure, and the palsy usually resolves once pressure is controlled.

Reference: Kanski's Clinical Ophthalmology, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.