A 45-year-old man with idiopathic intracranial hypertension develops unilateral horizontal diplopia. On examination the affected eye cannot abduct past the midline. Neuroimaging is otherwise unremarkable apart from signs of raised intracranial pressure. The sixth nerve palsy in this setting is described as a false localising sign because:

• A The abducens nerve passes through the foramen ovale where it is compressed
• B The sixth nerve nucleus lies in the midbrain where pressure effects are greatest
• C Raised intracranial pressure directly compresses the cavernous sinus segment of the nerve
• D The abducens nerve has the longest intracranial course over the clivus, making it vulnerable to stretch by raised pressure ✓

Explanation

The abducens nerve has the longest intradural course of any cranial nerve, ascending along the clivus from the pontomedullary junction to the petrous apex and Dorello's canal. Generalised raised intracranial pressure stretches the nerve against the petrous tip, causing a palsy unrelated to a focal lesion at its nucleus, hence the term false localising sign. The abducens nucleus lies in the caudal pons, not the midbrain, killing option B.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.