A 40-year-old woman presents with complete ophthalmoplegia of the right eye, ptosis, and numbness over the right forehead and cheek. MRI shows a mass expanding the right cavernous sinus. Which structure lies most medially within the cavernous sinus and is therefore the first cranial nerve compressed by an expanding pituitary tumour breaking through the sella?

• A Oculomotor nerve
• B Abducens nerve ✓
• C Trochlear nerve
• D Ophthalmic division of the trigeminal nerve

Explanation

The abducens nerve runs free within the cavernous sinus itself, lying medial and inferolateral to the internal carotid artery, so it is the nerve closest to the pituitary and the earliest affected by lateral expansion from the sella. The oculomotor, trochlear, ophthalmic and maxillary nerves all lie within the lateral wall of the sinus, embedded in dura, which spares them initially. This explains isolated sixth nerve palsy as the presenting sign of a pituitary apoplexy or intracavernous mass.

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

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Written and medically reviewed by the StethoPrep medical team.