A coronal MRI through the cavernous sinuses shows a mass in the right cavernous sinus sparing all cranial nerves embedded in its lateral wall. Which nerve, because it lies free within the venous blood of the sinus lumen rather than in the lateral wall, would be affected first?

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

Explanation

Within the cavernous sinus, oculomotor, trochlear, and the ophthalmic and maxillary divisions of the trigeminal nerve all run in the dural lateral wall, whereas the abducens nerve courses freely through the venous lumen, lateral to the internal carotid artery. An intracavernous lesion therefore strikes the abducens nerve earliest, giving lateral rectus palsy with diplopia before other ocular motor deficits appear.

Reference: Gray's Anatomy for Students, 4th ed.

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