Splitting of the optic nerve associated with ruptured anterior communicating artery aneurysm.

A 52-year-old man presented with an anterior communicating artery aneurysm associated with splitting, penetration, and visual symptoms of the optic nerve and manifesting as sudden onset of severe headache. Angiography disclosed a 10-mm anterior communicating artery aneurysm projecting supero-posteriorly, and operative findings revealed unexpected splitting of the optic nerve. Moreover, the optic nerve was not in direct contact with the aneurysm fundus. Splitting of the optic nerve without the presence of a penetrating aneurysm is extremely rare. The pathogenesis of optic nerve penetration may involve congenital fenestration of the optic nerve.