De novo fenestration of the optic nerve case illustration.

Fenestration of the optic nerve or chiasm due to the presence of aneurismal dilation of the internal carotid artery (ICA) has been described. In three out of five cases reviewed recently, the optic nerve was penetrated by an ICAΓÇôophthalmic artery aneurysm.3 Penetration of the optic nerve was due to a ruptured anterior communicating artery aneurysm or an aneurysm of the anterior wall of the ICA in the other cases published to date.4,2 Splitting of the optic nerve without the presence of a penetrating aneurysm has not been identified previously.