Fenestration of the middle cerebral artery associated with an aneurysm--case report.

A 36-year-old female suddenly developed a severe headache. Computed tomography demonstrated subarachnoid hemorrhage in the left Sylvian fissure. Left carotid angiography revealed a fenestration at the distal portion of the M1 portion of the left middle cerebral artery, and an aneurysm arising from the proximal end of the fenestration. These findings were confirmed when the aneurysm neck was clipped. The embryonic origin of this lesion indicates that congenital factors may result in aneurysm formation.