Aneurysm and fenestration of the azygos anterior cerebral artery--case report.

A 78-year-old female suffered sudden onset of nausea and headache. Computed tomography demonstrated diffuse subarachnoid hemorrhage. Cerebral angiography demonstrated an aneurysm arising from the right A1 segment of the azygos anterior cerebral artery, and the hypoplastic left A1. A right frontotemporal craniotomy was performed to obliterate the aneurysm by neck clipping. Surgical exploration found the fenestration of the right A1 and showed that the aneurysm had originated from the bifurcation of the fenestrated A1 and a small perforating artery arising from the fenestration. This unusual combination of an aneurysm associated with a fenestration of the right A1 and contralateral A1 hypoplasia may have been caused by local hemodynamic stress.