Intracranial dissection of the distal middle cerebral artery as an uncommon cause of distal cerebral artery aneurysm. Case report.

An aneurysmal dissection of a right middle cerebral artery (MCA) branch is described in a 56-year-old woman. The abnormality was an incidental finding on computerized tomography and subsequently appeared on magnetic resonance imaging performed to evaluate the patient for subjective pulsatile tinnitus. The intracranial aneurysm was documented to have enlarged on serial angiography over a 6-week interval. Treatment was believed to be necessary because of the unknown etiology of the aneurysm, with the differential diagnosis including mycotic or neoplastic aneurysm with a risk of hemorrhage. The lesion was excised and flow to the distal MCA branch was preserved with an anastomosis of the superficial temporal artery to the MCA. The aneurysm, which developed at the level of the sylvian fissure, proved on pathological study to be related to a focal dissection of the MCA branch. The radiographic appearance and pathological findings are presented. Focal dissection must henceforth be included in the differential diagnosis of peripheral cerebral artery aneurysms.