[Ruptured Dissecting Aneurysm of M2 Portion of the Middle Cerebral Artery:A Case Report].

A 71-year-old woman presented with a sudden onset of headache and vomiting. Computed tomography(CT)showed diffuse subarachnoid hemorrhage(SAH)that was more severe on the right side. Three-dimensional CT angiography and right carotid angiography(CAG)demonstrated 2-mm microaneurysms at the middle cerebral artery(MCA)bifurcation and anterior communicating artery, with slight narrowing and dilatation of the M2 inferior trunk. Each microaneurysm was smooth, making it difficult to identify the bleeding source. Thus, surgery was postponed at the acute stage, and further investigation was planned. Repeated CAG was diagnostically unsuccessful, finding no source of the bleeding. On day 45 after the onset, exploratory craniotomy was performed to confirm the cause of the SAH. During the operation, both microaneurysms were found to be unruptured. However, the distal portion of the M2 inferior trunk was dark purplish and red and enlarged in a fusiform shape, suggesting a dissecting aneurysm. Residue of the SAH observed near the enlarged vessel identified it as the bleeding source. The enlarged vessel was wrapped with Bemsheets, and the Bemsheets was clipped to secure it. A dissecting aneurysm of the distal MCA is rare, as is the onset of bleeding. Based on a review of the literature related to dissecting aneurysms of the distal MCA, we recommend exploratory craniotomy if CT demonstrates laterality of the sylvian fissure on the SAH and CAG reveals stenosis or occlusion of the distal MCA in cases of SAH for which no bleeding source is detected.