Histopathologic characteristics of a saccular aneurysm arising in the non-branching segment of the distal middle cerebral artery

Currently, the characteristics of aneurysms arising in the distal middle cerebral artery (dMCA) are not well understood. Here, we report the case of a 56-year-old woman with a ruptured saccular aneurysm in the M2 segment of the middle cerebral artery (MCA). The patient presented with a disturbance of consciousness, and computed tomography revealed a subarachnoid hemorrhage that angiography disclosed as a result of a saccular-type aneurysmal dilatation in the M2 segment of the left MCA. We excluded infection, inflammation, trauma, and neoplasia as causes of the aneurysm. Pathologic examination confirmed a ruptured saccular aneurysm of the dMCA that was unrelated to a branching zone. The aneurysmal wall was composed primarily of αSMA-positive and calponin/desmin-negative spindle cells and lacked internal elastic lamina, which was completely disrupted at the neck of the aneurysm. There was a strong positive immunoreactivity for matrix metalloproteinases (MMPs) 1, 2, and 9 in the spindle cells of the aneurysmal wall. In contrast, tests for MMP-8 were negative. The parent artery showed thickening of both the intima and media, with preservation of the internal elastic lamina. Atherosclerosis was not detected in either the parent artery or aneurysm. These findings suggest that an overexpression of MMPs may contribute to the development of saccular aneurysms in regions of the arterial trunk unrelated to branching zones.