Pathological Considerations for Ruptured and Fusiform Aneurysms at the Distal Posterior Inferior Cerebellar Artery : Two Case Reports

Summary: We report 2 surgical cases of distal posterior inferior cerebellar artery (PICA) aneurysms and mainly discuss pathological findings. Seventy-one and 61-year-old women experienced a subarachnoid hemorrhage and were found to have fusiform aneurysms showing a pearl-and-string appearance at the distal PICA peripheral to the choroidal point. Both aneurysms were trapped and pathologically examined. The aneurysms show similar pathological findings. The aneurysm walls, which lost both the internal elastic lamina and the vascular smooth muscle layer, had an area of marked fibrous thickening as well as an area of thinning. Infiltration of inflammatory cells, primarily monocytes, was seen localized within the wall of the rupture area. There was no evidence of an acute dissecting aneurysm even though the radiological results are compatible with a dissecting aneurysm. Arteriosclerotic factors can also be excluded because no fat cells or degenerated cells were observed. In both aneurysms, we speculate, hemodynamic factors in areas where a congenital defect of the internal elastic lamina in the peripheral PICA may have led to aneurysm formation and rupturing.