Fusiform (atherosclerotic) aneurysm of basilar artery: A less frequent type of aneurysm is the fusiform aneurysm demonstrated in this slide. The basilar artery, crossing the ventral surface of the brain stem in the lower part of this slide, is markedly dilated, and its course is deviated to one side. It is diffusely atherosclerotic, as is invariably seen with fusiform aneurysms. (It is not clear, however, to what extent other factors, such as developmental, may also contribute to the genesis of this lesion). In contrast to saccular aneurysms, fusiform aneurysms seldom bleed. The lesion in this slide was an exception, as evidenced by the blood in the subarachnoid space. Fusiform aneurysms also occasionally cause symptoms by compression of adjacent structures.