[Small saccular aneurysms in moyamoya disease].

Histopathological studies on the cerebral aneurysm in cerebrovascular moyamoya disease have been limited to only a few cases and the pathological details still remain obscure. Six moyamoya disease autopsy cases that satisfied the criteria of this disease proposed by Nishimoto in 1976 are studied. In addition to the routine neuropathological examination of the brains, detailed observa tion of the circle of Willis and its branches was carried out under a dissecting microscope and all of the bulged areas or locally thin portions of the vessel wall were histologically examined. All the pa tients had cerebral aneurysms. Seventeen aneurysms in total were discovered and their diameter ranged from 450 to 1, 300 ,um (mean diameter of 860 μm) . The principal histology of these aneurysms was of the saccular type in which the dome was composed of endothelial and adventitial layers, accompanied by the disappearance of the internal elastic lamina and the media. These small saccular aneurysms of the cerebral arteries in moyamoya disease were revealed to have the following characteristic features: 1) They were located at the bifurcating sites of arteries, and were distributed predominantly on the posterior communicating and posterior cerebral arteries, and on the abnor mal vascular networks branching from these arteries. 2) They were usually small, about 1 mm in diameter in size, and were multiple. 3) The lumen of the aneurysms was often occluded by fibrous thickening of the intima, which is believed to be the most characteristic disease process in moyamoya disease, and is commonly noticed in various cerebral arteries, most significantly at the terminal portion of bilateral internal carotid arteries. Based on these results, the pathogenesis of these small saccular aneurysms in moyamoya disease, and a possible mechanism in cerebral aneurysm formation were discussed.