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Arterial bifurcations in the circle of Willis were observed three-dimensionally in 44 cases of cerebral aneurysm and 45 control cases. Variations of the circle of Willis were also studied in relation to cerebral aneurysm. The following results were obtained : 1. Aneurysm always occurred at the apex of the cerebral arterial bifurcation where axial flow in the parent artery immediately proximal to the aneurysm was assumed to impinge. 2. A statistically valid correlation was found between cerebral aneurysm and anterior middle cerebral artery. A statistically valid correlation was also noted between the external diameter of the parent arteries and the side of aneurysms in 19 anterior communicating aneurysms and 5 posterior communicating aneurysms. When the proximal portion of the anterior cerebral artery or when the posterior communicating artery was thicker than the contralateral one, the aneurysm tended to occur on the thicker side. 3. One aneurysm developed on a dilated feeding artery of an associated cerebral arterio-venous malformation. Another one developed at the proximal branching point from an associated accessory middle cerebral artery. These results show the preponderence of the occurrence of cerebral aneurysm at sites where blood flow is relatively abundant because of some variation in the form of the circle of Willis. They also suggest that hemodynamic stress plays an important role in the pathogenesis of cerebral aneurysm.

Variation of the Circle of Willis Related to the Pathogenesis of Cerebral Aneurysm