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Anomalous internal carotid-posterior cerebral artery circulation: one form of congenital incomplete circle of Willis.

Introduction Anomalies of the cerebral circulation have proven to be the rule rather than the exception. In a study of 994 autopsies of patients with clinical manifestations of neural dysfunction, Riggs and Rupp [1] found anomalies in 79%. Most of these variants were related to failure of one or more of the primary branching stems forming the circle of Willis to develop to adult size. A search of nadiologic and pathologic literature failed to reveal a case in which the posterior cerebral artery arose as the only branch of one of the internal carotid arteries. We report such a case.