Magnetic resonance angiographic evidence of sex-linked variations in the circle of Willis and the occurrence of cerebral aneurysms

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Abstract
In this study the authors investigated the relationship between variations in the circle of Willis observed on magnetic resonance (MR) angiograms and locations of cerebral aneurysms, and evaluated the risk of aneurysm formation. One hundred thirty-one patients with cerebral aneurysms were retrospectively selected from a series of 4518 patients who underwent MR angiography at one neurosurgical institute. Variations in the anatomy of the circle of Willis were simply classified into Type A, in which there was no visualization of a unilateral A1 segment, and Type P, in which there was a fetal type of posterior cerebral artery that was continuously delineated from the internal carotid artery (ICA) through the posterior communicating artery. All other variations in the circle of Willis were defined as Type O (ordinary type of variations). An additional 440 patients who did not harbor cerebral aneurysms were randomly selected for a comparison. Anterior communicating artery aneurysms were significantly related to the Type A anatomy and ICA aneurysms to Type P anatomy. Male patients who did not harbor aneurysms tended to have Type A anatomy, whereas women had a significantly greater incidence of Type P. This sex-linked difference in anatomical variations may be correlated to the well-known sex-linked difference in aneurysm distribution.