Variations in the configuration of the circle of Willis.

Brains were obtained from 1000 medicolegal autopsy subjects of varying ages. The circle of Wills was examined at the base of the brain. The cerebral vessels were observed with regards to their origin, caliber and typical configuration. Variations were noted and grouped into different categories. Out of 1000 specimens examined, 452 (45.2%) conformed to the typical pattern. In the rest of the specimens (54.8%) there were variations in the circulus arteriosus. The circle was deficient in 32 (3.2%). The anterior cerebral artery was absent in 0.4%; hypoplastic in 1.7%; duplicated in 2.6%; triple in 2.3% and single in 0.9%. The anterior communicating artery was absent in 1.8%, duplicate in 10%, triplicate in 1.2% and plexiform in 0.4%. Multiplication of posterior cerebral artery was observed in 2.4% cases while it was hypoplastic in 10.6% brains. Posterior communicating artery was absent in 1% and hypoplastic in 13.2%. Seventy-four brains (7.4%) had multiple variations. Intracranial saccular aneurysm was present in 10 (1%). These figures are compared with the available literature. Persistence of some embryonic vessel that normally disappear, disappearance of vessels that would normally persist or sprouting of new vessels due to hemodynamic and genetic factors are the usual causes for such anomalies. These variations are discussed with regard to development and other hemodynamic factors.