The anatomy of aneurysm-bearing circles of Willis.
The circle of Willis is known to exhibit considerable anatomical variability. The incidence of variations of the circle is said to be greater when an aneurysm is present, particularly one of the anterior communicating artery. In this study 40 circles of Willis bearing 51 saccular aneurysms were dissected and examined macroscopically. Forty circles from patients with other neurological diseases served as controls. Ninety-seven per cent of circles bearing aneurysms showed variations from a hypothetical "normal" structure, as did 85% of controls. The aneurysm group as a whole, and circles bearing anterior communicating aneurysms, showed a higher incidence of anterior cerebral artery asymmetry than controls (p less than 0.001), as did the latter when compared with circles bearing aneurysms at other sites (p less than 0.01), but there were no other significant differences.