Widening and High Inclination of the Middle Cerebral Artery Bifurcation Are Associated With Presence of Aneurysms

Background and Purpose-The middle cerebral artery (MCA) bifurcation is a preferred site for aneurysm formation. Wider bifurcation angles have been correlated with increased risk of aneurysm formation. We hypothesized a link between the presence of MCA aneurysms and the angle morphology of the bifurcation. Methods-Three-dimensional rotational angiography volumes of 146 MCA bifurcations (62 aneurysmal) were evaluated for angle morphology: parent-daughter angles (larger daughter Phi(1), smaller daughter Phi(2) ), bifurcation angle (Phi(1)+Phi(2)), and inclination angle (gamma) between the parent vessel axis and the plane determined by daughter vessel axes. Statistics were evaluated using Wilcoxon rank-sum analysis and area under the receiver operator characteristic curve. Results-Aneurysmal bifurcations had wider inclination angle gamma (median 57.8 degrees versus 15.4 degrees; P<0.0001). Seventy-five percent of aneurysmal MCAs had gamma > 10 degrees, compared with 25% nonaneurysmal Phi(1) and Phi(2), but especially Phi(1)+Phi(2), were significantly larger in aneurysmal bifurcations (median 171.3 degrees versus 98.1 degrees; P<0.0001). Sixty-seven percent of aneurysmal bifurcations had Phi(1)+Phi(2) > 161 degrees, compared with 0% nonaneurysmal MCAs. An optimal threshold of 140 degrees was established for Phi(1)+Phi(2) (area under the curve, 0.98). Sixty-eight percent of aneurysms originated off the daughter branches. Seventy-six percent of them originated off the branch with the largest branching angle, specifically if this was the smaller daughter branch. Wider Phi(1)+Phi(2) correlated with aneurysm neck width, but not dome size. Conclusions-MCA bifurcations harboring aneurysms have significantly larger branching angles and more often originate off the branch with the largest angle. Wider inclination angle is strongly correlated with aneurysm presence, a novel finding. The results point to altered wall shear stress regulation as a possible factor in aneurysm development and progression.