Widening and high inclination of the middle cerebral artery bifurcation are associated with presence of aneurysms

Abstract

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 Ф1, smaller daughter Ф2), bifurcation angle (Ф1+Ф2), and inclination angle (γ) 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 γ (median 57.8° versus 15.4°; P<0.0001). Seventy-five percent of aneurysmal MCAs had γ >10°, compared with 25% nonaneurysmal. Ф1 and Ф2, but especially Ф1+Ф2, were significantly larger in aneurysmal bifurcations (median 171.3° versus 98.1°; P<0.0001). Sixty-seven percent of aneurysmal bifurcations had Ф1+Ф2 >161°, compared with 0% nonaneurysmal MCAs. An optimal threshold of 140° was established for Ф1+Ф2 (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 Ф1+Ф2 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.

Keywords: intracranial aneurysm.

Figure 1

(A) Measurement of branching angles in a control MCA bifurcation (patient with no aneurysms), a non-aneurysmal MCA bifurcation (patient with aneurysms at other locations), and an aneurysmal MCA bifurcation. (B) Schematic drawing of a bifurcation showing measurement of branching angles: Φ₁ between parent vessel main axis and largest daughter vessel (branch 1) axis and Φ₂ with the axis of the smallest branch (branch 2). (C) Schematic drawing of a bifurcation (lateral view) which shows measurement of the inclination angle (γ) between the parent vessel plane and the daughter branches plane.

Figure 2

Statistical differences between control, non-aneurysmal, and aneurysmal MCA bifurcation subgroups for angles (A) Φ₁, (B) Φ₂, (C) total bifurcation angle Φ₁ + Φ₂ and (D) inclination angle γ. ** represents p<0.001, whereas * represents p<0.04.

Figure 3

(A) Receiver operator characteristics plot showing the performance of all bifurcation angles in discriminating between aneurysmal and non-aneurysmal MCA bifurcations. AUC represents area under the ROC curve. (B) Graph showing significantly larger Φ₁ for bifurcations in which the aneurysm originated off of the larger branch compared to bifurcations where the aneurysm originated off of the smaller branch. (C) Graph showing significantly larger Φ₂ for bifurcations in which the aneurysm originated off of the smaller branch compared to bifurcations where the aneurysm originated off of the larger branch or the middle of the bifurcation.

Figure 3

(A) Receiver operator characteristics plot showing the performance of all bifurcation angles in discriminating between aneurysmal and non-aneurysmal MCA bifurcations. AUC represents area under the ROC curve. (B) Graph showing significantly larger Φ₁ for bifurcations in which the aneurysm originated off of the larger branch compared to bifurcations where the aneurysm originated off of the smaller branch. (C) Graph showing significantly larger Φ₂ for bifurcations in which the aneurysm originated off of the smaller branch compared to bifurcations where the aneurysm originated off of the larger branch or the middle of the bifurcation.

Figure 3

(A) Receiver operator characteristics plot showing the performance of all bifurcation angles in discriminating between aneurysmal and non-aneurysmal MCA bifurcations. AUC represents area under the ROC curve. (B) Graph showing significantly larger Φ₁ for bifurcations in which the aneurysm originated off of the larger branch compared to bifurcations where the aneurysm originated off of the smaller branch. (C) Graph showing significantly larger Φ₂ for bifurcations in which the aneurysm originated off of the smaller branch compared to bifurcations where the aneurysm originated off of the larger branch or the middle of the bifurcation.