This configuration—indicating a larger bifurcation angle—and an aspect ratio greater than 1.6 both point to increased rupture risk, researchers say.
Both greater aspect ratio and larger bifurcation angle appear to strongly predict rupture of bifurcation-type basilar artery tip aneurysms, according to a small, retrospective Japanese study. The added risk stems from these factors’ effects on wall shear stress and aneurysmal hemodynamics.
Advancing technology is making unruptured aneurysms easier to identify. What lags behind is the ability to determine which of these aneurysms will cause patients no significant difficulty during their lifetimes and which will rupture, producing substantial mortality and morbidity risk. Lacking a reliable method of identifying the most at-risk aneurysms, clinicians are often faced with considerable uncertainty over how to counsel patients whose unruptured aneurysms are identified incidentally.
“The treatment of unruptured aneurysms is not a 100% risk-free option,” write the authors, led by Sherif Rashad, MD, (Kohnan Hospital, Sendai, Japan). It carries “3% mortality and 15% morbidity rates for surgical treatment and 2% morbidity and 5% mortality rates for endovascular intervention,” they say.
Factors that have been shown to influence rupture risk include the patient-related features of age, ethnicity, and smoking status, as well as morphometric features such as aspect ratio, bottleneck ratio, bleb formation, neck deviation, inflow angle, and aneurysm volume/ostium ratio.
Identifying Static Aneurysmal Features
“Measuring the simple bifurcation configuration may be a useful predictor of a cerebral aneurysm’s hemodynamics and tendency to rupture,” write the authors, who note that previous research has only looked at the predictive value of the bifurcation angle in combination with other aneurysmal features that change after rupture. This, they say, can limit the interpretation of those findings.
In their study published online March 10, 2017, ahead of print in the Journal of Neurosurgery, Rashad et al set out to develop a classification system of bifurcation aneurysms that would help predict the risk of rupture based on features that do not change once rupture occurs. They retrospectively reviewed their hospital database to identify 71 patients with basilar tip cerebral aneurysms (22 ruptured and 19 unruptured) treated surgically or endovascularly between 2008 and 2015. They acquired 3D rotational angiographic data and used both 3D stereolithography models and computational fluid dynamic analysis to determine multiple morphometric and hemodynamic features of the aneurysms.
Univariate analysis revealed that aspect ratio, bifurcation angle, bottleneck ratio, and inflow coefficient were all significantly correlated with a ruptured status. Logistic regression analysis showed, however, that only aspect ratio (P = 0.002) and bifurcation angle (P = 0.034) independently predicted rupture status.
The aneurysm features interacted with each other. Notably, bifurcation angle was inversely correlated with inflow coefficient (P < 0.0005), which in turn correlated directly with mean (P = 0.028) and maximum (P = 0.014) wall shear stress. The authors interpreted this to mean that “a larger bifurcation angle (“hands up” configuration) causes aneurysms to rupture at a smaller size as compared with a smaller bifurcation angle (“hands down” configuration).”
In addition, aspect ratio was inversely correlated with mean (P = 0.012) and minimum (P = 0.018) wall shear stress. Rashad and colleagues write that this “may explain why a larger aspect ratio is related to the rupture of aneurysms.”
“Aspect ratio and bifurcation angle had the strongest effects on rupture status mediated via [wall shear stress] lowering inside the aneurysm, whether as a direct consequence in the case of the aspect ratio or via the effect on inflow in the case of the bifurcation angle,” they write. “Given our results, the presence of [a hands up] configuration and/or aspect ratio more than 1.6 should prompt immediate treatment in patients with bifurcation aneurysms.”
Rashad S, Sugiyama SI, Niizuma K, et al. Impact of bifurcation angle and inflow coefficient on the rupture risk of bifurcation type basilar artery tip aneurysms. J Neurosurg. 2017;Epub ahead of print.
Rashad reports no relevant conflicts of interest.
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