Editorial: Aneurysm geometry

Abstract

Treatment of small, unruptured intracranial aneurysms is controversial, and the decision of whether to intervene is a difficult one. While a very small subset of unruptured aneurysms go on to rupture, the vast majority remain quiescent for years. Different factors (related to patient, aneurysm, and surgeon/technique) are taken into consideration to guide the decision-making process. The development of 3D imaging techniques has triggered an expanding number of reports on the predictive role of various geometric and hemodynamic measurements on the formation, growth, and rupture of intracranial aneurysms. While estimation of the putative role of these measurements is appealing and sophisticated, most of the studies suffer from a major methodological flaw: unruptured aneurysms are compared with ruptured aneurysms. As most surgeons who have operated on patients with ruptured and unruptured aneurysms are well aware, the 2 entities are often completely different. In their article, Lin and coworkers1 analyze a relatively large cohort of middle cerebral artery (MCA) aneurysms and compare unruptured and ruptured aneurysms with respect to various geometric measurements calculated on 3D CT angiography reconstructions. The authors conclude that aspect ratio, flow angle, and parentdaughter angle were more common in the ruptured rather than in the unruptured cohort. The authors of this study make an attempt to consider a homogeneous group of aneurysms by selecting only patients with MCA aneurysms. However, the 2 groups under consideration are still quite different, and the study is not devoid of the significant methodological flaws of similar studies. I suspect that composition and strength of the aneurysm wall and time elapsed from aneurysm formation to its detection are much more important differences between ruptured and unruptured aneurysms than are geometric differences. Most aneurysms that go on to rupture most likely do so shortly after their formation at a time when the aneurysm wall is more vulnerable to hemodynamic stress.2,3 It is possible that aneurysm geometry plays a role in this early stage of vulnerability, hence the differences between ruptured and unruptured aneurysms. While the growing body of literature on fluid dynamics and aneurysm geometry has had important repercussions in designing newer treatments for intracranial aneurysms (such as flow diverters), the ever-increasing list of complex geometric factors associated with “rupture” has done little to change our decision making in patients with unruptured aneurysms or our understanding of the formation, growth, and rupture of intracranial aneurysms. A prospective study of conservatively treated unruptured aneurysms with analysis of geometric factors associated with rupture risk may provide useful information, but even such a study could be inconclusive because of the small number of end points (that is, ruptures) and the interaction with other important variables (aneurysm size, patient age, aneurysm location, and correctable risk factors for aneurysm rupture). Despite the growing body of literature on aneurysm/parent artery geometry and computational fluid analysis, the decision making in treating unruptured intracranial aneurysms continues to be based on art rather than science. (http://thejns.org/doi/abs/10.3171/2012.3.JNS112260)