Hemodynamics and natural history outcome in unruptured intracranial aneurysms

There is increasing interest in assessing the role of hemodynamics in aneurysm growth and rupture mechanism. Identification of the indicators of rupture risk can prove very valuable in the clinical management of patients. If rupture risk of aneurysms can be predicted, immediate preemptive treatments can be done for the high risk patients whereas others can avoid the risky intervention. Retrospective studies have been performed in the past to filter out indices that differentiate ruptured aneurysms from unruptured aneurysms. However, these differences may not necessarily translate to differences between aneurysms that present unruptured but fork towards growth/rupture and unruptured aneurysms that are invariably stable. The hypothesis of the present study is that hemodynamic indices of unruptured aneurysms when they first presented can be used to predict their longitudinal outcome.

A prospective longitudinal cohort study was designed to test this hypothesis. Four clinical centers participated in this study and a total of 198 aneurysms were recruited. These aneurysms were chosen by the physicians to be kept under watchful waiting. Three-dimensional models of aneurysms and their contiguous vasculature generated using the initial scans of patients were used for computational fluid dynamic (CFD) simulations. Both pulsatile and steady flow analyses were performed for each patient. By collating all the prominent hemodynamic indices available in aneurysm literature and developing a few new indices, 25 hemodynamic indices were estimated for each subject. For statistical analysis, it was hypothesized a priori that low wall shear area is different between stable and unstable aneurysms. All other indices were tested in a post-hoc manner.

The longitudinal outcome information of these patients was recorded at the clinical centers and the author was blinded until all analyses were complete. Aneurysms that grew during the follow up period were labeled as "grown" and otherwise they were called "stable" by the radiologists. After the hemodynamic analysis was complete, a non-parametric Mann Whitney U test was performed to determine if any index can statistically differentiate the two groups ("grown" versus "stable"). It was found that none of the indices distinguished the two groups with statistical significance. Comparison of the steady and pulsatile flow analysis suggested that the patient population is stratified in the same order by an index, irrespective of whether the index is computed using a steady or pulsatile flow simulation. Pearson correlation coefficient was obtained between basic geometric indices and hemodynamic indices of this population. No strong correlation was found in between morphology and hemodynamics, suggesting uniqueness of the hemodynamic indices.

The hypothesis motivating the present study is that aneurysm blood flow based indices can be used as prognostic indicators of growth and/or rupture risk. This study is the first to analyze intracranial aneurysm hemodynamics of a large cohort in a longitudinal prospective manner. Results of the present study indicate that quantitative hemodynamics cannot be used to predict the longitudinal outcome of an aneurysm. Further studies are needed to gain additional clinical insights.