Abstract TP369: Carotid Siphon Aneurysms have a Milder Hemodynamic Environment than Intradural Aneurysms during the Growth Process.

Background: It is known that carotid siphon aneurysms (CS-ANs) are less likely to rupture than cerebral aneurysms in other regions and more likely to become large. One of the reasons for this is assumed to be the hemodynamic environments caused by the characteristic bent vascular geometry, which is not yet clear. In a prospective observational study using computational fluid dynamics (CFD) analysis, we recently reported that two hemodynamic mechanisms exist for intradural aneurysm growth, depending on size, with a maximum diameter of 4 mm (Fukuda S, et al. J Cereb Blood Flow Metab). In the present study, we compared the hemodynamic environments involved in growing CS-ANs with those of intradural aneurysms. Methods: CFD analysis was performed using the unique arterial geometry and flow velocities of the enrolled patients of the study. Hemodynamic metrics were compared by multivariate analysis between aneurysms that grew more than 1 mm and those that did not during the 3-year observation period, using known growth risks as confounding factors. Results: Ten of the 69 CS-ANs were enlarged for 3 years. In CS-ANs, the two hemodynamic mechanisms for growth by aneurysm size, bounded by 4 mm, were less distinct, as seen in intradural aneurysms; only gradient oscillatory number (GON) was significantly higher in the dome area at 4 mm or greater. We then compared distributions of hemodynamic metrics in 69 CS-ANs with those of 202 intradural aneurysms in a multivariate analysis. In aneurysms <4 mm, CS-ANs had significantly lower wall shear stress (WSS) in the whole aneurysm, body region, dome region, and parent artery, and significantly higher oscillatory shear index (OSI), GON, and normalized transverse WSS (NtransWSS) in the parent artery, compared to intradural aneurysms. In aneurysms ≥4 mm, GON and NtransWSS were significantly lower for CS-ANs in the whole aneurysm and neck region. Furthermore, in aneurysms <4 mm, there was a reversal of the increase or decrease in hemodynamic metrics between growth and non-growth aneurysms in CS-ANs and intradural aneurysms, including transWSS in the whole aneurysm and in the neck region, and WSSG in the neck region. Conclusions: The hemodynamic environments involved in the growth of CS-ANs differed significantly from that of intradural aneurysms. The data suggest that hemodynamic environment inside CS-ANs is more moderate in magnitude and disturbance of the WSS compared to that inside intradural aneurysms.