Background and PurposeHemodynamics play adriving role in the life cycle of brain aneurysms from initiation through growth until eventual rupture. The specific factors behind aneurysm growth, especially in small aneurysms, are not well elucidated. The goal of this study was to differentiate focal versus general growth and to analyze the hemodynamic microenvironment at the sites of enlargement in small cerebral aneurysms.Materials and MethodsSmall aneurysms showing growth during follow-up were identified from our prospective aneurysm database. Three dimensional rotational angiography (3DRA) studies before and after morphology changes were available for all aneurysms included in the study, allowing for detailed shape and computational fluid dynamic (CFD) based hemodynamic analysis. Six patients fulfilled the inclusion criteria.ResultsTwo different types of change were observed: focal growth, with bleb or blister formation in three, and global aneurysm enlargement accompanied by neck broadening in other three patients. Areas of focal growth showed low shear conditions with increased oscillations at the site of growth (a low wall shear stress [WSS] and high oscillatory shear index [OSI]). Global aneurysm enlargement was associated with increased WSS coupled with a high spatial wall shear stress gradient (WSSG).ConclusionFor different aneurysm growth types, distinctive hemodynamic microenvironment may be responsible and temporal-spatial changes of the pathologic WSS would have the inciting effect. We suggest the distinction of focal and global growth types in future hemodynamic and histological studies.