R esearch on the development of intracranial aneurysms has been performed in numerous fields such as hemodynamics, 18 biomechanics, 4 histology, 6 and computational fluid dynamics. 21 Although these studies have provided new information regarding factors involved in aneurysm development, the results cannot be directly related to the growth rates of aneurysms. Given that an aneurysm's size is a main determinant in its risk of rupture, 26 lesion growth rates are a key factor in the relation among aneurysm prevalence, the risk of aneurysm rupture, and the observed incidence of SAH. Although aneurysm growth has been suggested to be erratic, 12 existing models of aneurysm screening or treatment generally incorporate a constant time-independent growth rate and risk of rupture. Aneurysms that grow fast in a short period of time [14][15][16]30,31 are likely to have high risks of rupture for short periods of time.11 Variable growth rates with corresponding time-variable risks of rupture will change predictions of the SAH risk over different time intervals. Evidence regarding the actual growth process may support or challenge current assumptions and perhaps evoke new screening and/or treatment strategies for unruptured aneurysms.
24,33Limited empirical data exist on the change in, or the consistency of, aneurysm growth rates over time. Extensive data collection on growth rates is not feasible, because it would require frequent screening-for example, with MR angiography-of large groups of healthy individuals and persons with unruptured aneurysms. Thus, insight into aneurysm growth rates can be attained only by modeling the effects of different growth rates on aneurysms sizes and SAH incidence and comparing these results with the observed incidence of SAH in an actual population. We constructed a model suitable for the simulation of individual patient histories. Drawing from a parametric distribution, aneurysm growth rates were assigned to individuals. As aneurysm formation and Object. The annual rate of rupture of intracranial aneurysms is often assumed to be constant, but it is unknown whether this assumption is true. Recent case reports have suggested that aneurysms grow fast in a short period of time. The authors of the present report investigated the plausibility of a constant growth rate for intracranial aneurysms.Methods. Assuming a constant aneurysm growth rate within an individual and varying rates between individuals, a hypothetical cohort was simulated. Individuals with high growth rates will display aneurysm formation and rupture at a young age; such persons disappear early from the hypothetical cohort. As a result the mean lesion growth rate varies over time. In hypothetical cohorts with different initial mean growth rates, the authors calculated age-specific incidence rates (per 100,000 person-years) of subarachnoid hemorrhage and compared these rates with populationbased data on the incidence of subarachnoid hemorrhage (per 100,000 person-years).Results. A hypothetical cohort with a mean initial growth rate o...