Background: In many studies a constant annual rate of rupture for intracranial aneurysms is used, based on the characteristics of the patients and aneurysms being evaluated.However, it is unknown to what degree growth rates and rupture rates of intracranial aneurysms are constant. We assessed the plausibility of a constant aneurysm growth rate. Knowledge of the nature of aneurysm growth is important in the determination of optimal screening and treatment strategies.

Methods: A discrete event system was constructed to simulate a model in which all patients have one intracranial aneurysm and all aneurysms have a constant time-independent growth rate. Since no information is available on the distribution of growth rates for aneurysms in individual patients the shape of this distribution was defined using a Weibull distribution. The parameters a and b of this distribution were then defined as input for the simulation. The output from the simulation was compared to results from the literature on the incidence of subarachnoid hemorrhage.

Results: Population-independent observational data on rupture rates could not be reproduced under the assumption of constant growth rates. Regardless of the actual distribution of growth rates in populations significant differences exist between results from the literature and results from the simulation. It was determined that this result did not depend heavily on the distribution of the initial age of the patients or the risk of aneurysm rupture as used in the model, two model parameters around which uncertainty exists.

Conclusions: It is highly unlikely that intracranial aneurysms grow at a constant, time-independent rate. It is hypothesized that the actual growth process is irregular and discontinuous, allowing for both periods of growth and periods of non-growth. Such a growth process could explain the notion that aneurysms may exhibit fast growth over a short period of time that has appeared in recent years.