Growth rates of intracranial aneurysms: exploring constancy

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Abstract
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. 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 population-based data on the incidence of subarachnoid hemorrhage (per 100,000 person-years). A hypothetical cohort with a mean initial growth rate of 0.18 mm/year reproduced most closely the incidence rates observed in the population. However, even for this most plausible hypothetical cohort, age-specific incidence rates in the model differed substantially and statistically significantly from those observed in the population. Based on the results of this study, it is unlikely that intracranial aneurysms in general grow at a constant time-independent rate. The authors hypothesized that the actual growth process is irregular and discontinuous, which results in periods with and without aneurysm growth and with high and low risks of rupture.