Growth rates of intracranial aneurysms: exploring constancy

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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 population-based data on the incidence of subarachnoid hemorrhage (per 100,000 person-years).

Results

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.

Conclusions

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.

Abbreviations used in this paper: ACoA = anterior communicating artery; ASTRA = Aneurysm Screening After Surgical Treatment for Ruptured Aneurysms; CI = confidence interval; ICA = internal carotid artery; IDR = incidence density ratio; IMGR = initial mean growth rate; MCA = middle cerebral artery; SAH = subarachnoid hemorrhage; VBS = vertebrobasilar system.

Article Information

Address correspondence to: Hendrik Koffijberg, Ph.D., Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, P.O. Box 85500, Room STR 6.131/7.113, 3508 GA Utrecht, The Netherlands. email: h.koffijberg@umcutrecht.nl.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Line graphs showing the continuous function relating aneurysm size and location to the annual risk of rupture for Scenarios 1 and 2 (upper) and 3 and 4 (lower). The annual risk of rupture increases smoothly and exponentially with aneurysm size. It is highest for aneurysms in the VBS (VB) and lowest for those in the ACoA (ACA) and MCA.

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    Graphs depicting the IDRs for the SAH incidence rates for 8 age categories in the hypothetical cohort and the actual population together with their CIs. The IDRs for Scenarios 1 (A), 2 (B), 3 (C), and 4 (D) are shown. The horizontal line at IDR = 1 indicates the level at which the results from the simulation and the actual incidence agree perfectly.

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    Graphs depicting the number of SAH incidents during follow-up (A) and as function of patient age (B). Graphs illustrating the annual risk of rupture, on a logarithmic scale, during follow-up (C) and as a function of patient age (D), for 4 illustrative IMGRs. The solid black line in panel D represents the annual SAH incidence rates derived from the literature. FU = follow-up.

  • View in gallery

    Graphs showing for 4 illustrative IMGRs the average size of aneurysms at the time of rupture during follow-up (left) and the average aneurysm size as function of patient age (right). The peaks in the left panel for the high IMGRs are artifacts of the simulation model. For high mean growth rates almost all aneurysms rupture in the first 40 years of follow-up and too few rupture thereafter to provide a reliable estimate of the average size of ruptured aneurysms.

  • View in gallery

    Schematic showing the Markov model for the hypothetical cohort of patients. The transition probability qg,x denotes the mortality quotient for an individual of sex g and age x, FTreated indicates whether or not a particular person was treated after surviving an episode of SAH in the last 12 cycles (months), and FProbSAH(s, l) denotes the risk of rupture of an aneurysm of size s mm at location l.

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