Epidemiology of the size distribution of intracranial bifurcation aneurysms: smaller size of distal aneurysms and increasing size of unruptured aneurysms with age.

OBJECTIVE: To explore the epidemiology of the size distribution of intracranial bifurcation aneurysms at different locations and to specifically test the hypothesis that distal vessels develop, on average, smaller aneurysms.

METHODS: A database detailing all aneurysm cases admitted to Massachusetts General Hospital from 1991 to 2003 was reviewed. Aneurysms were classified by location and size. The distribution of aneurysms by sizes at differing origin sites was then compared.

RESULTS: We identified 1673 aneurysms for study; 854 were ruptured and 819 were unruptured; 58 lesions were classified as distal middle cerebral artery and anterior cerebral artery, of which 26 were unruptured and 32 were ruptured. Analysis of the kernel density estimates for the distribution of aneurysm sizes revealed that aneurysms at distal locations and the posterior-inferior cerebellar artery location were smaller than those at other locations in the circle of Willis. The mean size of ruptured distal aneurysms at 5.7 mm (95% confidence interval 4.8-6.5), or ruptured posterior-inferior cerebellar artery aneurysms at 7.1 mm (95% confidence interval 6.3-7.8) was smaller than the average size for basilar, middle cerebral artery, or ICA aneurysms occurring proximally in the Circle of Willis. The decrease in the mean size of the distal lesions is caused by a relative paucity of aneurysms above 10 mm in size. Although ruptured aneurysms showed no change in size with age, unruptured lesions at most intracranial locations increased in size with age.

CONCLUSION: The distribution of aneurysm sizes differs according to location in the intracranial vasculature in this single institution series. Smaller aneurysm sizes are observed for "distal" aneurysms than at other locations in the Circle of Willis. We hypothesize that this may be related to Laplace's Law, which states that the "critical" size for aneurysm rupture is related to the parent artery wall thickness. The larger size of unruptured aneurysms in older patients in this study may reflect referral bias or a biological model in which a subset of smaller "unstable" aneurysms are prone to rupture. Because distal aneurysms present at smaller sizes compared with aneurysms originating proximally on the Circle of Willis, prospective studies that focus on the rupture risk of this subset of intracranial aneurysms are appropriate for future investigation.