Background Intracranial aneurysms more often occur in the same arterial territory within families. Several aneurysm locations are associated with specific circle of Willis variations. We investigated whether the same circle of Willis variations are more likely to occur in first-degree relatives than in unrelated individuals. Methods We assessed four circle of Willis variations (classical, A1-asymmetry, incomplete posterior communicating artery and fetal circulation) in two independent groups of families with familial aneurysms and 2 first-degree relatives with circle of Willis imaging on MRA/ CTA. In each (index) family we determined the proportion of first-degree relatives with the same circle of Willis variation as the proband and compared it to the proportion of first-degree relatives of a randomly selected unrelated (comparison) family who had the same circle of Willis variation as the index family’s proband. Concordance in index families and comparison families was compared with a conditional logistic events/trials model. The analysis was simulated 1001 times; we report the median concordances, odds ratios (ORs), and 95% confidence intervals (95%CI). The groups were analysed separately and together by meta-analysis. Results We found a higher overall concordance in circle of Willis configuration in index families than in comparison families (meta-analysis, 244 families: OR 2.2, 95%CI 1.6–3.0) mostly attributable to a higher concordance in incomplete posterior communicating artery (meta-analysis: OR 2.8, 95%CI 1.8–4.3). No association was found for the other three circle of Willis variations. Conclusions In two independent groups of families with familial aneurysms, the incomplete PcomA variation occurred more often within than between families suggesting heritability of this circle of Willis variation. Further studies should investigate genetic variants associated with circle of Willis formation.
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