ABSTRACT NUMBER: ESOC2026A106 ANATOMICAL VARIATIONS OF THE CIRCLE OF WILLIS AND THEIR ASSOCIATION WITH INTRACRANIAL ANEURYSM PRESENCE AND LOCATION

Abstract Background and aims Anatomical variations in the circle of Willis (CoW) can influence hemodynamic stress and contribute to intracranial aneurysm (IA) development. However, findings – including those regarding the role of IA location – remain inconclusive due to small sample sizes. This study aimed to investigate the association of CoW arterial diameters, anatomical variants, and bifurcation angles with the presence and location of IAs in a large cohort. Methods In this observational, case-control study, we analyzed Time-of-Flight Magnetic Resonance Angiography (TOF-MRA) scans from 1,021 IA patients and 1,052 controls without vascular abnormalities. CoW arterial diameters, anatomical variants, and bifurcation angles were measured semi-automatically. Differences between patients and controls were assessed with Generalized Linear Models and logistic regression models, adjusted for relevant covariates. Subgroup analyses were conducted by IA location and sex. Results IA patients had larger arterial diameters in the M1 segment of the middle cerebral artery, the internal carotid artery, and the basilar artery. They also more frequently exhibited anatomical variants, including aplasia or hypoplasia of the anterior communicating artery. Bifurcation angles between the proximal and distal branching vessels were smaller in IA patients. These differences persisted across different IA locations and were largely consistent in sex-stratified analyses. Conclusions Anatomical features of the CoW, including larger arterial diameters, anterior incompleteness, and smaller proximal-distal bifurcation angles, are consistently associated with the presence of IAs, independent of IA location or sex. These findings support the potential of CoW anatomical variations as imaging-based markers for assessing IA risk. Conflict of interest Phebe J. Groenheide, Iris N. Vos, Mark K. Bakker, Robin Bülow, Irene C. van der Schaaf, Ynte M. Ruigrok: nothing to disclose Figure 1 - belongs to Conclusions