Multiple aneurysms: an insight through the mirrors.

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Mirror intracranial aneurysms (MIAs) are intracranial aneurysms (IAs) located bilaterally and symmetrically either side of the circle of Willis. This study explored the characteristics of MIAs and examined their prevalence at various intracranial locations in a large international population with multiple IAs, aimed at shedding light on the concept of MIA. Data were collected from cohorts of patients in France and Finland with ≥ 2 definite saccular arterial dilatations at intracranial vessel bifurcations. Patients were classified as 'MIA patients' if they had bilateral, symmetrically located IAs and further classified as having a pure phenotype (i.e., no other IAs present) or a mixed phenotype if non-mirror additional IAs were present. Statistical analysis used logistic regression models to assess the association of IA location with MIA status and conditional probabilities were calculated for paired locations. In a population of 2,124 patients with 5,459 IAs, 754 patients (33.5%) with a mean age 55.5 (SD 11.3) years had 798 MIAs. MIA patients were predominantly female (541; 72%). We found no relevant differences in clinical characteristics between MIA and non-MIA patients. Middle cerebral artery (MCA) location was the only variable independently associated with MIA status (OR 1.36 (95%CI 1.07%-1.71%), P=.0101). MIAs on MCA is a distinct condition among patients with multiple IAs, indicating a potential focal vulnerability related to a predisposing anatomical factor. Particular care is therefore needed during IA screening and follow-up to identify and manage MIAs.