MIAs (Mirror Intracranial Aneurysms) – symmetrical coincidence or a true independent risk factor?

Purpose To determine whether mirror intracranial aneurysms (MIAs) confer risk beyond aneurysm multiplicity and to describe their distribution and longitudinal change. Methods Retrospective two-centre UK cohort of unruptured intracranial aneurysms (UIAs) diagnosed 2006–2020; outcomes to 2022 (N=1,438). Endpoints: first rupture, SAH-specific/all-cause mortality, time to treatment, and lesion-level growth/morphology change. Rates used Poisson models with person-time offsets; lesion-level risks used GEE (modified Poisson). Survival used inverse-probability-weighted Kaplan–Meier. Results We identified 1,985 UIAs; 289 (14.6%) were within the MIA group. MIAs clustered at the MCA bifurcation (57.8%) and ICA terminus (34.6%). First-rupture incidence was higher in MIAs (1.74/100 person-years [PY]) than aMIAs (0.76/100 PY) or SIAs (0.39/100 PY); MIA>SIA IRR 4.46 (q=0.0003), MIA>aMIA IRR 2.29 (q=0.0044). SAH-specific mortality incidence was higher in MIAs (1.21/100 PY) than SIAs (0.36/100 PY; IRR 3.36, q=0.0057) and aMIAs (0.19/100 PY; IRR 6.37, q=0.0002). IPW survival was poorer for MIAs vs aMIAs (weighted log-rank χ²=9.95, p=0.0016) and vs SIAs (χ²=18.09, p=2.11×10⁻⁵). Lesion-level growth ≥1 mm (RR 1.67, q=0.0380) and morphology change (RR 2.10, q=0.0121) were higher in MIAs. Conclusion MIAs mark a higher-risk phenotype with excess rupture and SAH-specific mortality and greater lesion-level instability, supporting vigilant contralateral review and closer surveillance at MCA bifurcation/ICA terminus.