MIAs (Mirror Intracranial Aneurysms): symmetry-related patient risk or consequence of multiplicity?

Abstract

Determine whether mirror intracranial aneurysms (MIAs) confer risk beyond aneurysm multiplicity and describe their distribution and longitudinal change. Retrospective two-centre UK cohort of unruptured intracranial aneurysms (UIAs) diagnosed 2006-2020; outcomes to 2022. 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). Rupture-free survival used inverse-probability-weighted Kaplan-Meier. Models adjusted for baseline aneurysm count. 1,985 UIAs were identified; 289 (14.6%) were MIAs. 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 GEE showed no symmetry-specific increase in rupture risk (omnibus p = 0.72). Lesion-level growth ≥ 1 mm (RR 1.67, q = 0.0380) and morphology change (RR 2.10, q = 0.0121) were higher in MIAs. With aneurysm count adjustment, effects attenuated with wide CIs, consistent with limited power. MIAs were associated with higher patient-time rupture and SAH-specific mortality and greater lesion-level instability, but not with an independent per-aneurysm rupture hazard. The excess patient-level risk is largely explained by exposure (multiplicity); a symmetry-related effect remains plausible but unconfirmed. Larger, prospectively harmonised datasets are needed.