Comparative analysis of the geometric morphology and local hemodynamics of small and medium intracranial aneurysms: a two-center retrospective study in a Chinese Han population.

BACKGROUND: Small (<4 mm) and medium (≥4 and <15 mm) intracranial aneurysms (IAs) account for the majority of lesions detected on routine cerebrovascular imaging, yet their true rupture propensity and mechanistic pathways remain controversial, particularly in Asian populations. Emerging evidence indicates that geometry-dependent hemodynamic disturbances may modulate rupture risk, but size-specific patterns have not been systematically compared. Therefore, we aimed to compare the clinical, geometric, and hemodynamic characteristics of small and medium IAs and to clarify the rupture-related mechanisms.

METHODS: We retrospectively reviewed computed tomography angiography (CTA) and digital subtraction angiography (DSA) in 214 patients with ruptured or unruptured IAs. Geometric parameters [location, dimensions, aspect ratio (AR), size ratio (SR), bottleneck ratio (BN), height-to-width ratio (HWR), and incidence angle] and key hemodynamic metrics [oscillatory shear index (OSI), relative residence time, time-averaged wall shear stress (TAWSS), time-averaged wall shear stress gradient (TAWSSG), and aneurysm formation indicator (AFI)] were calculated. Receiver operating characteristic analysis determined the aneurysm size threshold associated with rupture.

RESULTS: Medium aneurysms ruptured more frequently than did small aneurysms (67.4% vs. 44.7%; P=0.001). Small aneurysms occurred most often in the internal carotid and anterior cerebral arteries and along arterial side walls, whereas medium aneurysms predominately occurred at the anterior and posterior communicating arteries, the middle cerebral artery (MCA), and arterial bifurcations (P<0.001). Medium aneurysms exhibited larger aspect, size, and BNs; higher HWRs; and greater incidence angles (all P values <0.001). Small aneurysms situated at communicating arteries, the MCA, or bifurcation sites, as well as those with high SR, incidence angle, or aneurysm angle, were more likely to rupture. Irregular shape and bifurcation location predicted rupture in medium aneurysms. Hemodynamically, small aneurysms had a higher OSI and relative residence time, whereas medium aneurysms had a lower TAWSS and TAWSSG but higher values for AFIs (all P values <0.05). An aneurysm length greater than 3.87 mm was associated with rupture [area under the receiver operating characteristic curve (AUC) =0.613; 95% confidence interval (CI): 0.535-0.692; P=0.015].

CONCLUSIONS: Small and medium IAs exhibit distinct geometric and hemodynamic profiles linked to rupture. Prospective longitudinal studies are needed to confirm these associations and clarify causal pathways.