BACKGROUND: The plasticity of the Circle of Willis represents an underexplored yet intriguing dimension of vascular anatomy in cerebrovascular disorders. We outline distinct patterns of change in response to aneurysm treatment using flow diversion (FD) after covering major branches. METHODS: Retrospective analysis of digital subtraction angiographies from intracranial aneurysms treated with FD from 2013 to 2023. Vessel diameters, including those covered by the stent and adjacent arteries, were measured. Angiographic changes were evaluated at last imaging follow-up. RESULTS: Of the 622 patients, 49 had angiographic follow-up for pattern assessment. The median age was 62 years
females represented 71.4%. The median size of the treated aneurysms was 4.7 mm. Four patterns of angiographic change were identified: 1) Patients with supraclinoid aneurysms, A1-anterior cerebral artery (ACA) caliber increased (hypoplastic: 1.05-2.00 mm
nonhypoplastic: 2.45-2.75 mm) after FD coverage of the contralateral ACA. 2) Patients with paraclinoid aneurysms and hypoplastic-fetal posterior cerebral artery (PCA), the diameter increased from 0.80 to 1.7 mm (P <
0.01) after covering the ipsilateral posterior communicating artery origin. 3) Patients with basilar-tip and proximal PCA aneurysms showed increased ipsilateral posterior communicating artery size from 1.2 to 2 mm (P <
0.01) after PCA origin coverage. 4) Patients with anterior communicating aneurysms, the diameter of the contralateral hypoplastic A1 segment increased from 1.0 to 1.35 mm (P = 0.39) or nonhypoplastic A1-ACA from 2.75 to 3.05 mm (P = 0.10) after FD coverage. CONCLUSIONS: The circle of Willis displays both hemodynamic and anatomic plasticity after major branch coverage with a flow diverter. This phenomenon is aimed at preserving blood flow in the distal territory of the covered vessel.