1138 The Dynamic Adaptability of the Circle of Willis in Response to Major Branch Artery Coverage With a Flow Diverter

INTRODUCTION: The plasticity of the circle of Willis is an under-reported yet fascinating aspect of vascular anatomy in cerebrovascular disorders. Many clinicians see changes in vessel diameter and flow, which are qualitative and thus more difficult to report in the literature. METHODS: A retrospective analysis of digital subtraction angiographies (DSA) from intracranial aneurysms treated with FD from 2013 to 2023 was conducted. Vessel diameters, including those covered by the stent and adjacent arteries, were measured. Angiographic changes were evaluated at last imaging follow-up. RESULTS: Four hemodynamic patterns were identified: (1) Patients with a hypoplastic A1-ACA (N=33), increased the caliber of this vessel from 0.8mm (95 %CI=0.37-1.23) to 1.65 mm (95 %CI=0.79-2.60), in response to having the contralateral ACA segment covered. (2) Patients with distal ICA aneurysms treated covering the ACA-origin show increase in diameter of the contralateral A1 (mean increase of 0.45 mm; 95%CI=0.4-1.2) and ICA (mean increase of 0.55 mm; 95%CI=0.1-1.2; N=58). (3) Patients with hypoplastic P1-PCA or fetal PCA can increase the P1-PCA diameter after having the origin of the ipsilateral PComA covered (mean increase of 0.68 mm; 95 %CI=0.25-1.18; N=7). (4) Patients with basilar-tip and proximal PCA-aneurysms treated covering the origin of one of the PCAs demonstrated an increase in size of the ipsilateral PComA (mean increase of 0.44 mm, 95%CI=0.14-1.02; N=15). Physiologic and hemodynamic factors that appear to influence these changes will be discussed. CONCLUSIONS: The circle of Willis displays both hemodynamic and anatomic plasticity after major branch coverage with a flow diverter. This captivating and real phenomenon is aimed at preserving blood flow in the distal territory of the covered vessel. The biologic substrate of these changes is still not known but under investigation.