Abstract
BACKGROUND: De novo aneurysms occurring at the anastomotic site are extremely uncommon, especially following side-to-side (STS) bypass for anterior cerebral artery (ACA) revascularization. Their etiology remains uncertain; however, it is believed to be related to hemodynamic shear stress and inherent vascular fragility, potentially linked to genetic anomalies. Furthermore, surgical intervention is mandatory to avert aneurysm growth or rupture. OBSERVATIONS: A 38-year-old-male first presented with a large fusiform aneurysm of the left distal ACA and underwent ACA-ACA STS bypass with aneurysm excision. Nonetheless, the de novo aneurysm was discovered to arise at the bypass site 7 months after the operation. Subsequently, a strategic radical intervention was performed. At the 26-month follow-up, the patient's modified Rankin Scale score was 0, and there was good bypass patency without aneurysm recurrence. The aneurysm formation was attributed to the fragility of the underlying arterial wall, which might be aggravated by STS bypass as evidenced by the histological findings and computational fluid dynamics (CFD) analysis. LESSONS: The revascularization strategy must be executed cautiously given the existence of wall disease, which poses a risk for aneurysm formation, particularly following modification by STS bypass, as demonstrated by CFD and histological findings. Careful monitoring and long-term vascular assessment are crucial. https://thejns.org/doi/10.3171/CASE25100.