Abstract
<title>Abstract</title> <p>Background Spontaneous regression of unruptured intracranial aneurysms is uncommon and is typically attributed to hemodynamic alterations that promote intra-aneurysmal thrombosis. The role of proximal parent artery stenosis (PPAS) on aneurysm progression remains unclear. We report a case of bilateral middle cerebral artery (MCA) bifurcation aneurysms in a single patient that exhibited divergent outcomes under conservative management, providing a unique within-patient comparison of aneurysm behavior with and without proximal stenosis. Case presentation: A 61-year-old woman with hypertension and type 2 diabetes presented with left-sided weakness. Imaging revealed acute cerebral infarction and two unruptured MCA bifurcation aneurysms: a right-sided aneurysm (5.3×2.8 mm on CTA; 5.5×3.0 mm on DSA) with severe PPAS, and a left-sided aneurysm (1.6×2.1 mm on CTA; 1.5×2.2 mm on DSA) without stenosis. DSA confirmed irregular morphology of the right aneurysm, suggesting elevated rupture risk; intervention was recommended but the patient opted for chose conservative management. At the first follow-up (March 2025, 2.3 years after diagnosis), the right aneurysm had regressed to 2.8×2.8 mm with suspected intra-aneurysmal thrombosis, while the left aneurysm had progressed to 2.3×2.8 mm, prompting initiation of dual antiplatelet therapy for 90 days, followed by long-term single antiplatelet therapy. At the second follow-up (March 2026, 3.2 years after diagnosis), both aneurysms remained stable, and no new ischemic stroke or intracranial hemorrhage had occurred. Conclusions This bilateral comparison provides direct clinical evidence that PPAS may protect against aneurysm progression, likely by reducing antegrade flow and promoting intra-aneurysmal thrombosis. In selected patients with PPAS and evidence of chronic hypoperfusion, conservative management with close imaging surveillance may be a reasonable alternative to intervention.</p>