A ruptured de novo anastomotic aneurysm shortly after superficial temporal artery-to-middle cerebral artery bypass surgery in a patient with moyamoya disease: illustrative case.

backgroundMoyamoya disease (MMD) is a progressive cerebrovascular disorder characterized by stenosis of the distal internal carotid arteries and development of collateral vessels. While surgical revascularization through direct superficial temporal artery-to-middle cerebral artery (MCA) bypass is the primary treatment for adults with MMD, rare but potentially fatal hemorrhagic complications can occur due to hemodynamic changes. OBSERVATIONS: A 34-year-old male with MMD who had previously undergone combined direct and indirect bypass surgery developed a massive intracerebral hemorrhage 2.5 months postoperatively. Imaging revealed an aneurysm at the anastomosis site as the bleeding source. Emergency surgery was performed to evacuate the hematoma and clip the aneurysm while preserving the bypass. The aneurysm originated from the MCA wall opposite to the anastomotic site, suggesting a hemodynamic etiology. Postoperative angiography confirmed complete clipping with preserved anastomotic patency. The patient was discharged with severe left-sided hemiparesis but without speech or cognitive impairment. LESSONS: This case represents the shortest documented interval between combined bypass surgery and a ruptured de novo anastomotic aneurysm. Genetic predisposition, inflammatory processes, and hemodynamic alterations likely contribute to aneurysm formation. Standardized postoperative surveillance and individualized management strategies are essential to mitigate these risks. https://thejns.org/doi/10.3171/CASE25429.