Edoardo Maria Barbieri

BACKGROUND: Surgical revascularization is the cornerstone of treatment for moyamoya angiopathy (MMA), yet postoperative contralateral progression and the timing of surgery after acute ischemia remain controversial. While current guidelines recommend delaying bypass after a recent ischemic event, rapidly progressive symptoms may necessitate urgent intervention. OBSERVATIONS: A 26-year-old woman with unilateral MMA underwent a left superficial temporal artery–middle cerebral artery (STA-MCA) bypass for recurrent ischemic events and impaired cerebrovascular reserve. Despite initial improvement and confirmed bypass patency, she developed contralateral ischemic symptoms 3 weeks later, with new right internal carotid artery–MCA stenosis and vessel wall enhancement. CSF analysis revealed herpes simplex virus 2 positivity, and a heterozygous factor V Leiden mutation was identified. Despite antiviral and corticosteroid therapy, ischemic deficits worsened. An emergency right STA-MCA bypass was performed under dual antiplatelet therapy (DAPT), resulting in immediate symptom resolution and sustained graft patency. LESSONS: MMA may rapidly progress from unilateral to bilateral involvement. When medical therapy fails, urgent revascularization—even under DAPT—can be safe and effective. Vigilant postoperative surveillance and readiness for contralateral bypass are crucial in atypical, rapidly evolving cases. https://thejns.org/doi/10.3171/CASE25903