Aneurysm appearing at the anastomosis site 11 years after superficial temporal artery–middle cerebral artery bypass surgery: moyamoya disease with a rapidly growing aneurysm. Illustrative case

BACKGROUND: Superficial temporal artery–middle cerebral artery (STA-MCA) bypass surgery is performed to prevent ischemia and hemorrhage in patients with moyamoya disease. Only a few reports have described aneurysms appearing around the anastomosis site after bypass surgery, and the underlying mechanism remains unknown. OBSERVATIONS: The present case involved a 62-year-old woman who underwent STA-MCA bypass surgery for ischemic quasi-moyamoya disease at 46 years of age. Postoperatively, she underwent annual magnetic resonance imaging examinations. At 11 years after STA-MCA bypass surgery, a 3-mm aneurysm appeared at the anastomosis site. Four years later, headache developed and the aneurysm had grown to 5 mm. Craniotomy clipping was performed to prevent rupture. The patient was discharged home 2 weeks after surgery without any apparent complications. LESSONS: Long-term observation is crucial after direct bypass surgery for moyamoya disease. Measures to prevent rupture should be considered for cases involving aneurysm complications.