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A chronic cerebral vasculopathy primarily occurring in childhood and characterized by slowly progressive carotid artery narrowing and occlusion at the base of the brain. An extensive network of anastomoses forms between branches of the carotid arteries, resulting in a characteristic angiographic appearance. The condition may be idiopathic (classic moyamoya disease) or occur in association with ANEMIA, SICKLE CELL; DOWN SYNDROME; CRANIOCEREBRAL TRAUMA; RADIOTHERAPY induced arterial injury; NEUROFIBROMATOSIS; and other diseases. Clinical manifestations include hemiparesis, headache, seizures, and delayed mental development. In older individuals, this disease may present as SUBARACHNOID HEMORRHAGE. (From Adams et al., Principles of Neurology, 6th ed, p831; Clin Neurol Neurosurg 1997 Oct;99 Suppl 2:S36-8)