A 60-year-old Japanese woman had a history of ischemic stroke. She had no risk factors including a history of transient ischemic attack (TIA), hypertension, myocardial infarction, atrial fibrillation, left atrial enlargement, smoking, heavy alcohol use, diabetes mellitus, obesity, dyslipidemia, and carotid artery stenosis. Cerebral angiography showed steno-occlusive changes at the proximal middle cerebral arteries with abnormal vascular networks (Fig. 1). What is the diagnosis?
