In light of the increasing number of patients with unilateral involvement as well as evidence that a substantial number of unilateral cases progress to bilateral presentation, the diagnostic criteria for definitive MMD were revised to include patients with both bilateral and unilateral presentation of terminal ICA stenosis with an abnormal vascular network at the base of the brain (statement by the Research Committee of MMD of the Japanese Ministry of Health, Labour, and Welfare in 2015). Moyamoya disease (MMD), also called spontaneous occlusion of the circle of Willis, is a chronic, occlusive cerebrovascular disease of unknown etiology characterized by steno-occlusive changes at the terminal portion of the internal carotid artery (ICA) and an abnormal vascular network at the base of the brain. High-resolution vessel wall MR imaging also helps diagnose MMD by revealing concentric vessel wall narrowing with basal collaterals. Magnetic resonance (MR) angiography and computed tomographic angiography are noninvasive diagnostic methods. Catheter angiography is the diagnostic method of choice.

These different age peaks and different clinical presentations in each age group are also observed in MMD patients in the USA. In patients with MMD, intracerebral hemorrhage is more often accompanied by intraventricular hemorrhage than in patients with hypertensive intracerebral hemorrhage. In contrast, adult patients present with intracranial hemorrhage more often than pediatric patients. Intellectual decline, seizures, and involuntary movements are also more common in this age group. In children, ischemic symptoms, especially transient ischemic attacks, are predominant. The peak appears to occur later in women than men. There are two peaks of incidence with different clinical presentations, at around 10 years and 30-40 years. The prevalence of MMD appears to be slightly lower among Chinese, compared to Koreans or Japanese. Possibly because of genetic differences, MMD is relatively common in people living in East Asian countries such as Korea and Japan, compared to those in the Western Hemisphere. Although its etiology remains unknown, recent genetic studies identified RNF213 in the 17q25-ter region as an important susceptibility gene of MMD among East Asian populations. Moyamoya disease (MMD) is a chronic, occlusive cerebrovascular disease characterized by progressive stenosis at the terminal portion of the internal carotid artery and an abnormal vascular network at the base of the brain.

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