Moyamoya disease is a rare progressive cerebrovascular disorder characterized by a slowly progressive stenosis and obliteration of the large vessels at the base of the brain, affecting mainly the supraclinoid segment of the internal carotid artery and the initial portion of the anterior or middle cerebral arteries and the posterior cerebral arteries. As a result of the chronic obliteration, small collateral vessels arise distal to the blocked vessels visible as 'puff of smoke' on angiography. In fact, the name of the disease is derived from the Japanese word moyamoya, which means 'something hazy like a puff of cigarette smoke, drifting in the air'. The steno-occlusive areas are usually bilateral, but unilateral involvement does not exclude the diagnosis.
Moyamoya disease occurs primarily in Asians, but it can also occur in other populations. Females are more frequently affected than males. The idiopathic or primary form of moyamoya disease, which is sometimes familial, has to be distinguished from the secondary form, referred to as moyamoya syndrome, which can be associated with certain systemic conditions such as sickle cell disease, chronic basilar meningitis, neurofibromatosis, X-ray irradiation, homocystinuria, and Down syndrome.