Moyamoya, meaning a "hazy puff of smoke" in Japanese, is a chronic, occlusive 
cerebrovascular disease involving bilateral stenosis or occlusion of the 
terminal portion of the internal carotid arteries (ICAs) and/or the proximal 
portions of the anterior cerebral arteries and middle cerebral arteries (MCAs). 
The Ministry of Health and Welfare of Japan has defined 4 types of moyamoya 
disease (MMD): ischemic, hemorrhagic, epileptic, and "other." The ischemic type 
has been shown to predominate in childhood, while the hemorrhagic type is more 
often observed in the adult population. The highest prevalence of MMD is found 
in Japan, with a higher female to male ratio. Studies have shown a possible 
genetic association of MMD linked to chromosome 17 in Japanese cases as well as 
in cases found in other demographics. During autopsy, intracerebral hematoma is 
found and most commonly serves as the major cause of death in patients with MMD. 
Moyamoya vessels at the base of the brain are composed of medium-sized or small 
muscular arteries emanating from the circle of Willis, mainly the intracranial 
portions of ICAs, anterior choroidal arteries, and posterior cerebral arteries, 
forming complex channels that connect with distal positions of the MCAs. Off of 
these channels are small tortuous and dilated vessels that penetrate into the 
base of the brain at the site of the thalamoperforate and lenticulostriate 
arteries. On angiography, there is the characteristic stenosis or occlusion 
bilaterally at the terminal portion of the ICAs as well as the moyamoya vessels 
at the base of the brain. Six angiographic stages have been described, from 
Stage 1, which reveals a narrowing of the carotid forks, to Stage 6, in which 
the moyamoya vessels disappear and collateral circulation is produced solely 
from the external carotid arteries. Cases with milder symptoms are usually 
treated conservatively; however, more severe symptomatic cases are treated using 
revascularization procedures. Surgical treatments are divided into 3 types: 
direct, indirect, and combined/other methods. Direct bypass includes superficial 
temporal artery-MCA bypass or use of other graft types. Indirect procedures 
bring in circulation to the intracranial regions by introducing newly developed 
vasculature from newly approximated tissues. These procedures may not be enough 
to prevent further ischemia; therefore, a combination of direct and indirect 
procedures is more suitable. This article will give a review of the 
epidemiology, natural history, pathology, pathophysiology, and diagnostic 
criteria, including imaging, and briefly describe the surgical treatment of MMD.