The natural history of moyamoya is not completely understood, although there is inevitable progression in the majority of patients. The rate of progression, however, is unpredictable. Two-thirds of patient have symptomatic progression over 5 years and the outcome is poor without treatment [6, 19, 20]. Those with unilateral presentation often develop moyamoya on the other side [14] and medical therapy alone is not effective in preventing progression. The 5-year stroke rate is 20–65 % with medical treatment only [5, 10, 28]. “Asymptomatic” moyamoya may not be entirely accurate [18], as 20 % of patients have silent infarcts (magnetic resonance imaging [MRI]-positive) and a 3.2 %/year stroke risk.

Moyamoya diagnosis involves taking careful histories and evaluating several radiology studies. Patients are asked in-depth questions about the onset, frequency, location, duration and severity of any neurological symptoms in order to determine if they are related to moyamoya and which side of the brain might be affected. Moyamoya can initially be confused with other neurological disorders, such as multiple sclerosis [7], until radiological studies confirm findings consistent with narrowing or closure of brain arteries.

MRI scans reveal brain structures and identify locations of infarcts or areas of bleeding (Fig. 10.3), MRI or computed tomography (CT) angiograms noninvasively screen blood vessels of the brain (Figs. 10.4 and 10.5) and cerebral angiograms show the structure of brain arteries (Fig. 10.6). Most common findings include narrowing or closure of the ICA or its branches. In addition, compensatory thin-walled arteries that develop over time may be present. These fragile “moyamoya” vessels, which led Japanese doctors to initially describe the “puff of smoke” appearance on angiograms [29], can bleed. If treatment is indicated, external carotid (scalp) arteries are examined to choose the best donor artery and the optimal technique for surgery.

Several different brain perfusion studies help determine how well blood vessels deliver oxygen to tissues of the brain. Patients may undergo one or more of these studies to evaluate brain perfusion, such as MRI, CT, PET and Xenon CT (Fig. 10.7) which, when combined with clinical history and physical evaluation, help guide treatment recommendations. Cognitive evaluation is also frequently done to assess for executive dysfunction (disorganization and impaired focus), which has been shown to be present at higher rates in adult moyamoya patients even without prior stroke [4, 12, 13].

Moyamoya is sometimes treated with antiplatelet or anticoagulation therapy in an attempt to improve blood flow to the brain. However, as previously mentioned, medical therapy alone is ineffective. The goal of treatment is to improve blood flow to the brain and prevent future stroke. Placement of stents, often effective against intracranial atherosclerosis, are ineffective against moyamoya long-term due to its lack of atherosclerotic etiology and progressive nature [16]. As most patients are young when moyamoya is diagnosed, surgery is usually recommended when symptomatic.

Surgical procedures include direct and indirect revascularization techniques. The direct bypass is known as the superficial temporal artery to middle cerebral artery (STA-MCA) bypass. This involves attaching a scalp artery (STA) directly into a brain artery (MCA) on the outer surface of the brain. Patients benefit from an immediate improvement in their blood flow when the bypass is completed as well as reduced risks of stroke. Indirect bypasses (encephalo-duro-synangiosis [EDAS], encephalo-duro-arterio-myo-synangiosis [EDAMS], pial synangiosis) involve placing a scalp artery or vascular muscle or tissue on the outer surface of the brain for gradual growth of blood supply inwards to areas in need. At least several months are required for ingrowth of new blood supply as well as stroke risk reduction using this method alone.