The etiology of this pathological entity is unknown. The gradual progression of cerebral ischemia results in the development of various collateral pathway formations. Moyamoya vessels, themselves, are also abnormally dilated perforating arteries that act as collateral vessels. Transdural communicating arteries are also frequently detected. This combination of progressive occlusive lesions and abnormally developed perforating arteries as collaterals is also a feature detected in the posterior half of the circle of Willis.
Some patients suffer from hemorrhagic episodes.
The long-standing hemodynamic stress placed on the abnormally dilated moyamoya vessels has been considered a cause of hemorrhage. The age at onset is under 10 years in approximately half of the patients. The age of onset is intimately correlated with the severity of the disease in pediatric patients. A higher CBF volume is required to maintain the active cerebral metabolism along with brain development. The younger the child, the larger the difference between the required CBF and the compromised CBF state in patients.