It is well known that more than one-half of all adult patients with moyamoya disease suffer intracranial hemorrhage, whereas most nonadult patients present with cerebral ischemia [1]. Typically, the hemorrhage involves the thalamus and basal ganglia, and it frequently involves perforation to the ventricles (Fig. 1)
in rare instances, subcortical or subarachnoid hemorrhage can also be observed. Such bleeding attacks, which are potentially fatal, seriously affect the patient's prognosis [2]. It is speculated that chronic and exceptionally high hemodynamic stress might induce vascular wall pathologies such as microaneurysms or vessel fragility, which lead to hemorrhagic attacks (Fig. 2a). On occasion, subarachnoid hemorrhage is caused by rupture of saccular aneurysms located on the circle of Willis, especially on the posterior cerebral artery, which acts as the collateral pathway to the anterior circulation (Fig. 2b). © 2010 Springer-Verlag Tokyo.