Current status of revascularization surgery for moyamoya disease: Special consideration for its ‘internal carotid-external carotid (IC-EC) conversion’ as the physiological reorganization system

Moyamoya disease is a chronic cerebrovascular disease with unknown etiology, which is characterized by bilateral steno-occlusive changes at the terminal portion of the internal carotid artery and an abnormal vascular network formation at the base of the brain. Moyamoya disease is known to have unique and dynamic nature to convert the vascular supply for the brain from internal carotid (IC) system to the external carotid (EC) system, as indicated by Suzuki’s angiographic staging established in 1969. Insufficiency of this ‘IC-EC conversion system’ may result in cerebral ischemia, as well as in intracranial hemorrhage from inadequate collateral vascular network, both of which represent the clinical presentation of moyamoya disease. Therefore, surgical revascularization by extracranial-intracranial bypass is the preferred procedure for moyamoya disease to complement ‘IC-EC conversion’ and thus to avoid cerebral infarction and/or intracranial hemorrhage. Long-term outcome of revascularization surgery for moyamoya disease is favorable, but rapid increase in cerebral blood flow on the affected hemisphere could temporarily cause unfavorable phenomenon such as cerebral hyperperfusion syndrome. We would review the current status of revascularization surgery for moyamoya disease based on its basic pathology, and sought to discuss the significance of measuring cerebral blood flow in the acute stage and intensive perioperative management.