moyamoya disease /moi″əmoi″ə/ , a cerebrovascular disorder in which the main cerebral arteries at the base of the brain are replaced by a fine network of vessels. It is caused by progressive stenosis of the large-caliber vessels and development of collateral network. It tends mainly to affect Japanese children and young adults and is characterized by convulsions, hemiplegia, cognitive impairment, and subarachnoid hemorrhage. Patients who survive into adulthood are susceptible to massive intracerebral hemorrhage caused by rupture of the fragile network of new vessels. Few patients live beyond 30 years of age.
