Progression of disease in unilateral moyamoya syndrome. 
Progression of vasculopathy associated with moyamoya syndrome is extremely variable.  The authors review their experience in patients with unilateral moyamoya syndrome to identify factors predictive of contralateral clinical and imaging-documented disease progression.  The authors reviewed the clinical and imaging records of all patients with moyamoya syndrome and unilateral disease who underwent cerebral revascularization surgery between January 1985 and June 2006 by using a standardized surgical procedure, pial synangiosis.  Of 235 surgically treated patients with moyamoya syndrome, 33 (14%) presented with unilateral disease (4 adults and 29 children).  There were 16 female and 17 male patients, with an average age of 10.4 years (26.8 years for adults and 8.1 years for children; range 1.5-39 years).  Twenty patients presented with left-sided disease and 13 with right-sided disease.  The average follow-up after surgery was 5.3 years (3.1 years for adults and 5.6 years for children; range 1-16 years).  During this period, 10 (30%) of 33 patients progressed to bilateral disease.  The mean time until disease progression was 2.2 years (range 0.5-8.5 years).  Factors associated with progression in this series included contralateral abnormalities on initial angiography, previous history of congenital cardiac anomaly, cranial irradiation, Asian ancestry, and familial moyamoya syndrome.  Young age at diagnosis was associated with a more rapid rate of progression (age < 7 years, 0.9 years to progression and age >or= 7 years, 3.1 years to progression).  Of patients with unilateral moyamoya syndrome, 30% will have progression of arteriopathy during long-term follow-up.  In this series, the average time of progression from unilateral to bilateral angiographic disease was 2.2 years.  Several factors, including contralateral abnormalities on initial imaging, congenital cardiac anomaly, previous cranial irradiation, Asian ancestry, and familial moyamoya syndrome, were associated with an increased risk of progression.  Patients with known unilateral angiographic disease should undergo continued monitoring by using MR imaging and MR angiography at regular intervals.  Treatment with pial synangiosis is safe and confers durable protection against stroke in patients with both bilateral and unilateral moyamoya syndrome.