Are the strokes in moyamoya syndrome associated with Down syndrome due to protein C deficiency?

Moyamoya syndrome has occasionally been seen in association with Down syndrome. We report a child with moyamoya syndrome and Down syndrome who was admitted with repeated episodes of strokes; his magnetic resonance imaging and magnetic resonance angiography findings confirmed the presence of occlusive cerebrovascular disease with basal collateral vessels. His protein C levels were significantly decreased during the stroke. Complete clinical recovery was seen during follow-up. This raises the possibility of a link between protein C deficiency and Down syndrome with moyamoya syndrome.