Moyamoya disease is a rare but very serious condition affecting one or both of the internal carotid arteries. These arteries are located at the base of the brain. Moyamoya disease causes them to become narrowed over time. This restricts blood flow to the brain. As a result, blood clots may form, and the risk of having a stroke, mini-stroke or seizure may increase. 

Moyamoya develops over the course of months or years. It is most common during childhood. As the arteries become narrower, the brain forms tiny blood vessels to create new pathways for blood to flow. The tiny blood vessels look like puffs of smoke on a cerebral angiogram. The word “moyamoya” means “puff of smoke” in Japanese.