Also known as Moyamoya syndrome, a form of brain disease involving "the spontaneous occlusion of one or usually both internal carotid arteries in the intracranial region resulting in the formation of a fine network of neovascularity (blood vessel growth) at the base of the brain."1 In plain language, this means that large blood vessels, almost always the carotid arteries, become blocked, causing the formation of many very small blood vessels at the brain base. This network appears on an angiograph as a dull cloud, hence the name moyamoya, which means "hazy" or "misty" in Japanese2.
It is difficult to diagnose moyamoya disease, because the symptoms are easily confused with those of other disorders. The symptoms include:
Moyamoya is more prevalent in Japan than in the rest of the world. It was first described in Japan in the 1950's, and therefore it may well be that its symptoms are simply misdiagnosed more often in other countries. Incidence is roughly one per million people, with victims most often being under 10 years of age, or aged between 30 and 50.
The best treatment is currently even more unclear than the diagnosis. Patients are sometimes treated with steroids or aspirin to alleviate pressure on the major blocked arteries. Bypasses can be performed for the same reason, but with limited success. Some patients reach a stable condition, mostly after developing some disabilities, while others simply continue to deteriorate due to the difficulty of preventing further haemorrhaging from the 'cloud' of small blood vessels.
1: UCLA Neurosurgery website
2: The Harvard neurosurgery website gives the translation as "Puff of smoke" - thanks to gn0sis who informed me that moyamoya is an adjective, not a noun.