Moyamoya disease is an idiopathic, non inflammatory, non atherosclerotic progressive vasculo-occlusive disease involving the circle of Willis, typically the supraclinoid internal carotid arteries. The term moyamoya disease should be reserved for an idiopathic, sometimes familial, condition, which leads to characteristic intracranial vascular changes. Numerous entities have been described which mimic the appearance, in which case the term moyamoya phenomenon, syndrome or pattern is used. 

The name “moyamoya” means “puff of smoke” in Japanese and describes the look of the tangle of tiny vessels formed to compensate for the occlusion. Moyamoya disease was first described in Japan in the 1960s and it has since been found in individuals in the other countries around the world; its incidence is higher in Asian countries than in Europe or North America.

Moyamoya is a disease of children and young people, with a bi-modal age distribution:

- early childhood: peak ~4 years of age (two-thirds)

- middle age: 30-40 years of age (one-third)

The condition was initially described in Japanese patients, where it is still most common, in which 7-10% of cases are familial.

Presentation is to some degree age dependent. In children hemispheric ischaemic strokes are most pronounced, whereas in adults haemorrhage from the abnormal vessels is more common. Watershed infarcts are also very commonly identified.

Moyamoya disease affects the bilateral distal ICAs and circle of Willis. Up to 18% of patients with moyamoya may present with unilateral angiography-documented disease.

Small abnormal net-like vessels proliferate giving the characteristic "puff of smoke" appearance on direct angiography. CTA and MRA is not always able to demonstrate this appearance on account of lower flow and spatial resolution. Although classically described affecting the ICA, over 50% of patients also have involvement of the posterior cerebral arteries. Generalised cerebral atrophy is a common finding. 

Collateral circulation forms from a number of sources:

- via the abnormal moyamoya vessels: lenticulostriate, thalamoperforating, leptomeningeal, and dural arteries appear as multiple tortuous flow voids on T1 and T2 weighted sequences

- pial collaterals from less affected vessels (especially PCA): forming the so called ivy sign (high serpentine sulcal FLAIR signal intensity due to slow flow and also high signal on T1 post contrast enhanced MRI) 

- multiple foci of microbleeds and also prominent deep medullary veins "brush sign" on susceptibility sequences 

- transdural branches of the middle meningeal and other dural branches

Bypassing the occlusive segments is the aim of most surgical therapy. 

In adults external carotid artery to middle cerebral artery (ECA-MCA) anastomoses can be performed as the vessels are larger. One of the surgical option is superficial temporal artery to middle cerebral artery (STA-MCA) bypass. Encephaloduroarteriosynangiosis is the treatment of choice in paediatric patients as their vessels are too small to allow direct anastomosis.