Moyamoya disease (MMD) is a rare disease affecting the blood vessels to the brain, generally on both sides. The disease is characterized by the progressive narrowing and occlusion of the intracranial internal carotid arteries and their major branches due to progressive growth of the intima (the inner lining of arteries). In order to compensate for the narrowed/occluded middle cerebral and anterior cerebral arteries, the small branches of the vessels dilate and establish further connections inside the brain (called collateral circulation).

The brain also develops other collateral vessels through the pial circulation. As the disease advances, the dilated arteries in the base of the brain may develop small aneurysms, which can lead to rupture and brain hemorrhage.

In a more advanced stage, even the collateral vessels become occluded. Ischemia (loss of blood flow at the microvascular level) occurs when the collateral circulation is unable to compensate for the metabolic demands of the brain. This is when patients develop symptoms of transient ischemic attacks (mini-strokes) or brain infarction (permanent brain stroke).

In advanced stages of the disease, as the collateral vessels shut down, very severe brain ischemia and severe strokes occur. Moyamoya (Japanese for "puff of smoke") refers to the angiographic appearance of these abnormal vascular collateral networks. The steno-occlusive areas are usually bilateral, but unilateral involvement does not exclude the diagnosis.

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