Moyamoya disease (MMD) is a rare, progressive cerebrovascular disorder that affects the arteries in the brain. It involves the gradual narrowing and blockage of the large arteries supplying blood to the brain, most notably the internal carotid arteries. The name “moyamoya” originates from a Japanese term meaning “puff of smoke,” which describes the characteristic appearance of the tiny, abnormal blood vessels that form as the body attempts to compensate for the blockages. This disorder directly impacts the brain’s blood supply, carrying an inherent risk of stroke and hemorrhage.
The Mechanism of Moyamoya Disease
Moyamoya disease begins with a progressive narrowing (stenosis) of the distal internal carotid arteries and the proximal segments of the anterior and middle cerebral arteries. These major blood vessels deliver oxygen-rich blood to the front two-thirds of the brain, and their constriction severely limits blood flow. This reduction in cerebral blood flow is the primary driver of the disease’s effects.
In an attempt to bypass these blockages, the body stimulates the growth of a network of small, fragile collateral vessels, known as the “moyamoya” network. While they temporarily increase blood flow, they are inherently weak and insufficient for long-term needs. Their delicate nature makes them prone to two life-threatening events: occlusion leading to ischemic stroke, or rupture leading to hemorrhagic stroke.
The clinical presentation is a direct result of these circulatory failures. Ischemic events, such as transient ischemic attacks (TIAs) or full strokes, are more common in children due to insufficient blood flow. Adults frequently experience hemorrhagic strokes when the fragile moyamoya vessels rupture, causing bleeding into the brain.
Survival Rates and Long-Term Outlook
Moyamoya disease is a serious, progressive condition that significantly impacts long-term health and survival, particularly without timely intervention. The risk of death is most closely tied to the occurrence of severe strokes or intracranial hemorrhages, which can cause permanent neurological deficits. Studies have indicated that the overall mortality rate for untreated MMD patients is approximately 10% in adults and 4.3% in children.
Prognosis is heavily influenced by the patient’s age at diagnosis and the type of stroke presentation. Adult patients generally face a more severe prognosis than children, reflected in their lower 5-year survival rates (92.9% compared to 99.3% for the pediatric group). Adult patients who present with a hemorrhagic stroke have the worst survival outcomes. For instance, the 10-year survival rate for adult patients presenting with hemorrhage was found to be 76.3%, significantly lower than the 88.9% for the ischemic group.
The long-term outlook is also complicated by the risk of progressive neurological and cognitive decline, even in patients who survive the initial events. Without treatment, between 50% and 66% of patients may experience progressive neurological deficits. Early diagnosis is paramount because the goal is not only survival but also the preservation of functional independence and quality of life.
Treatment Strategies to Mitigate Risk
The primary goal of treating Moyamoya disease is to mitigate the risk of future ischemic and hemorrhagic events, thereby improving survival and long-term function. Medications such as antiplatelet agents (like aspirin) are sometimes used, particularly for children with preserved cerebral blood flow, to help prevent blood clots and strokes. However, medical management alone cannot stop the underlying blood vessel narrowing from progressing.
Surgical revascularization is considered the definitive treatment for MMD, as it aims to restore adequate blood flow to the brain. These procedures are categorized into two main types: direct and indirect revascularization.
Direct Revascularization
Direct revascularization, most commonly the Superficial Temporal Artery to Middle Cerebral Artery (STA-MCA) bypass, immediately connects a scalp artery to a brain artery to provide augmented blood flow.
Indirect Revascularization
Indirect revascularization procedures, such as Encephaloduroarteriosynangiosis (EDAS), involve laying a blood-rich tissue, often an artery from the scalp, directly onto the brain’s surface. This technique encourages the gradual growth of new blood vessels into the brain over time.
Both approaches relieve the hemodynamic stress on the fragile moyamoya vessels, reducing the risk of further ischemic events and hemorrhage. Successful revascularization has been shown to result in a significant reduction in the risk of additional strokes, with some studies indicating a decrease of 65% to 85% after surgery. Surgical intervention significantly alters the natural history of the disease, moving the long-term outlook toward a more favorable outcome and a normal life expectancy for many patients.