Moyamoya disease is a rare condition in which the major arteries at the base of the brain slowly narrow and eventually close off, starving the brain of blood. The name comes from Japanese and means “puff of smoke,” describing the tangle of tiny blood vessels the brain grows in a desperate attempt to reroute its own blood supply. These fragile replacement vessels show up on imaging scans as a hazy cloud, and while they partially compensate for the blockage, they’re never as effective as the original arteries.
Which Arteries Are Affected
The narrowing targets a very specific location: the terminal portion of the internal carotid arteries, the two large vessels that travel up through the neck and into the skull to supply most of the brain. As these arteries close off, the blockage spreads outward to involve the front and middle cerebral arteries that branch from them. The back of the brain’s blood supply, which comes from a separate set of arteries (the vertebrobasilar system), is typically spared, at least in earlier stages.
The disease progresses through six recognized stages, first described by the Japanese neurosurgeon Suzuki. It begins with narrowing at the carotid fork, followed by the appearance of the characteristic moyamoya collateral vessels. These collaterals intensify as the main arteries disappear, then the collaterals themselves begin to fade as the disease advances. In the final stage, the entire front circulation is gone and the brain relies entirely on blood rerouted from the external carotid arteries, which normally supply the face and scalp.
Who Gets Moyamoya Disease
Moyamoya disease is most common in East Asian populations, particularly in Japan and Korea. Incidence in Korea runs between 1.7 and 2.3 per 100,000 people per year, while China reports roughly 0.43 per 100,000. Prevalence across East Asia ranges from about 4 to 16 per 100,000 inhabitants. The disease is far less common in people of European descent, and reliable epidemiologic data for Western populations remain scarce. The first European clinical guidelines were based largely on East Asian research.
Symptoms most commonly appear in two age windows: children between 5 and 10, and adults between 30 and 50. About 15% of cases run in families, and a specific genetic variant on the RNF213 gene is strongly linked to the disease. A founder mutation in this gene has been identified in 95% of familial cases and 73% of non-familial cases in East Asian populations. Carrying this variant increases the risk roughly 190-fold compared to the general population, though it also appears in about 1.4% of unaffected people, meaning additional triggers are likely involved.
Moyamoya Syndrome
When the same pattern of arterial narrowing develops alongside another medical condition, it’s called moyamoya syndrome rather than moyamoya disease. Conditions associated with moyamoya syndrome include Down syndrome, sickle cell disease, and neurofibromatosis type 1. The distinction matters because treating the underlying condition can sometimes influence how the vascular disease behaves.
Symptoms in Children vs. Adults
In children, the first sign is usually a transient ischemic attack, often called a mini-stroke. These episodes happen when blood flow to part of the brain is briefly interrupted, causing sudden weakness or paralysis on one side of the body. Symptoms typically resolve within a day, but they tend to recur. Children may also develop seizures, headaches, involuntary movements, vision problems, trouble speaking, or cognitive and developmental delays.
Adults experience many of the same symptoms, but with one critical difference: they are significantly more likely to suffer a hemorrhagic stroke, where one of the fragile collateral vessels bursts and bleeds into the brain. This type of stroke can be sudden and severe. Adults may also have transient ischemic attacks, headaches, and difficulty with speech or understanding language.
Cognitive Effects of Reduced Blood Flow
Even without a full stroke, the chronic shortage of blood flowing to the brain causes measurable cognitive problems. Research comparing moyamoya patients who had never experienced a stroke to healthy controls found significant deficits across multiple areas: processing speed, executive function (the ability to plan, organize, and switch between tasks), visuospatial skills, and verbal memory.
These cognitive changes don’t appear suddenly. They develop gradually as blood flow to key brain regions decreases over time. The areas most affected include the frontal lobes, the temporal lobes, and the deep white matter tracts that connect different parts of the brain. Older age, lower education levels, and existing white matter damage all raise the risk of cognitive decline in moyamoya patients. This means the disease can quietly erode mental sharpness long before a dramatic event like a stroke draws attention to it.
How Moyamoya Is Diagnosed
A definitive diagnosis requires seeing the characteristic narrowing on imaging. Digital subtraction angiography, a type of X-ray that captures blood flowing through arteries in real time, remains the gold standard. It provides the sharpest view of both the narrowed arteries and the wispy collateral network that gives the disease its name.
To confirm moyamoya disease specifically, doctors need to see bilateral narrowing of the terminal internal carotid arteries and the nearby cerebral arteries, evidence of the moyamoya collateral vessels, and no signs that the narrowing is caused by inflammation or another systemic vascular condition. MRI and MR angiography are also used, particularly for screening or follow-up, but angiography offers the spatial resolution needed to stage the disease precisely and plan treatment.
Surgical Treatment
There is no medication that can stop or reverse the arterial narrowing. The primary treatment is surgery to restore blood flow to the brain, and several approaches exist.
Direct bypass is essentially plumbing: a surgeon connects a scalp artery directly to a brain artery, creating an immediate new route for blood. The most common version uses the superficial temporal artery, which runs along the side of the head, and connects it to a branch on the brain’s surface. The advantage is that blood flow improves right away on the operating table.
Indirect bypass takes a different approach. Instead of stitching vessels together, the surgeon places blood-rich tissue from the scalp, the lining of the skull, or even the covering of a muscle directly onto the brain’s surface. Over the following months, new blood vessels grow from this tissue into the brain, a process called angiogenesis. The most commonly used indirect technique is encephaloduroarteriosynangiosis, where the superficial temporal artery is laid along the brain’s surface with its surrounding tissue intact. The tradeoff is that it takes several months to a year before blood flow meaningfully improves.
Many surgeons use a combined approach, performing a direct bypass for immediate blood flow while also placing tissue to encourage long-term vessel growth. The choice depends on the patient’s age, the stage of the disease, and the anatomy of their vessels. Indirect techniques are often preferred in young children, whose small arteries make direct connections technically difficult.
Long-Term Outlook After Surgery
Revascularization surgery substantially lowers the risk of future strokes. In studies tracking patients after successful surgery, the annual stroke rate beyond the first 30 postoperative days dropped to 1.2%. Without surgery, the disease tends to progress, with worsening narrowing and increasing risk of both ischemic and hemorrhagic strokes over time.
Outcomes are generally better when the disease is caught and treated earlier, before significant strokes have occurred or cognitive decline has set in. Because the disease is progressive and has no medical cure, patients who have surgery still need long-term follow-up with regular imaging to monitor for progression on the treated side or new narrowing on the other side. Children who are treated early often do well developmentally, though some may have lasting effects from strokes or chronic low blood flow that occurred before diagnosis.