Moyamoya disease is a rare condition in which the major arteries at the base of the brain slowly narrow and can eventually close off, reducing blood flow to the brain. The body tries to compensate by growing a tangle of tiny replacement blood vessels, which on an angiogram look like a “puff of smoke,” the literal translation of “moyamoya” in Japanese. These fragile collateral vessels are not adequate substitutes for normal arteries, leaving people at significant risk of stroke.
What Happens Inside the Brain
The disease targets the internal carotid arteries, the two large vessels that carry blood up through the neck and into the brain. Over time, the walls of these arteries thicken and the passageway for blood narrows. This process typically affects both sides of the brain, though it can start on one side before progressing to the other.
As these main arteries narrow, the brain becomes starved for blood. In response, a network of tiny, abnormal vessels sprouts to reroute blood flow around the blockage. These collateral vessels are small and delicate. They can’t deliver the same volume of blood as a healthy carotid artery, especially during moments of high demand, like exercise, crying, or hyperventilation. And because their walls are thin, they’re prone to rupturing and bleeding.
Who Gets Moyamoya
Moyamoya is most common in Japan, South Korea, and other East Asian countries. In Japan, the standardized prevalence has risen to roughly 17.6 per 100,000 people, with an incidence of about 2.4 per 100,000. In Western populations, the disease is considerably rarer, though it’s increasingly recognized worldwide as awareness and imaging technology improve.
The condition has two peak ages of onset: one in children around ages 5 to 10, and another in adults in their 30s and 40s. Women are affected more often than men. A genetic component plays a clear role. Variants in a gene called RNF213 are a major susceptibility factor, particularly in people of East Asian descent. This gene appears to help regulate the integrity of blood vessel walls in the brain, and when it malfunctions, it may set the stage for the abnormal narrowing that defines the disease. However, moyamoya likely requires a combination of genetic predisposition and other triggering factors to develop, not a single gene alone.
When the same pattern of arterial narrowing occurs alongside another condition, such as sickle cell disease, Down syndrome, or radiation therapy to the head, it’s called moyamoya syndrome rather than moyamoya disease. The distinction matters for treatment planning, but the brain changes look the same on imaging.
Symptoms in Children vs. Adults
Moyamoya often announces itself differently depending on age. In children, the first sign is usually a stroke or a transient ischemic attack (TIA), sometimes called a “mini-stroke.” A child might suddenly develop weakness on one side of the body, slurred speech, or difficulty walking. These episodes can be triggered by crying, blowing on hot food, or playing a wind instrument, all of which cause rapid breathing that temporarily reduces blood flow to an already compromised brain. Some children experience seizures, headaches, or a gradual decline in cognitive function before a stroke occurs.
Adults experience strokes and TIAs too, but they also face a second threat: hemorrhagic stroke, or bleeding in the brain. This happens when those fragile collateral vessels burst under pressure. A sudden, severe headache, loss of consciousness, or new neurological symptoms in someone with moyamoya should be treated as a medical emergency.
How Moyamoya Is Diagnosed
Diagnosis typically begins with brain imaging after a stroke or TIA. An MRI can reveal evidence of reduced blood flow or prior strokes, and a specialized type of MRI called magnetic resonance angiography (MRA) can show the narrowed arteries and the characteristic tangle of small vessels. Conventional angiography, where dye is injected into the blood vessels and X-rays are taken, remains the gold standard for confirming the diagnosis and assessing severity.
Doctors use a six-stage classification system developed by Suzuki and Takaku to describe how far the disease has progressed. In stage 1, only the fork of the carotid artery is narrowed. By stages 2 and 3, the moyamoya vessels are forming and intensifying as the major brain arteries narrow further. In stages 4 and 5, the moyamoya vessels themselves begin to fade as the carotid arteries close off almost entirely. By stage 6, the internal carotid arteries have essentially disappeared, and whatever blood reaches the brain comes from external carotid artery branches. Understanding the stage helps guide treatment decisions.
Surgical Treatment Options
Surgery is the primary treatment for moyamoya disease. The goal is to restore blood flow to the brain by creating new pathways that bypass the blocked arteries. Without surgery, the recurrence rate for ischemic stroke is roughly 39%, a risk that makes watchful waiting a difficult choice for most patients.
There are three general approaches to revascularization surgery:
- Direct revascularization involves physically connecting a healthy artery from the scalp to an artery on the brain’s surface, creating an immediate new route for blood. This technique, first developed in the 1970s, has been shown to significantly reduce future stroke events compared to conservative treatment. It provides blood flow right away but requires a skilled microsurgeon and a recipient vessel large enough to connect to, which can be a limitation in young children.
- Indirect revascularization takes a different approach. Instead of directly connecting vessels, the surgeon places blood-rich tissue from the scalp or its lining onto the surface of the brain. Over weeks to months, new blood vessels naturally grow from this tissue into the brain. Because it doesn’t require stitching tiny vessels together, this method is often preferred for young children whose arteries are too small for a direct bypass.
- Combined revascularization uses both techniques in the same operation, providing the immediate blood flow of a direct bypass along with the long-term vessel growth of an indirect approach. Many surgical centers now favor this combined strategy.
Recovery from revascularization surgery typically involves a hospital stay of about a week, with close monitoring for stroke and careful management of blood pressure and hydration. The new blood supply from indirect procedures takes time to mature, so the full benefit may not be apparent for several months.
Long-Term Management
Surgery addresses blood flow, but most people with moyamoya need ongoing medical care. For patients whose disease presented with strokes or TIAs rather than bleeding, expert guidelines recommend long-term use of antiplatelet medication (blood thinners like aspirin) to reduce the risk of clot-related strokes. This recommendation is based on expert consensus, since large randomized trials in moyamoya are difficult to conduct given how rare the disease is.
Regular follow-up imaging is important to track whether the disease is progressing, whether the surgical bypass is functioning, and whether new collateral vessels are developing. Many patients undergo MRI and MRA annually or every few years. Children in particular need monitoring for cognitive effects, since even small strokes or chronic low blood flow can affect learning, memory, and attention over time.
With successful surgery and careful follow-up, many people with moyamoya live full, active lives. The disease doesn’t go away, and the underlying tendency for arterial narrowing persists, but revascularization can dramatically lower the risk of the strokes that make it dangerous. Early diagnosis makes a meaningful difference, catching the disease before a major stroke occurs gives surgery the best chance of preserving brain function long term.