Moyamoya disease is extremely rare. It affects roughly 1 to 9 people per million worldwide, placing it firmly in the category of rare diseases recognized by international health authorities. But “rare” doesn’t mean the same thing everywhere. In East Asian countries, moyamoya is diagnosed far more frequently than in Europe or the United States, and rates have been climbing steadily over the past two decades.
Incidence Rates by Region
The geography of moyamoya disease is striking. South Korea has the highest documented rates, with annual incidence rising from 1.7 per 100,000 people in 2007 to 4.3 per 100,000 by 2013. Japan follows closely, with incidence climbing from 0.35 per 100,000 in 1994 to 0.94 per 100,000 by the mid-2000s. China’s nationwide hospital data from 2016 to 2018 puts the rate at about 1.14 per 100,000 per year.
Western countries see far fewer cases. In the United States, incidence sits around 0.09 per 100,000, or roughly 1 in every 1.1 million people. Denmark, the source of one of the better European population studies, recorded just 0.07 per 100,000 annually. To put this in perspective, a person in South Korea is roughly 50 times more likely to be diagnosed with moyamoya than someone in Denmark.
Japan has the most complete long-term data, and its prevalence (the total number of people living with the disease at any given time) is estimated between 1 in 30,000 and 1 in 9,500. That’s considerably higher than the global average, reflecting both higher incidence and better detection in a country where clinicians are more familiar with the disease.
Why Rates Are Rising
Across every country that tracks moyamoya, the numbers are going up. This likely reflects improved brain imaging technology rather than a true surge in the disease itself. Moyamoya is defined by a distinctive pattern on angiography, where major arteries at the base of the brain slowly narrow and a network of tiny collateral vessels develops to compensate. As MRI and magnetic resonance angiography have become more widely available, more cases are being caught, including milder ones that might have gone undetected in earlier decades.
Hospital admission data in the United States also shows an upward trend, though the disease remains exceptionally uncommon there compared to East Asia.
A Strong Genetic Connection
The dramatic difference between Asian and Western populations points to genetics. A specific variant in the RNF213 gene is the strongest known risk factor. In Japanese populations, this variant appears in roughly 2 to 5 percent of people with certain types of stroke, compared to about 1 to 2 percent of healthy controls. In European populations, the same variant is essentially absent, with a frequency of just 0.06 percent.
This gene variant doesn’t guarantee someone will develop moyamoya, but it significantly raises the odds. Its concentration in East Asian populations explains much of the geographic gap in incidence. Family members of moyamoya patients carry higher risk than the general population, and about 10 to 15 percent of cases in Japan involve more than one family member.
Who Gets It
Moyamoya has a distinctive pattern when it comes to age. Diagnoses cluster around two peaks: the first between ages 10 and 20, and the second between 35 and 50. The childhood peak tends to present with stroke-like episodes caused by reduced blood flow, while adults more often experience bleeding in the brain from the fragile collateral vessels that form as the disease progresses.
Women are affected more often than men, with most studies reporting a female-to-male ratio of roughly 1.8 to 2.2 to 1. The reasons for this aren’t fully understood, though hormonal factors have been proposed as a possibility.
Frequently Misdiagnosed
Being rare makes moyamoya easy to miss, especially outside East Asia where doctors encounter it less often. In a study of 192 Caucasian patients eventually diagnosed with moyamoya at a specialized center, 62 percent had initially received a wrong diagnosis. The most common misdiagnoses were inflammation of the brain’s blood vessels (31 percent of misdiagnosed cases) and vaguely defined stroke diagnoses (30 percent). A small number were told they had multiple sclerosis.
The delays were significant. While some patients were correctly diagnosed within a year, 49 out of the misdiagnosed group waited more than three years. The average delay was over five years, and the longest recorded gap between first symptoms and correct diagnosis was 26 years. These delays matter because moyamoya is progressive. The narrowing of the arteries continues over time, and surgical treatment to restore blood flow is most effective before serious strokes occur.
In East Asian countries, where moyamoya is more common and clinicians are trained to recognize it, diagnostic delays tend to be shorter. If you live in a Western country and have unexplained stroke symptoms, particularly at a young age, moyamoya is worth raising with a neurologist, even though the odds of having it are low.
Putting the Numbers in Context
Moyamoya is rare by any measure, but rarity is relative. In the United States, it is far less common than conditions like multiple sclerosis (which affects about 1 in 300 people) and less common than many other rare diseases. In South Korea and Japan, it is uncommon but well within the range that most neurologists will encounter during their careers.
The practical takeaway: if you’re of East Asian descent, particularly with a family history of stroke or moyamoya, your baseline risk is meaningfully higher than global averages suggest. If you’re of European descent, the disease is genuinely rare, but that rarity itself contributes to diagnostic delays when it does occur.