Moyamoya disease is rare everywhere, but how rare depends heavily on where you live. In East Asian countries like Japan and South Korea, the incidence runs several times higher than in Western populations, where the disease affects fewer than one person per 100,000 each year. South Korea, for example, has seen incidence climb to roughly 4 per 100,000 annually, while a large recent U.S. study put the figure at about 0.75 per 100,000. That gap is not just a curiosity of medical geography; it traces back to specific genetic variants concentrated in East Asian populations, and it shapes how quickly the disease gets recognized and treated.
East Asian Incidence Stands Apart
Japan and South Korea report the highest rates of moyamoya disease in the world, and the numbers have been rising steadily. In South Korea, the standardized incidence climbed from about 2.7 per 100,000 in 2005 to 4.3 per 100,000 in 2013, while the overall prevalence jumped from 6.5 to 18.1 per 100,000 in the same period.1PubMed Central. Epidemiology of Moyamoya Disease in Korea: Based on National Health Insurance Service Data Japan reports similar or slightly higher figures, consistently placing both countries at the top of global rankings.
China tells a more nuanced story. A large nationwide retrospective study calculated the incidence in mainland China at about 0.59 per 100,000 person-years and the prevalence at roughly 1.01 per 100,000. That makes the disease more common in China than in the United States or Europe, but considerably less common than in Japan or South Korea.2BMJ Open. Incidence and prevalence of moyamoya disease in urban China: a nationwide retrospective cohort study The gap within East Asia itself is substantial: Korean and Japanese rates may be four to seven times higher than Chinese rates, depending on the year and the study.3PubMed Central. Moyamoya Disease: Epidemiology, Clinical Features, and Diagnosis This intra-regional difference matters because it undermines the simple narrative that “East Asians get moyamoya.” The reality is more specific than that.
Incidence in the United States and Europe
Western populations have historically been considered low-incidence, and that remains broadly true. A recent large U.S. study calculated the overall age- and sex-standardized incidence of moyamoya angiopathy at 0.75 per 100,000 per year. That number includes both isolated moyamoya disease, at about 0.57 per 100,000, and moyamoya syndrome (cases triggered by an identifiable underlying condition), at about 0.18 per 100,000.4PubMed. Demographic Disparities in the Incidence of Moyamoya Angiopathy in the United States Earlier U.S. data from hospital discharge records in California and Washington state during 1987–1998 put the incidence lower, at about 0.086 per 100,000, suggesting either a genuine increase in disease frequency over the decades or, more likely, substantially better detection.5Neurology / PubMed Central. Moyamoya disease in Washington State and California
European data is thinner but generally falls in the same range or slightly below U.S. figures. The disease affects people of all ethnic backgrounds in Europe and North America, though it disproportionately shows up in certain groups, as the next section addresses.
Ethnic Disparities Within Multiethnic Countries
Living in the West does not erase the ethnic gradient. The California and Washington state study found that Asian Americans were diagnosed at about 4.6 times the rate of white Americans. African Americans also had an elevated rate, roughly 2.2 times that of whites. Hispanic Americans, on the other hand, had a lower incidence than whites.5Neurology / PubMed Central. Moyamoya disease in Washington State and California The rate among U.S. Asian Americans was in the ballpark of what had been reported in Japan, suggesting that ancestry carries much of the risk regardless of where a person lives.
One study from a primarily white midwestern U.S. population evaluated 94 patients with moyamoya and found that about 85% were white and roughly 72% were female.6Stroke / AHA/ASA Journals. Moyamoya disease in a primarily white, midwestern US population: increased prevalence of autoimmune disease That confirms the disease does occur in white populations, just at lower rates. Clinicians in Western countries who only associate moyamoya with East Asian patients can miss it entirely in other groups, which feeds into the diagnostic delay problem discussed later in this article.
Why the Regional Gap Exists
The single biggest reason moyamoya clusters in East Asia is a genetic variant called p.R4810K in a gene named RNF213. This variant is overwhelmingly common in moyamoya patients of East Asian descent, and its strength of association with the disease is extraordinary. In Japanese patients, carrying the variant increased the odds of having moyamoya by a factor of roughly 339. In Korean patients, the odds ratio was about 136.7PubMed. Distribution of moyamoya disease susceptibility polymorphism p.R4810K in RNF213 in East and Southeast Asian populations In Chinese patients the association was present but much weaker, which tracks with China’s lower incidence compared to Japan and South Korea.8PLOS ONE. Molecular Analysis of RNF213 Gene for Moyamoya Disease in the Chinese Han Population
A large case-control analysis across East Asian populations confirmed this pattern, reporting an overall odds ratio of about 112 for the p.R4810K variant.9PLOS ONE. Identification of RNF213 as a Susceptibility Gene for Moyamoya Disease and Its Possible Role in Vascular Development These are among the strongest genetic associations reported for any complex disease, which is partly why moyamoya is so much more common in populations where the variant circulates at higher background frequencies.
The p.R4810K variant is vanishingly rare in people of European or African descent, which raises the question of what drives moyamoya in non-East Asian patients. Researchers have identified variants in a different gene, DIAPH1, in a subset of non-East Asian patients with sporadic moyamoya. In a discovery cohort, about 12.5% of cases carried damaging DIAPH1 variants, and additional rare variants turned up in a validation group that was roughly three-quarters European.10JAMA Neurology. DIAPH1 Variants in Non–East Asian Patients With Sporadic Moyamoya Disease This hints that moyamoya may have partially distinct genetic underpinnings across populations, rather than being one disease with one cause that simply varies in frequency.
Age Peaks and Sex Differences
Moyamoya tends to appear in two age windows. A study from Nanjing, China found a dual peak: one in children aged 5 to 9 and another in adults aged 35 to 39.11PubMed. Epidemiological and clinical features of Moyamoya disease in Nanjing, China This bimodal pattern has been replicated across multiple populations and is one of the disease’s distinctive epidemiological signatures. Childhood-onset and adult-onset moyamoya often present differently: children tend to show up with symptoms from blocked blood flow to the brain, while adults are more prone to bleeding events, though the pattern varies by region.
Women are affected more often than men in most cohorts. The female-to-male ratio typically runs about 2:1, and in some studies female patients make up over 70% of cases. One Brazilian review noted that in recent decades the male-to-female gap has narrowed or even reversed in some registries, though this shift may reflect changing diagnostic patterns rather than a real change in who gets the disease.12Radiologia Brasileira. Moyamoya disease and syndrome: a review
How Symptoms Differ by Population
The disease does not present the same way everywhere. East Asian patients with moyamoya appear more prone to intracerebral hemorrhage, which is a bleed inside the brain and often a more immediately dangerous event. In contrast, a study looking at Southeast Asian and European Caucasian patients found that ischemic strokes (caused by blocked blood flow rather than bleeding) dominated. Among Southeast Asian patients in that cohort, about 71% presented with ischemic strokes, and among European Caucasians the figure was 82%.13PubMed Central. Moyamoya disease in Southeast Asians: genetic and autopsy data, new cases, systematic review, and meta-analysis of all patients from the literature This lines up with a broader observation that hemorrhagic presentations are particularly characteristic of Japanese and Korean moyamoya and less common in Western patients.
The practical consequence is that moyamoya gets misread in populations where doctors are not expecting it, or where the typical presentation differs from textbook descriptions drawn largely from East Asian data. A patient presenting with ischemic stroke in Europe may not immediately trigger the suspicion of moyamoya the way a hemorrhagic stroke in a young Japanese patient would.
Rising Rates or Better Diagnosis
Across virtually every country that tracks moyamoya, reported incidence and prevalence have gone up over recent decades. South Korea saw its prevalence nearly triple in under a decade.1PubMed Central. Epidemiology of Moyamoya Disease in Korea: Based on National Health Insurance Service Data The U.S. incidence measured in the 2020s is several times higher than what was calculated from 1980s and 1990s hospital data. Does that mean the disease itself is becoming more common?
Probably not, or at least not entirely. Modern noninvasive brain imaging, particularly magnetic resonance angiography (MRA), has made it far easier to spot the characteristic narrowing of internal carotid arteries without putting patients through catheter-based angiograms. That means more mild or asymptomatic cases get caught incidentally. Improved survival after surgical treatment also inflates prevalence by keeping diagnosed patients alive longer.12Radiologia Brasileira. Moyamoya disease and syndrome: a review A genuine increase in true incidence is harder to prove or disprove, but the weight of opinion leans toward diagnostic improvement as the main driver.
Even asymptomatic moyamoya carries real risk. A Japanese multicenter survey found that among patients with moyamoya who had no symptoms at diagnosis and were managed without surgery, the annual risk of any stroke was about 3.2%. Disease progression on imaging was associated with developing ischemic events or silent infarctions.14PubMed. Radiological findings, clinical course, and outcome in asymptomatic moyamoya disease: results of multicenter survey in Japan That statistic matters because it shows that finding the disease incidentally is not the same as finding something harmless.
Misdiagnosis and Diagnostic Delay
Outside of East Asia, the biggest practical problem with moyamoya’s rarity is that doctors often do not think of it. A large study of 192 Caucasian patients eventually diagnosed with moyamoya at a single European center found that 62% had initially received a wrong diagnosis. The average time from symptom onset to correct diagnosis was more than five years, with a median of three years and some patients waiting as long as 26 years.15PubMed. Misdiagnoses and delay of diagnoses in Moyamoya angiopathy-a large Caucasian case series
That is a striking number for a progressive vascular disease. Moyamoya does not spontaneously improve; the arterial narrowing tends to worsen over time. Each year of delayed diagnosis is a year during which the patient risks stroke or cognitive decline without access to the revascularization surgery that is the standard treatment. The study was the first systematic report documenting this delay, and the authors attributed it directly to low clinician awareness. In Japan or South Korea, where the disease is far more common and every neurologist has seen cases, this kind of delay is much less likely.
Moyamoya Syndrome and Associated Conditions
When moyamoya-pattern arterial changes occur alongside another recognized condition, clinicians call it moyamoya syndrome rather than moyamoya disease. The distinction matters for epidemiology because some underlying conditions substantially increase the risk. Down syndrome is one of the best-documented associations: moyamoya is roughly three times more common in people with Down syndrome than in the general population.16PubMed Central. A Better Understanding of Moyamoya in Trisomy 21: A Systematic Review Other conditions linked to moyamoya syndrome include sickle cell disease, neurofibromatosis type 1, thyroid disorders, and prior cranial radiation therapy.
In the U.S. data, moyamoya syndrome accounted for roughly a quarter of all moyamoya angiopathy cases, with an incidence of about 0.18 per 100,000.4PubMed. Demographic Disparities in the Incidence of Moyamoya Angiopathy in the United States Whether moyamoya disease and moyamoya syndrome share identical vascular mechanisms or simply look alike on imaging is still debated, but the treatment approach, surgical revascularization, tends to be similar in both.
Familial Cases and Inheritance
About 10% of moyamoya cases occur in families rather than appearing sporadically.17PubMed Central. Mapping of a familial moyamoya disease gene to chromosome 3p24.2-p26 The familial form is particularly common in Japan, where multigenerational families have been documented in enough detail to study how the disease passes down. One analysis of Japanese families with three or more affected generations found evidence for autosomal dominant inheritance with incomplete penetrance, meaning a single copy of a susceptibility gene can cause disease but does not always do so. Among 135 offspring of affected parents, about 44% were either patients or obligatory carriers. The study also noted a possible effect of parent-of-origin: affected mothers were more likely to have daughters who developed the disease later in life or remained asymptomatic.18PubMed Central. Inheritance pattern of familial moyamoya disease: autosomal dominant mode and genomic imprinting
For families where one member is diagnosed, the question of whether other family members should be screened is not standardized across guidelines but comes up frequently in clinical practice, especially in East Asian countries where familial clustering is more common. The incomplete penetrance complicates screening: a family member might carry the same genetic variant yet never develop symptomatic disease, making it hard to know how aggressively to monitor them.
When Screening and Awareness Actually Matter
Moyamoya sits in an awkward zone for rare diseases. It is common enough in East Asia that pediatric neurologists there encounter it regularly, and public awareness exists. It is rare enough in Europe and the Americas that the average emergency physician may never see a case, leading to the years-long diagnostic delays documented in the literature. The mismatch between where the disease is well understood and where it also occurs, just less frequently, is the core practical problem.
If you or a family member has had unexplained strokes or transient ischemic attacks, especially at a young age or without conventional risk factors like high blood pressure or atrial fibrillation, moyamoya belongs on the differential list regardless of ethnicity. The disease does not require East Asian ancestry to appear, and the non-East Asian genetic contributors identified so far probably account for only a fraction of cases in Western populations. Much of the genetic architecture in non-Asian moyamoya remains unmapped, which means clinicians cannot rely on genetic testing to rule it out. Brain imaging, specifically angiography showing the characteristic bilateral narrowing of the internal carotid arteries and the wispy collateral vessels that give the disease its Japanese name (“a puff of smoke”), remains the diagnostic cornerstone.