**3. Epidemiology**

For a very long time, Moyamoya disease was thought to be a disease of Asian lineage, but now it has been observed to be prevalent across the world in people with many ethnic backgrounds. MMD has been most extensively studied in Japan, where it is the most common pediatric cerebrovascular disease [8]. It shows a prominent East–West gradient, with a in East Asian countries ten times higher than the Western countries [9]. MMD is most frequently seen in Japan, with an incidence of 0.35–1.13/1,00,000/year and a prevalence of 3.16–10.5/1,00,000 [10]. In a study done in Hokkaido, Japan, 267 new cases were diagnosed between 2002–2006 [8]. The incidence and prevalence were also found to be high in other Asian countries like Korea, China and Taiwan [11]. The incidence in all these countries is found to be increasing over the years, most likely due to advancements in diagnostic modalities and a better understanding of the genetic factors linked to the disease [12]. Studies from outside Asia are very few. The incidence in Washington state and California was 0.086/1,00,000, but in them the incidence in Asian Americans was 4.6 times that of White [9]. In Europe, the incidence of MMD was 1/10th of that in Japan. North America's incidence was as low as 0.09/1,00,000 individuals, although an increasing trend is now being noted [13].

A similar bimodal age distribution is seen across the world, with the first peak occurring at 5–14 years in the pediatric population and around 4th decade in

adulthood [5]. In Japan, family history is present in 10–15% of cases, and the risk of the disease in a family member is about 30–40 times higher than the general population. A familial predisposition was less commonly seen in European countries. In most countries, the disease was more frequently seen in females, with male to female ratio ranging from 1:1.8 to 1:2.2 [10, 14]. These epidemiological parameters remained constant from East to West as evident in the literature review from across the world by Kim et al. [6].
