A Historical Overview of Epidemiological Studies on Multiple Sclerosis in Japan

ABSTRACT Multiple sclerosis (MS) was long considered rare in Japan until the 1950s, but epidemiological evidence has since accumulated. This review outlines the development of MS epidemiology in Japan and summarizes nationwide and regional trends in prevalence, clinical and laboratory features, genetic and environmental risk factors, and disease burden. Across five nationwide surveys (1972–2018), the estimated number of patients and crude prevalence have increased substantially, although prevalence remains lower than that in Western countries. Distinct features have been reported in Japanese MS, including relatively mild disability and a low oligoclonal IgG band positivity rate. Although many genetic and environmental risk factors are shared with MS in Western countries, HLA‐DRB1*04:05 , which is most frequent in Japanese patients with MS but rare in Western populations, may partly explain phenotypic differences in a subset of patients. Nationwide surveys and analyses of health insurance claims databases suggest improving disease severity and declining hospitalization rates in recent years, likely reflecting broader availability and increased use of disease‐modifying drugs. Despite mild disability and improving disease severity, the burden of MS remains substantial, with cognitive impairment, reduced quality of life, work productivity loss, and increasing healthcare costs. Further understanding of population‐specific factors and features is needed to translate epidemiologic insights into better symptom control and outcomes. Prospective longitudinal cohort studies that enroll a larger number of patients and distinguish MS, neuromyelitis optica spectrum disorder, and myelin oligodendrocyte glycoprotein antibody‐associated disease are warranted.