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[Epidemiology of Multiple Sclerosis: Is Multiple Sclerosis on the Rise?]

Multiple sclerosis (MS) is a worldwide disease with an uneven geographic distribution. There has been a sharp increase in MS prevalence over time almost throughout the world, including Japan. The reasons for the increase in the prevalence of MS are unknown. However, evidence suggests that genetic and environmental factors and their interaction contribute to the etiology of MS. Therefore, the increase in prevalence can be attributed in part to a greater exposure to certain environmental risk factors in genetically susceptible individuals and also to increased survival rates and improved assessment. To clarify whether the increase in MS prevalence reflects a real increase in disease frequency, it is essential to assess temporal and geographical differences in MS incidence and to compair incidence in different ethnic populations. However, epidemiological data on incidence are limited, and there are marked geographical disparities in available data, most of which were obtained from studies in Europe and North America. In addition, there are marked variabilities in methodology, objectives, and study periods. Further epidemiological studies with appropriate standardization are needed to determine whether the risk of MS has changed over time.