| Summary: | Purpose and aims: Multiple sclerosis (MS) is an autoimmune disease that leads to damage in the central nervous system. Although the cause of MS is still unknown, the generally accepted view is that environmental and life-style factors influence the risk of MS in genetically predisposed individuals. The frequency of MS varies between countries. Countries with a more distant position from the equator have the highest reported incidence and prevalence rates. This association may be partially explained by lower exposure to sunlight and low vitamin D levels, two of the established risk factors for MS. According to previous studies the incidence and prevalence of MS appear to have increased since the middle of the past century. Knowledge of changes in disease epidemiology is important to dimension the need for health care resources, especially in light of new and expensive treatment alternatives, but also to identify risk factors of disease. We aimed to assess the incidence (Study I), prevalence (Study II), and mortality (Study IV) of MS in Iceland. In addition, we wanted to assess the influence of birth month in Sweden and Iceland on the risk of being diagnosed with MS later in life (Study III). Such an association has been noted in previous studies and has been hypothesized to be linked with low vitamin D levels during the winter season in pregnant women. Subjects and methods: Studies I, II and IV are population-based, nationwide studies on the epidemiology of MS in Iceland. Cases were identified by searching in multiple sources: administrative databases of both hospitals, private offices and difference government authorities such as the Directorate of Health, and the Social Insurance Administration. When applicable the search was based on diagnosis codes from: ICD10 (G35, G37.9), ICD9 (340, 341) and ICD8 (340, 341). Inclusion criteria for studies I and II were diagnosis of clinically definite MS or primary progressive MS according to the Poser diagnostic criteria. In addition, Study II included patients with ...
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