Prevalence of multiple sclerosis in rural and urban districts in Telemark county, Norway

Objective: To explore the trends in prevalence and incidence of multiple sclerosis (MS) in Telemark, Norway (latitude 58.7-60.3˚N), over the past two decades, with focus on differences between rural and urban areas. Methods: Data from all patients with a confirmed diagnosis of MS in Telemark since 1993 were prospectively recorded and collected in a retrospective chart review. Prevalence estimates on January 1st 1999, 2009 and 2019, and incidence rates at five-year intervals between 1999 and 2018 were calculated and all results were adjusted to the European Standard Population. The study population was divided into urban and rural residency using a Norwegian governmental index. Results: We registered 579 patients with MS in Telemark between 1999 and 2019. The adjusted prevalence estimates for January 1st 1999, 2009 and 2019 were 105.8/105, 177.1/105 and 260.6/105, respectively. In 2019, the prevalence estimates were 250.4/105 in urban and 316.2 /105 in rural areas. Between 1999 and 2018, the yearly incidence increased from 8.4/105 to 14.4/105. Conclusions: The prevalence of MS in Telemark is among the highest ever reported in Norway, consistent with an increasing incidence in the county over the past twenty years. The even higher prevalence in the rural areas is unlikely to be explained by possible risk factors like latitude, exposure to sunlight and diet. Further studies on differences between urban and rural areas are required to reveal possible new risk factors