High nationwide prevalence of multiple sclerosis in Sweden

BACKGROUND: Few nationwide multiple sclerosis (MS) prevalence studies have been published. In Scandinavia, the nationwide MS prevalence was 173/100,000 in Denmark 2005 and 100/100,000 in Iceland 1990. OBJECTIVE: Our aim with the present study was to determine the first population-based nationwide MS prevalence in Sweden, based on observed, registered patients and to investigate the presence of a north-south gradient of MS prevalence. METHODS: By linking the Swedish National Patient Register, the Swedish Multiple Sclerosis Registry and the Swedish Total Population Register we obtained the number of patients who were diagnosed with MS before 2009, and who were registered, alive and resident in Sweden on the prevalence date 31 December 2008. We calculated the gender-specific nationwide MS prevalence in 1-year age intervals. The relationship between MS risk and latitude was studied in a logistic regression model including all individuals in the population of Sweden. RESULTS: The number of registered MS patients in 2008 was 17,485 out of the Swedish population of 9,256,347. The overall MS prevalence was 188.9/100,000 (95% CI 186.1-191.7), 113.4 (95% CI 110.3-116.5) for men and 263.6 (95% CI 258.9-268.3) for women. The female to male ratio was 2.35:1. The prevalence of MS significantly increased for each degree of north latitude with 1.5% in men (p = 0.013) and 1% in women (p = 0.015). CONCLUSIONS: The MS prevalence of 188.9/100,000 in Sweden is among the highest nationwide prevalence estimates in the world. In Sweden, the risk of MS increases with increasing north latitude for both men and women.