Multiple sclerosis in the far north - incidence and prevalence in Nordland County, Norway, 1970–2010

BACKGROUND: The risk of multiple sclerosis (MS) increases with increasing latitude. Taking into consideration that Norway has a large latitude range, a south-to-north gradient would be expected. However, previous studies have reported an uneven distribution of the disease in Norway, with a relatively low prevalence in the most northern parts of the country. We describe the incidence and prevalence of MS in a county in the north of Norway over a period of 40 years. METHODS: All patients with MS living in Nordland County in the period 1970-2010 were identified by reviewing hospital charts. The patients were included if they met the criteria of definitive or probable MS according to Poser [Ann Neurol 13:227-231, 1983] or MS according to McDonalds [Ann Neurol 50:121-127, 2001]. Point prevalence at the beginning of the decades was calculated. The average annual incidence was calculated for 5-year periods. RESULTS: The total crude prevalence on January 1, 2010 was 182.4 per 100 000. The annual incidence continuously increased from 0.7 per 100 000 in 1970 - 1974 to 10.1 per 100,000 in 2005 - 2009. The time delay from the first symptom to diagnosis was stable from 1975 to 2010. The proportion of primary progressive MS in the prevalence numbers was 38.2% in 1980, and decreases continuously, to 18.6% in 2010. The female to male prevalence ratio has been stable since 1990 at 2.2 to 1. CONCLUSION: The prevalence and the incidence of MS have steadily increased over a 40 year period. Nordland County is a high-risk area for MS.