Multiple sclerosis in Finland: incidence trends and differences in relapsing remitting and primary progressive disease courses

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Abstract
Objective: To compare the secular trends and geographical differences in the incidence of relapsing-remitting (RRMS) and primary progressive multiple sclerosis (PPMS) in Finland, and to draw inferences about aetiological differences between the two forms of the disease. Methods: New multiple sclerosis cases in southern Uusimaa and the western districts Vaasa and Seinäjoki of Finland in 1979–1993 were verified from hospital records and classified into RRMS and PPMS. Patients met the Poser criteria for definite multiple sclerosis or otherwise satisfied the criteria for PPMS. Disease course was categorised by the same neurologist. Crude and age adjusted incidence in 1979–1993 was estimated. Results: During 1979–1993 the age adjusted incidence was 5.1 per 100 000 person-years in Uusimaa, 5.2 in Vaasa, and 11.6 in Seinäjoki. The rates in Uusimaa remained stable, while a decrease occurred in Vaasa and an increase in Seinäjoki. Between 1979–86 and 1987–93 the incidence of PPMS increased in Seinäjoki from 2.6 to 3.7 per 105 and decreased in Vaasa from 1.9 to 0.2 per 105; the trends were similar for RRMS. Conclusions: There are significant differences in secular trends for multiple sclerosis incidence in Finland by geographical area, but these are similar for PPMS and RRMS. The recent changes point to locally acting environmental factors. The parallel incidence trends for RRMS and PPMS suggest similar environmental triggers for the two clinical presentations of multiple sclerosis.