Increasing prevalence of multiple sclerosis in Finland Sumelahti M-L, Tienari PJ, Wikstro Èm J, Palo J, Hakama M. Increasing prevalence of multiple sclerosis in Finland. Acta Neurol Scand 2001: 103: 153±158. # Munksgaard 2001. Objectives ± To follow-up the prevalence trends of MS from 1983 to 1993 in western and southern Finland. MS epidemiology has been previously followed from 1964 to 1978 in these regions. The updated prevalences were correlated with incidence trends in the same period. Methods ± Age- adjusted and age-speci®c MS prevalence rates were calculated for cases classi®ed by Poser's criteria. Results ± In the western health-care districts, Seina Èjoki and Vaasa, prevalences in 1993 were 202/10 5 and 111/10 5 . In the southern district Uusimaa the respective ®gure was 108/10 5 . In Seina Èjoki a signi®cant 1.7-fold increase was found in 1993 as compared to 1983, mainly due to increased incidence. In Uusimaa a signi®cant 1.2- fold increase in prevalence was found in the presence of stable incidence. In Vaasa prevalence was stable, although incidence was declining. Conclusion ± The prevalence of MS is increasing in Seina Èjoki and Uusimaa but not in Vaasa. Both the prevalence and incidence in Seina Èjoki are now among the highest reported. M.-L. Sumelahti 1,2 , P. J. Tienari 3 , J. Wikstro Èm 3 , J. Palo 3 , M. Hakama 1 1 School of Public Health, University of Tampere; 2 Department of Clinical Neurophysiology, Tampere University Hospital; 3 Department of Neurology, University of Helsinki Key words: multiple sclerosis; prevalence Dr Marja-Liisa Sumelahti, School of Public Health, P.O. Box 607, FIN-33101 Tampere, Finland Tel.: 358 40 5874149 Fax: 358 03 2156057 e-mail: memasu@uta.® Accepted for publication October 24, 2000 One of the epidemiological hallmarks of multiple sclerosis (MS) is its uneven geographic distribution often classi®ed as areas of low, medium and high prevalence (1). Uneven geographic distribution, especially the areas of high prevalences (prevalence >30/10 5 ) in Scandinavia (2±6) and areas populated by Scandinavian ancestry in southern Canada (7) and northern parts of the US (8), may aid in the search for the aetiology of MS. The same is true in Scotland (9, 10) and British Isles (11±14). However, several causes, such as changes in incidence and survival (2, 9, 15) may affect prevalence, whereas signi®cantly changing incidence with valid diagnos- tics is likely to be a more clear indicator of locally affecting etiological factors (11±17), as was found in the Orkney islands (12). In Finland regional variation in MS occurrence from 1964 to 1979 was found between the western and southern parts of Finland (2, 18±20). A recent incidence study of de®nite cases from 1979 to 1993 (21) demonstrated the persisting gradient: incidence was 5.1/10 (5) in southern Uusimaa, 11.6 in western Seina Èjoki and 5.2 in the neighbouring Vaasa. This suggests that the overall regional differences in Finland are due to a high MS occurrence in Seina Èjoki, where also an exceptionally high familial clustering of the disease has been found earlier (20). We have here estimated the prevalence trends in western Vaasa and Seina È joki and southern Uusimaa regions and assessed the contribution of incidence changes on the prevalence trends. Material and methods Demography The genetic background of the Finnish population, 5.1 million in 1993, is homogenous (22). There is an increase of older age-groups and a decreasing birthrate in all parts of the country. The southern largely urban population of Uusimaa is expanding due to immigration from other parts of Finland while the western, rural populations of Vaasa and Seina È joki are stable in size and socio-economically similar. In 1979, 91% of the MS patients living in the western region were also born there. In 1993, the ®gure was 87% in Seina Èjoki illustrating the stability of these western populations. Acta Neurol Scand 2001: 103: 153±158 Printed in UK. All rights reserved Copyright # Munksgaard 2001 ACTA NEUROLOGICA SCANDINAVICA ISSN 0001-6314 153