Studies on the clustering of multiple sclerosis in Finland.

The epidemiological investigation of multiple sclerosis (MS) in Finland revealed a prevalence rate of 40.3 per 100.000 inhabitants. The geographic distribution of MS was uneven in the country. MS seemed to occur more often in the western and southwestern parts of the country, where the prevalences varied between 51.7 and 62.1 This phenomenon became even more pronounced when the distribution of the disease was investigated in small geographic units. Optic neuritis showed a geographic distribution similar to that of MS, with clustering in the same parts of the country and even in the same narrow districts. The correlation between the birthplaces of MS and optic neuritis patients was very significant (p less than 0.001), which suggest that they share a common aetiology. The possible aetiological role of genetic and exogenous factors was discussed in the western cluster of MS, where the percentage of familial cases was 13 and an accumulation of muscular dystrophy of cattle was observed.