The epidemiology of optic neuritis in Finland.

Evidence has been presented that optic neuritis partially reflects benign cases of MS which are lost in the epidemiological investigation of the disease. As part of a large epidemiological investigation of MS, 221 patients with pure optic neuritis were identified during the period from January 1, 1967 to December 31, 1971. The mean annual incidence for the whole of Finland was 0.94 per 100,000 population. The female to male ratio was 1.7. The mean age at onset was 31.2 years. The distribution of optic neuritis by counties showed the highest mean annual incidence in the southwestern county of Turku and Pori (1.69) and in the western county of Vaasa (1.68). The prevalence data for MS were highest in these counties. A highly significant deviation from a random distribution according to place at onset and place of birth was obtained. Even the geographical distribution by smaller units, i.e. the combined clerical districts, revealed a firm accumulation to the western districts in the county of Vaasa and to the southwestern districts in the county of Turku and Pori. Thus, optic neuritis showed a similar geographical distribution of Jalasjärvi with several familial cases of MS did not increase the familial percentage when both conditions were considered as a single group. The risk of getting optic neuritis seems to depend on the influence of factors present during childhood. The epidemiological data point to a common factor in the aetiology of optic neuritis and MS.