Incidence of multiple sclerosis: a periodic or stable phenomenon.

For diseases of unknown aetiology, the question as to whether the incidence is constant or variable is very important. A study on multiple sclerosis in a defined northern area of the German Democratic Republic showed a prevalence of 68.6 and an incidence rate of 3.0. Retrospective and prospective investigations concerning an observation period of 22 years revealed cyclic periods (6-7 years) of high incidence rates (up to 4.5) interrupted by shorter intervals (4-5 years) with low rates (about 1.8). The differences (1963-1968 vs. 1969-1973, 1974-1978 vs. 1979-1983) are significant. In accordance with the findings of Kurtzke et al. on a cyclic outbreak of multiple sclerosis in the Faroes and Iceland, our results are considered to be a consequence of environmental factors, such as epidemic viral infections.