Epidemiology of multiple sclerosis in the county of Vestfold, eastern Norway: incidence and prevalence calculations.

The county of Vestfold in the South-eastern part of Norway has undergone two incidence and prevalence surveys on multiple sclerosis. The prevalence of definite/probable MS on January 1, 1963 was 61.6/100,000. Based on the same diagnostic criteria, the present study reports a slight increase in prevalence to 86.4/100,000 on January 1st 1983. The average annual incidence was calculated for 5 years periods from 1953 to 1983. The time periods 1953-1962 and 1973-1977 showed age-adjusted incidence rates between 4.50 and 5.49/100,000 while the 10-year period 1963-1972 showed significantly lower rates. The fluctuating pattern of MS incidence and prevalence over time supports the view that MS is not a stable disease, and that exogenous factors are influencing the disease pattern.