Epidemiological studies of multiple sclerosis suggest a trend of increasing disease prevalence in susceptible populations. The reasons for this are unclear and may be the results of methodological differences between studies, incomplete ascertainment or advances in technologies that allow the increased identification of early or mild disease. In addition, direct comparison of cross-sectional prevalence estimates performed in different epochs in ethnically and geographically distinct populations may be inappropriate.
Using detailed phenotypic information and standardised methodology we have resurveyed a geographically defined Welsh population after a significant interval, establishing contemporary prevalence rates and examining demographic and clinical data to determine causes of changing disease frequency.
Disease prevalence rose 45% from 101 to 146 per 100,000 population over 20 years. The greatest increase was observed in females between the ages of 45 and 54. No significant increase in disease frequency was observed in the male population overall, or within specific age groups. There was no demographic evidence for a pattern of earlier age at onset or diagnosis to explain increased disease frequency or decrease in mean age of the prevalent population. In addition we failed to identify a pattern of recognition of patients with less severe disability. Although there was a modest 13% increase of 2.2 years in mean disease duration, and 8 new previously prevalent patients were identified, the main cause of rising disease frequency was related to a 2.8 fold increase in disease incidence for females over 23 years from 2.65 to 7.30/100,000/year increasing sex ratio of incident patients from 1.8 to 4.3 F:M.
Recent change in disease incidence and prevalence in this population is likely to be the result of environmental factors that have been operative in the last few decades in females alone and infers avoidable risk factors. Modelling of current overall incidence suggests a further increase in prevalence to 260 per 100,000 population within the next 20-40 years. Further studies are needed in order to identify recent changes in sex specific environment and lifestyle that confer susceptibility.