Prevalence And Incidence Studies

Incidence and prevalence rates continue to be reported from all parts of the world, but one is cautioned by the words of Rosati (1). "The comparison of prevalence studies carried out in different areas and times is made difficult by the variability in surveyed population sizes, age structures, ethnic origins and composition, and the difficult quantification of numerators, especially regarding the recognition of benign and very early cases. Additionally, complete case ascertainment depends on access to medical care, local medical expertise, numbers of neurologists, accessibility and availability of new diagnostic procedures, the degree of public awareness about MS, and the investigators' zeal and resources.''

A summary of studies published recently is provided in Table 1. These studies have wide variation in methodology, but two conclusions can be drawn from the aggregate. First, although a general latitude gradient may still be perceived (Fig. 3) there are important differences in the rates reported at similar latitudes, possibly explicable in terms of racial or ethnic differences; and second, the incidence and prevalence rates reported have increased whenever a study was repeated.

It is regrettable that any comparison of prevalence between published regional studies has limited validity due to differences in age distribution, ethnic composition, and case ascertainment, as well as to changes in prevalence over time. Recently presented data have cast doubt upon the reliability of all the estimates published to date. The regional distribution of MS (Beck C, personal communication) showed prevalence rates between 180 and 350/105 in five transnational geographic regions in a population health survey of 131,535 Canadians in 2000/2001. The overall weighted estimate of MS prevalence in Canada was 240/105. Regional weighted prevalences ranged from 180 in Quebec to 350/105 in the Atlantic provinces. The odds of having MS in the Prairies and Atlantic regions were significantly elevated when compared with other regions. While the results were based upon self-report in a random telephone interview and not clinically confirmed, figures from the Alberta healthcare agency supported these estimates, doubling or tripling the prevalence rates reported hitherto in Canada. If this methodology is sound, the conclusions drawn from all previous studies must be questioned, as they would have been based upon findings in a limited sample.

Variation with Latitude

The correlation of prevalence with latitude is often quoted and still holds in the presence of updated prevalence studies (74). In Australia, the strong correlation with latitude (the disease becoming increasingly prevalent with increasing southern

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Table 1 Prevalence (or Incidence) Rates of Multiple Sclerosis Reported in Recent Years

Year

Place

Latitude (approx.)

Prevalence/10s

References

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