A cluster-focus of multiple sclerosis at Henribourg, Saskatchewan

Prevalence of Multiple Sclerosis in Canada: A Systematic Review

Background:

Studies of the prevalence of multiple sclerosis (MS) in Canada have generally been isolated to specific regions. Given the importance of multiple sclerosis as a cause of disability in adults, a comprehensive review of Canadian MS prevalence examining current data, interregional variation, deficiencies in knowledge and frontiers for research is timely.

Methods:

A systematic review of all studies addressing the prevalence of MS in Canada or regions within Canada, published in English or French since 1985, was conducted. Studies were identified using MEDLINE, EMBASE and bibliographic review. Ten studies were evaluated for methodological rigour and a test of heterogeneity across studies was performed and a measure of consistency (I2) estimated.

Results:

Studies were generally of high quality. Nine were restricted to regions within Canada and one provided an estimated national prevalence based on self-reported cases. All reported a high prevalence (>50 per 100 000). Latitude and longitude gradients were not striking while assessment of heterogeneity confirmed that regional differences were unlikely to be the result of sampling variability.

Conclusions:

This review confirms Canada as a country of very high MS prevalence and it is the first study to demonstrate that variation in regional estimates represents true differences in prevalence within Canada. Avenues for future MS prevalence research, including adoption of a national MS registry, are proposed.

A cluster of multiple sclerosis cases in Lysvik in the Swedish county of Värmland

OBJECTIVES

When surveying the county of Värmland in Sweden in order to determine the prevalence of multiple sclerosis (MS), we observed an aggregation of MS cases originating from the parish of Lysvik in the local region called Fryksdalen. Our intention was to analyse this cluster thoroughly, confirming the MS diagnosis and seeing if a hereditary or environmental background was plausible.

METHODS

The medical files were studied and the cases were classified by a neurologist according to Poser's criteria. Hereditary factors were analysed.

RESULTS

Sixteen living cases of MS were found, either living in the parish (n = 6) or born or raised there and had later moved to another place (n = 10). All patients had clinically definite MS. Eleven patients had relatives with MS, all of these being descendants of the Suhoinen family. Another two cases were Suhoinen descendants who did not have relatives with MS. Other common ancestors were also identified. Two cases were adopted. Eleven deceased MS patients from Lysvik were found, 10 of them had Suhoinen ancestry.

CONCLUSION

We report a cluster of MS cases with a common ancestry indicating heredity for MS in 85% of the cases. Lysvik is a parish where Finnish immigration was pronounced in the 17th century and there has been inbreeding to a certain extent through marriage between cousins. Thus, we interpret this aggregation as possibly being genetically based, and neurogenetic studies are now being performed. However, as two of the cases were adopted environmental factors must also be considered.

Responding to reported clusters of common diseases: the case of multiple sclerosis

Reports of disease clustering are becoming ever more common, and there is increasing pressure on public health agencies to respond rapidly and appropriately to these reports. We investigated a cluster of five cases of MS occurring in a small multidisciplinary hospital department. Data were collected by a variety of methods, including measurement and description of the workplace, completion of survey instruments by department staff, and construction of case histories of persons with MS. The results indicated that the department comprised a high-risk population and that only one case of MS could have any possible etiologic significance. Investigators should consider a number of factors when evaluating disease clusters, including the accuracy of diagnosis, the induction period and cause of the disease, and possible biases in the population at risk. Additionally, boundaries should not encircle the cases that led to identification of the cluster and should reflect environmental significance.

The epidemiology of multiple sclerosis

Although the cause of multiple sclerosis (MS) is unknown, epidemiologic studies support both genetic and environmental components of susceptibility. Reports of clusters, small "epidemics," geographic variation in prevalence, and alteration of MS susceptibility by migration support an environmental factor (or factors). The higher risk for MS in Europeans and in relatives of patients and the existence of MS-resistant ethnic groups support genetic predisposition. Findings from epidemiologic studies are most consistent with an environmental influence occurring on a background of genetic susceptibility as the cause of MS. Currently, there is little evidence for a single or unique environmental cause of MS.

Epidemiology of multiple sclerosis

The inhomogeneous distribution of MS may represent an important clue about the cause of the disease. The prevalence of MS appears to be increasing in many regions of the world, although it is likely that in a majority of regions surveyed, this increase is attributable to differences in ascertainment over the relatively short period that MS has been surveyed worldwide, which is almost exclusively the last 50 years. MS occurs most frequently in regions populated by northern Europeans, and in these areas, it is much more common in the northern European population than in the indigenous populations. MS occurs in relatives of MS patients at a rate 10- to 50-fold greater than in the general population. On average, the absolute risk to a first-degree relative of an MS patient is 2% to 5%, depending on the exact degree of relatedness. Although the mode of transmission is not certain, a multigenic pattern best fits the observed pattern of transmission. A majority of monozygotic twins are discordant for MS, indicating that a major component of MS susceptibility is environmentally determined, although recent observations in adopted relatives suggest that this risk is not transmissible. Rare epidemics of MS involving small numbers of individuals in geographically isolated regions have been reported. A number of migrant studies suggest that the risk of acquiring MS may be altered by migration, although some also support genetic factors. Analytic approaches to address the role of various risk factors include ecologic studies, case-control studies, and cohort studies. Ecologic studies are best suited for addressing risk factors that influence overall population risk, whereas case-control studies are best suited for addressing risk factors that pertain to individuals within a population. If a risk factor is distributed homogeneously in a given population, a case-control study is insensitive to its effect. Improved methodology to diminish the biases inherent in case-control studies and identification of other important risk factors from basic scientific studies undoubtedly will be important for analytic epidemiologic studies of the future. Furthermore, analysis of highly informative populations, such as discordant identical twins and adoptive siblings of MS patients, likely will improve the specificity of case-control studies by minimizing the vast number of potential differences between cases and controls.

The role of mast cells and diet in the onset and maintenance of multiple sclerosis: a hypothesis

Mast cells may invade the brain as a consequence of a childhood infection or predisposition, and it is proposed that multiple sclerosis arises due to the effect of various mediators (histamine and protease) released from the perivascular mast cells after stimulation by some diet factor.

Familial clustering of multiple sclerosis in a northern Swedish rural district

A small rural district in the most northerly province of Sweden was found to have a very high occurrence of multiple sclerosis. A total of 12 patients with multiple sclerosis among 4744 inhabitants were identified (five females, seven males), corresponding to a prevalence of 253/100,000. Many of the patients were related and a further 21 cases with multiple sclerosis (14 females, seven males), mostly living in the neighbouring area, have family ties to the district. A genealogical investigation showed that 22 of the 33 patients identified had ties of kinship and thus, to our knowledge, the largest aggregation of multiple sclerosis in a family is presented.