SYMPOSIUM ON MULTIPLE SCLEROSIS—Part IIIThe Epidemiology of Multiple Sclerosis
Section snippets
RACE
Of the three major human races (Caucasians, Blacks, and Orientals), the prevalence of MS in Caucasians of North America and Europe is more than an order of magnitude greater than that in Blacks of Africa and Orientals of Asia. Among Caucasians, certain ethnic populations are apparently relatively vulnerable to MS, whereas others are relatively resistant. Northern Europeans, especially Scandinavians, and their descendants are the most susceptible;13 accordingly, the prevalence is high in Sweden,
GEOGRAPHY
The incidence, prevalence, and mortality rates of MS vary with latitude.25 MS is rare in tropical and subtropical areas. Within temperature zones, disease rates increase with increasing latitude in both the Northern Hemisphere and the Southern Hemisphere. A north-south prevalence gradient has been detected in Europe, the United States, Japan, Australia, and New Zealand. In recent European studies with better case ascertainment, the finding of a higher prevalence in southern Europe than
MIGRATION
Migration studies of MS inherently have many potential sources of error and bias.5 Ideally, the incidence and prevalence rates should be well established for both the place of origin and the destination. Differences in demographics, ethnicity, religion, socioeconomics, and health between the migrants and their country of origin are important confounders. Patients with MS may have disabilities or health requirements that preclude migration, and some countries do not allow persons with medical
CLUSTERS
Several published reports have described the occurrence of MS in an unexpectedly high frequency within defined areas, including Key West (Florida),39 Los Alamos County (New Mexico),40 Vaasa (Finland),41 Hordaland (Norway),42 and Colchester County (Nova Scotia).43 These clusters are generally interpreted as evidence for a common environmental etiologic factor for MS in the affected region. Statistical chance cannot be excluded as an explanation for these apparent clusters. In fact, most
EPIDEMICS
Epidemics of MS allegedly have occurred in Iceland and the Faroe Islands.55, 56 Both instances were alleged to be related to the stationing of military troops during World War II. Further analysis of the data from Iceland has cast doubt on the existence of that epidemic.57 The gradual increase in the incidence of MS there was likely related to better case ascertainment by the first resident neurologist and began a decade before World War II.
The Faroe Islands data are more convincing. Although
FAMILY STUDIES
As many as 20% of patients with MS have at least one affected relative.11 In comparison with the general population, the first-degree relatives of patients with MS have a 20-to 40-fold increase in risk of eventually having MS. The risk varies with the relationship to the patient and is highest for sisters. Observed risks for various relatives have been published in epidemiologic studies.9, 10
Because close relatives would be expected to share more potential environmental influences than other
CONCLUSION
Epidemiologic studies have contributed substantially to our understanding of the etiologic factors involved in MS. Most of the epidemiologic support for environmental factors in the cause of MS is derived from migration studies and reports of MS clusters and epidemics. Because of the difficulty in performing migration studies, relatively few have been conducted. These studies have suggested that the risk for MS changes after migration from an area with a different prevalence. One apparent
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