Distribution and Incidence

Multiple sclerosis affects principally individuals of northern European Caucasoid origin living in temper ate zones, though it does occur infrequently in the tropics and in other racial groups. Because the course of the disease is so prolonged and the manifestations are so variable, it is difficult to determine its true incidence, and prevalence rates are generally used to compare the frequency of the disease in different populations. Females are affected about twice as commonly as men. The average age of onset is 30 years old, and it rarely starts over the age of 60 or before puberty. In Caucasoid populations, approximately 10 percent of patients have an affected relative. The concordance rate in twins (i.e., the frequency with which both members of a twin pair have the disease) is about 30 percent for identical twins and 2 percent for nonidentical twins, the latter being similar to the frequency in siblings. The difference in concordance rate between identical and nonidentical twins provides strong evidence for the implication of a genetic factor in the etiology of the disease, but the rather low concordance rate in identical twins suggests that an environmental factor is also involved. Further evidence implicating both environmental and genetic factors comes from a consideration of the geography and epidemiology of the disease.

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