Lectures: Epidemiology and Prevalence
There are about 300,000 patients suffering from Multiple Sclerosis in the North America today. The age of onset peaks between 20 and 30 years. Almost 70% of patients manifest symptoms between ages 21 and 40. Disease rarely occurs prior to 10 or after 60 years of age. However, patients as young as 3 and as old as 67 years of age have been described. Like other immuno- mediated diseases, females are affected more frequently than males (1.4 to 3.1 times as many women than men affected.)
There is a very specific geographic distribution of this disease around the world. A significantly higher incidence of the disease is found in the northernmost latitudes of the northern and the southern hemispheres compared to southernmost latitudes. This observation is based on the incidence of the disease in Scandinavia, northern United States and Canada, as well as Australia and New Zealand. The data from migration studies shows that if the exposure to a higher risk environment occurs during adolescence (before 15 years of age,) the migrant assumes the higher risk of the environment. This concept is nicely illustrated in studies of native-born South African white population with low incidence of the disease versus high incidence of MS among white immigrants from Great Britain, where the disease is much more prevalent (Saud A. Sadiq,James R. Miller et al.) "Epidemics" of MS have been reported and these provide further evidence of importance of environmental factors in MS. The most notable "epidemic" was described on the Faroe Islands after they were occupied by British troops in W.W.II. Similar increases in incidence of the disease were seen on Shetland and Orkney Islands, in Iceland, and in Sardinia. A specific "point agent" for these "epidemics" never was identified.
There are also population studies that show difference in susceptibility to MS between different populations. Lapps in Scandinavia appear to be resistant to the disease, contrary to the expectations based on their geographic distribution. Native Americans and Hutterites very infrequently suffer from MS, as opposed to other residents of the North America. MS is uncommon in Japan, China and South America. It is practically unknown among the indigenous people of equatorial Africa and among native Inuit in Alaska. When the racial differences are correlated, White populations are at greater risk than Asian or African populations. We can not yet explain these obvious inconsistencies in disease distribution, but the knowledge of them may be helpful in assessing specific patients.
The incidence of MS in first degree relatives is 20 times higher than in general population, suggesting the influence of genetic factors on the disease. Monozygotic twin studies show the concordance rate of 30%. Dizygotic twins show concordance rate of less than 5%. These results suggest that both the genetic factors and environmental exposure are important in disease expression.