As with many other presumed autoimmune and inflammatory conditions, the incidence of MS is 2 or 3 times higher in women than in men but the basis of this fact is unclear. The incidence in children is very low; only 0.3 to 0.4 percent of all cases appear during the first decade. In an analysis of a small number of childhood-onset cases, Hauser and colleagues (1982) found no phenotypic differences between childhood and adult cases, but Renoux and colleagues analyzed a cohort of 394 patients who had MS with an onset at 16 years or younger and found that these patients took longer to reach states of irreversible disability, but did so at a younger age than patients with adult-onset MS. Beyond childhood, the risk of first developing symptoms of the disease rises steeply with age, reaching a peak at about 30 years, remaining high in the fourth decade, then falling off sharply and becoming low in the sixth decade. On this basis it has been pointed out that MS has a unimodal age-specific onset curve, similar to that of infectious and connective tissue diseases.
In a smaller number, the disease appears to develop in late adult life (late fifties and sixties). In such patients, early symptoms may have been forgotten or may never have declared themselves clinically (we have several times found the typical lesions of MS in aged autopsied individuals who had no history of neurologic illness). Gilbert and Sadler report 5 such cases and from their pathologic findings suggest that the true incidence of MS may be 3 times higher than the stated figures.
Although the cause of MS remains undetermined, a number of epidemiologic facts have been established and will eventually have to be incorporated in any hypothesis. The disease has a prevalence of less than 1 per 100,000 in equatorial areas; 6 to 14 per 100,000 in the southern United States and southern Europe; and 30 to 80 per 100,000 in Canada, northern Europe, and the northern United States. Mayr and colleagues reported an incidence of 8 and a prevalence of 177 cases per 100,000 in Olmstead County, Minnesota; this prevalence has been stable for approximately 30 years. A less-well-defined gradient exists in the southern hemisphere. Kurland’s studies indicated that there is a threefold increase in prevalence and a fivefold gradient in mortality rate between New Orleans (30 degrees north latitude) and Winnipeg (50 degrees north). In Japan, there is a similar although less distinct latitudinal gradient (the prevalence of MS there is much lower than in corresponding latitudes of North America and northern Europe).
The increasing risk of developing MS with higher and lower latitude has been confirmed by many epidemiologists following the work of Kurtzke (1975). In the
United States, African Americans are at lower risk than whites at all latitudes, but both races show the same south-to-north gradient in risk, findings that invoked an environmental factor regardless of genetic predisposition. Supporting this view are the descriptions, by Kurtzke and Hyllested, of an “epidemic” of MS in the Faroe Islands of the North Atlantic. They found a much-higher-than-expected incidence of the disease, occurring as three separate outbreaks of decreasing extent between the years 1943 and 1973. (It should be pointed out that the largest outbreak consisted of only 21 cases.) It was their contention, confirmed by Poskanzer and colleagues, that the disease was the result of an unidentified infection introduced by British troops who occupied the islands in large numbers in the years immediately preceding the outbreak. Kurtzke and colleagues (1982) described a similar postwar epidemic in Iceland. The cause of these geographic distributions has been reinterpreted in terms of migration and population genetics rather than a number of other imputed causes, but they remain interesting (see Compston and Confavreux for a complete discussion).