Multiple sclerosis is an inflammatory demyelinating disorder and is one of the commonest causes of severe neurological disability in middle life in people of northern European origin. Classically, it begins with relapses and remissions of neurological impairment. In time, the remissions tend to be incomplete, and a phase of insidious progression with or without superimposed relapses, is entered (secondary progressive MS). In a small proportion of patients (approximately 10%) there is steady progression from onset, without clear cut relapses and remissions (primary progressive MS) (Thompson et al., 2000).
The prevalence of MS varies greatly throughout the world and appears to be influenced by a complex interaction between geographic position and genetic background (for reviews see Compston, 1998, 1999). In the temperate zones of northern Europe, North America and Australasia, rates in excess of 100 per 100000 population have been recorded and have reached 300 per 100000 in the Orkney and Shetland Islands (Kurtzke, 1993). By contrast, in Asia the prevalence is of the order of 5 per 100000; fewer than 10 cases have been described in black Africans.
A closer inspection of the epidemiological data reveals a number of important details departing from the overall picture. There are a number of reports that migration from a high prevalence zone (e.g. Northern Europe) to a low prevalence zone (e.g. South Africa) before puberty lowers the risk of MS while migration after this time does not. Migration in the reverse direction has the opposite effect. Another interesting detail is the occurrence of significant local variations in prevalence (see below) and the occurrence of clusters of cases over limited periods of time in confined geographical areas. This has been reported from Iceland, and most strikingly, for the Faroe Islands. The numbers involved in these studies are small, and caution should be exercised in their interpretion. It is of interest that the recent data on MS in Australia does not support the notion of an effect of age at migration on the risk of MS, though it does confirm strikingly the latitudinal gradient which is greater there than in any other country for which data is available (Hammond et al., 2000).