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Geographic variation in multiple sclerosis prevalence

 

Clinical bottom line

Multiple sclerosis occurs more frequently in higher latitudes than in places close to the equator, in both northern and southern hemispheres.


There is considerable variation in the occurrence of MS around the world. This has been ascribed to environmental factors, like exposure to viruses or ionising radiation, or to genetic factors. One constant, though, is that prevalence rates are higher in places closer to the poles compared to places closer to the equator. For instance, in the United States the prevalence is about twice as high in North Dakota than in Florida (Figure 1). Here we look at two recent papers from the UK and Australia.

Figure 1: Prevalence map of MS (cases/100,000) in the USA


References:

PM Rothwell, D Charlton. High incidence and prevalence of multiple sclerosis in south east Scotland: evidence of a genetic predisposition. J Neurol Neurosurg Psychiatry 1998 64: 730-735.

IAF van der Mei et al. Regional variation in multiple sclerosis prevalence in Australia and its association with ambient ultraviolet radiation. Neuroepidemiology 2001 20: 168-174.

Scotland and genetics

This was a study of incidence and prevalence of MS in the Lothian and Border region of Scotland in the mid-1990s, with a population of about 864,000.

Incidence was examined prospectively over three years with cases ascertained from all neurology and neurosurgery wards in the study area. Standard diagnostic criteria were applied. There were 310 definite cases (216 women and 94 men, sex ratio 2.3, mean age at diagnosis 34 years), giving an annual incidence of 12 (10.6 to 13.3) per 100,000. If probable cases were included also, the rate rose to 18 (16.5 to 19.8) per 100,000.

Prevalence was determined by defining a prevalent case as any person with a diagnosis of multiple sclerosis alive and normally resident in the area on 15 March 1995. Probable as well as definite cases were included. There were 1613 residents with a diagnosis of MS, giving a crude prevalence rate of 187/100,000. The sex ratio was 2.5 and the mean age was 49 years (Figure 2).

Figure 2: Prevalent cases in Lothian and Border by age and sex


There was a slight excess of people whose surnames suggested a Scottish ancestry than was expected from the population average.

Five studies of MS prevalence in Scotland have showed mean prevalence rates of about 150 to 200 cases/100,000, and have had 5-23% of names beginning with Mc or Mac. Six studies in England or wales have prevalence rates of 90-110 cases/100,000, and have 1-2% of names suggesting Scottish ancestry. This can be suggested as forming a link between MS and genetic susceptibility in populations.

However, geographical differences are also seen in the USA (Figure 1) and in the Southern hemisphere, in largely immigrant populations with considerable mixing. That would argue against a genetic factor and more in favour of an environmental factor or factors.

Australia and sunshine

The Lothian region of Scotland is about 55' of latitude north and has a prevalence of about 180 cases of MS/100,000. England and Wales, about 50' N have about 100 cases/100,000. Hobart is 42' S and has 74 cases/100,000 and tropical Queensland at about 18' S has 11/100,000.

That sort of describes the extent of the geographical association between latitude and MS. In Australia a study has tried to unpick any relationship between ambient UV radiation and MS.

Crude MS prevalence data, age standardised prevalence, number of MS patients and total population information was obtained for tropical Queensland, subtropical Queensland, Western Australia, New South Wales, South Australia and Tasmania (see Figure 3). Cases were found from hospital records, doctors, societies and statistical bureau. Diagnosis of MS used standard criteria.

Figure 3: Australian states


Monthly climate data was obtained for each largest city for the region with measurements from satellite observations of atmospheric ozone and UV irradiation calculated. Annual UV irradiation were lowest in Tasmania (about 2.2 kJ/sq metre/day) to 5.1 kj/sq m/day in tropical Queensland. There was a significant (1 in 100 significance) correlation between MS prevalence (and melanoma prevalence) and UV irradiation (and temperature, sunshine and latitude).

Figure 4: relationship between annual UV radiation and MS prevalence


Comment

This is not a comprehensive review of MS incidence and prevalence throughout the world, but rather a flavour to show that it is a complicated issue. One feature stands out, that of the relationship with latitude. There will undoubtedly be genetic factors relating to susceptibility. There may well be a relationship to UV radiation, and the mechanism for that makes interesting reading in the Australian paper. There is definitely more to come on this.

The problem for both papers is that while there may be statistical relationships, whether they causal is another matter. There are more "Macs" in Scotland, and less sunshine in Tasmania. But is this the cuase of MS?