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Multiple sclerosis in Malta in 1999
  1. N Koch-Henriksen
  1. Department of Neurology, Aalborg Hospital North, DK-9100 Aalborg, Denmark
  1. Correspondence to:
 Dr N Koch-Henriksen;
 niko{at}post3.tele.dk

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Examination of the low incidence of multiple sclerosis in Malta

The paper by Dean et al1 (this issue pp 256–60) is not just one of countless papers on the prevalence of multiple sclerosis that have been published over the last 70 years. For several reasons it deserves special attention.

The numerous studies on this topic have been of varying importance and quality, and have been driven largely by the hope that the geographical pattern of disease occurrence might reveal the cause of multiple sclerosis. The pay off of these efforts, in terms of strong or definite clues to the aetiology, has been disappointing, and during the last two decades the focus on multiple sclerosis epidemiology has weakened and given place to studies on immunology, genetics, and treatment.

Appropriate epidemiological studies may, however, still be of help in several areas—for example, is infection a possible contributory cause of multiple sclerosis?1 And what are the respective roles of environmental and genetic factors in the geographical variation in the prevalence of the disease?2 Investigations in areas with unusually high or low frequencies of multiple sclerosis may help solve these problems.

The paper by Dean et al is a good example of this. The observation of the rarity of multiple sclerosis among Maltese immigrants to London by Dean et al in 19763 led to the original 1978 survey of the disease in Malta,4 which indicated that the prevalence was far below that in mainland Italy, Sicily, Sardinia, and other places around the Mediterranean.

As shown in the new paper by Dean et al in this issue, the Maltese population is still a clear and interesting exception to the surrounding Mediterranean populations in terms of risk of multiple sclerosis, as the prevalence rate is only 16.6/100 000 when all cases of clinically probable disease are included, and 13.2/100 000 when the analysis is confined to clinically definite cases. Moreover, the incidence rate has remained low, at just 2/100 000 per year.

When a prevalence survey is repeated years after a primary survey, the prevalence invariably increases, because of better case ascertainment and possibly greater life expectancy of the patients. This is also true for the new Malta multiple sclerosis survey, but Dean has shown that the small rise in prevalence since 1978 can be attributed entirely to a shift of the population age distribution and to a better life expectancy. The study is comprehensive and there are several indicators of a persistently high ascertainment probability—for example, the average age at prevalence ascertainment was more than 43 years, and the prevalence among immigrants to Malta was much higher than among native Maltese and comparable to the prevalence in other parts of Europe.

As Dean et al point out, a low genetic susceptibility to multiple sclerosis is the most likely explanation for the low prevalence rate in Malta. This view is supported by the low risk among Maltese emigrants to London and the high prevalence among immigrants to Malta from places with a high risk of the disease. However, as migration studies indicate that environmental influences on the risk of multiple sclerosis act before adolescence, and as the age at migration of these groups is unknown, it is possible that environmental factors may also be involved in the low prevalence in Malta. A repeated study of the HLA profile of Maltese patients compared with the general population could clarify this important question.

Examination of the low incidence of multiple sclerosis in Malta

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