New epidemiological data for multiple sclerosis (part 1)

The prevalence of a disease is usually expressed in terms of the number of cases per 100,000 people at a given point in time. The annual incidence of a disease is the number of new cases occurring each year. French epidemiological studies are particularly interesting since they show an unequal distribution of MS cases, with medium- and high-risk geographical areas. There is a north-south gradient, as might be expected, with higher prevalence in the north, but also an east-west gradient, with higher prevalence in the east.

The above data originate from the records of the "Mutualité Sociale Agricole" (Social Agricultural Mutual Insurance), which provides healthcare for all farmers, agricultural workers and their families.  This is a relatively stable population, not prone to emigrate, comprising 4,098,477 people as of 1st January 2003. Prevalence is highest in the Nord, Picardie, Champagne, Lorraine and Alsace regions.  The figures are roughly twice as high as for Provence-Côte d'Azur, Languedoc-Roussillon and Midi-Pyrénées.  Similarly, the Atlantic seaboard regions show low prevalence (62, 59 and 47/100,000 respectively in Brittany, the Pays de Loire and Poitou-Charente) whereas the figure for Franche-Comté, at the same latitude, is as high as 95/100,000.

Partial confirmation of these trends comes from a study carried out in Flanders on the entire population of the Leuven region. The prevalence of MS was found to be 88 per 100,000 people, a figure very close to that for Nord-Pas de Calais (93). Further confirmation comes from a study published in the form of a Summary and a Communication at the recent Montreal Congress (September 2008). This time the study was based on the records of the "Caisse Nationale d'Assurances Maladies" (National Health Insurance Fund - CNAM), which covers 84% of the French population.  The same north-south and west-east gradients could be observed, but with higher average values: 95.6/100,000 overall; 134.9/100,000 among women; and 54.12/100.000 among men, as of 31 October 2004.  According to the CNAM records, there are 49,626 MS patients and, on the basis of the available figures, it can be estimated that the total number of cases in France is 57,360.

When we look at MS prevalence in relation to the age of patients, we find that it is highest in the 35-49 age group in all European countries with the exception of Ireland, the United Kingdom and Norway, where prevalence is highest in the 50-64 age group.

The incidence of the disease is more difficult to determine. It was accurately studied in two island populations: Iceland and Sicily.  It is 4.2/100,000 in the former and 2.3/100,000 in the latter.  A recent review of MS epidemiology in Europe reports an average yearly incidence of 4.3/100,000. This figure is identical to the one reported for the Dijon region in the 1993-1997 period. For the French population as a whole, a similar rate of incidence would imply the occurrence of 2,580 new MS cases per year.  For Belgium, MS incidence would amount to 430 new cases per year.

In European epidemiological studies, MS prevalence is always higher among women than among men, with sex ratios varying between 1.1 and 3.4 female patients to one male patient. Surprisingly, however, a major change was observed in the sex ratio in the course of the 20th century. While at the beginning of the last century - and up until the 1950s in the case of Scotland - the disease affected both sexes equally, it now affects women 3 to 3.5 times more frequently than men.  The year of birth is now predictive of the disease's sex ratio among new cases.  This change started before the introduction of contraceptive pills and refrigerators, but might be linked to the increased prevalence of smoking among women. The disproportionate increase of MS incidence among women is not associated with a lower incidence among men.  Rather, the rate has increased steadily over the 50-year observation period of the Canadian study.   
Nor is this trend due to a more rapid diagnosis in the case of women than in the case of men, nor to an earlier age of onset in the female population: the average difference in the onset age is only one year earlier for women. A number of American, Australian and Scandinavian studies report the same trend. Only environmental factors can adequately explain these significant changes in the sex ratio. We shall return to this issue in our next newsletter.

In the meantime, we can conclude that the prevalence of the disease is slowly increasing in the countries where it is endemic.

Prof. C. Sindic, Vice-President