NeuroepidemiologyEpidemiology of multiple sclerosis
Introduction
Multiple sclerosis (MS) is the most common demyelinating disease seen in high-income countries, and has a heterogeneous prevalence worldwide: it is highest in North America (140/100,000 population) and Europe (108/100,000), and lowest in East Asia (2.2/100,000 population) and sub-Saharan Africa (2.1/100,000). The global median prevalence of MS has increased from 30/100,000 in 2008 to 33/100,000 in 2013, according to a report by the MS International Federation. In Europe in particular, a North-South prevalence gradient has been described for distribution of the disease (higher in the North, lower in the South). France is located in the middle of Western Europe between zones of high MS prevalence (such as the Scandinavian countries and United Kingdom) and areas of low MS prevalence (Italy, Greece, Spain), and also appears to reflect the European epidemiology of the disease rather well. Given this situation, MS mortality, incidence and prevalence data and their evolution over time in France are important in that they provide a pertinent view of the epidemiology.
Analyzing such data is an essential first step towards taking into account the considerable knowledge and numerous hypotheses regarding the cause(s) of MS and, above all, the environmental risk factors. Also, extensive databases for large groups of MS patients provide accurate information on the natural history of MS as a two-stage disease (first the focal inflammatory process, and then the second, which is independent of focal inflammatory markers). All of these descriptive and analytical epidemiological data will lead to a better understanding of the risk factors for MS, and may even have implications for therapeutic strategies.
Section snippets
MS mortality
To our knowledge, around 40 epidemiological studies of mortality have been conducted in patients afflicted with MS. According to one published series [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], 70–88% of patients are still alive 25 years after clinical onset, and the median time from onset to death ranges from 24 years to > 45 years. These differences can be explained by differences in study periods, geographical areas and methodology (such as study population, duration,
Risk factors of MS
The cause of MS is multifactorial: both genetic and environmental risk factors contribute to disease risk. Several factors have been assessed and are reviewed below. However, the specific causes of MS are still largely unknown and, at present, there are no well-established factors to assist disease prevention [20], [21].
Prevalence and incidence
The distribution and frequency of MS are assessed by estimates of prevalence and incidence. ‘Prevalence’ refers to the proportion of a population that has the disease at or during a specified time, while ‘incidence’ refers to the proportion of new cases arising during a specific time period. Prevalence reflects the ‘stock’ of patients, while incidence reflects the dynamic dimension of the disease.
As mentioned above, assessment of the different incidences and prevalences across populations can
Gender ratio
Over the past 30 years, the MS gender ratio has changed due to an increase in the incidence of MS in women. Between 1960 and 2005, the gender ratio adjusted for the year of MS onset increased from 1.68 to 2.45 (P = 0.017), according to data from the Lyon MS database collected by the European Database for Multiple Sclerosis (EDMUS) system. This analysis was conducted in 4495 patients, including 3030 women (67.4%). However, the change in gender ratio over time was observed only for the relapsing
The natural history of MS in France
Only studies of large groups of MS patients using extensive databases can give accurate information on the natural history of MS. Specifically, the requirements for such studies include identifying a geographically defined population with MS at the early stages, achieving complete ascertainment and a complete follow-up for an appropriate period, and applying outcome measurements that detect all possible relevant outcomes. No large published cohorts have completely satisfied these requirements.
Lessons from the natural history of MS
Understanding the dissociation between early and late disability progression, and the role of relapses, is central to the debate on the putative mechanisms of disability progression in MS. Some studies [79], [81], [84] have suggested that age is a key factor (if not the only one) in the natural history of MS, leading to the concept of MS as a single-stage disorder with chronic age-related neurodegeneration following onset of the disease [85].
However, the Rennes cohort [86] suggests a two-stage
Conclusion
Given the mortality, prevalence, incidence, evolution of the gender ratio and geographical distribution of MS in France, this country can now be more precisely defined as a high-risk country for MS. Improved knowledge of the natural history of MS, with the dissociation between early and late disability progression and the impact of relapses on disability progression, has led to a better understanding of MS mechanisms and better guidance for therapeutic strategies.
As stated by Simpson et al. in
Disclosure of interest
The authors declare that they have no competing interest.
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