Multiple Sclerosis - Epidemiology

Epidemiology

Two main measures are used in epidemiological studies: incidence and prevalence. Incidence is the number of new cases per unit of person–time at risk (usually number of new cases per thousand person–years); while prevalence is the total number of cases of the disease in the population at a given time. Prevalence is known to depend not only on incidence, but also on survival rate and migrations of affected people. MS has a prevalence that ranges between 2 and 150 per 100,000 depending on the country or specific population. Studies on populational and geographical patterns of epidemiological measures have been very common in MS, and have led to the proposal of different etiological (causal) theories.

MS usually appears in adults in their thirties but it can also appear in children. The primary progressive subtype is more common in people in their fifties. As with many autoimmune disorders, the disease is more common in women, and the trend may be increasing. In children, the sex ratio difference is higher, while in people over fifty, MS affects males and females almost equally.

There is a north-to-south gradient in the northern hemisphere and a south-to-north gradient in the southern hemisphere, with MS being much less common in people living near the equator. Climate, sunlight and intake of vitamin D have been investigated as possible causes of the disease that could explain this latitude gradient. However, there are important exceptions to the north–south pattern and changes in prevalence rates over time; in general, this trend might be disappearing. This indicates that other factors such as environment or genetics have to be taken into account to explain the origin of MS. MS is also more common in regions with northern Europe populations. But even in regions where MS is common, some ethnic groups are at low risk of developing the disease, including the Samis, Turkmen, Amerindians, Canadian Hutterites, Africans, and New Zealand Māori.

Environmental factors during childhood may play an important role in the development of MS later in life. Several studies of migrants show that if migration occurs before the age of 15, the migrant acquires the new region's susceptibility to MS. If migration takes place after age 15, the migrant retains the susceptibility of his home country. However, the age–geographical risk for developing multiple sclerosis may span a larger timescale. A relationship between season of birth and MS has also been found which lends support to an association with sunlight and vitamin D. For example fewer people with MS are born in November as compared to May.