March 3, 2023

Epidemiology is the branch of medicine that deals with the incidence, distribution, and possible control of diseases and other factors relating to health. Epidemiological studies are challenging for several reasons:

MS can be difficult to diagnose. Since there is no single test for MS, the diagnosis can be missed, delayed, or even incorrect.
MS is not a "reportable" disease, which means that the government does not require physicians to inform any central database when they make the diagnosis. Without this kind of centralized reporting system, there is no easy way to count people with MS.
Data from earlier epidemiological studies may not accurately represent the current MS population because the investigators used different methods for identifying and counting people with MS, as well as different strategies for analyzing their data.

Therefore, all epidemiological numbers are estimates.

Incidence and prevalence

The incidence of a disease is the number of new cases occurring in a given period of time (usually a year) in a given population (usually 100,000). With the challenges inherent in promptly and correctly identifying people with MS, arriving at an accurate incidence figure has been virtually impossible.
The prevalence of MS is the number of people with MS at a particular point in time, in a particular place. Most epidemiological studies in MS focus on prevalence.

While prevalence is a bit easier to determine than incidence, diagnostic issues can distort these figures as well, since all persons with MS are included in prevalence figures regardless of how long they have had the disease. Of the MS prevalence studies that have been conducted worldwide, the data from the northern hemisphere are the most reliable thus far. The MS Society is working to improve reporting.

Epidemiological estimates

Although more people are being diagnosed with MS today than in the past, the reasons for this are not clear. Likely contributors, however, include greater awareness of the disease, better access to medical care, and improved diagnostic capabilities. There is no definitive evidence that the rate of MS is generally on the increase.

Age: Most people are diagnosed between the ages of 20 and 50, although MS can occur in young children and significantly older adults.

Geography: In general, MS is more common in areas farthest from the equator. However, prevalence rates may differ significantly among groups living in the same geographic area regardless of distance from the equator. For example, in spite of the latitude at which they live, MS is almost unheard of in some populations, including the Inuit, Yakutes, Hutterites, Hungarian Romani, Norwegian Lapps, Australian Aborigines, and New Zealanders --- indicating that ethnicity and geography interact in some complex way to impact prevalence figures in different parts of the world.

Migration from one geographic area to another seems to alter a person's risk of developing MS. Studies indicate that immigrants and their descendants tend to take on the risk level --- either higher or lower --- of the area to which they move. The change in risk, however, may not appear immediately. Those who move in early childhood tend to take on the new risk themselves. For those who move later in life, the change in risk level may not appear until the next generation. While underlining the complex relationship between environmental and genetic factors in determining who develops MS, these studies have also provided support for the opinion that MS is caused by early exposure to some environmental trigger in genetically susceptible individuals.

Gender: MS is at least two to three times more common in women than in men, suggesting that hormones may also play a significant role in determining susceptibility to MS, and some recent studies have suggested that the female-to-male ratio may be as high as three or four to one.

Genetics

Genetic factors are thought to play a significant role in determining who develops MS.

The average person in the United States has about one in 750 (.1%) chance of developing MS.
For first-degree relatives of a person with MS, such as children, siblings, or non-identical twins, the risk rises to approximately 2.5-5% --- with the risk being potentially higher in families that have several family members with the disease.
The identical twin of someone with MS (who shares all the same genes) has a 25% chance of developing the disease. If genes were solely responsible for determining who gets MS, an identical twin of someone with MS would have a 100% chance of developing the disease; the fact that the risk is only one in four demonstrates that other factors, including geography, ethnicity, and the elusive infectious trigger, are likely involved as well.

Research has demonstrated that MS occurs in most ethnic groups, including African-Americans, Asians, and Hispanics/Latinos, but is most common amongst Caucasians of northern European ancestry. Susceptibility rates vary among these groups, with recent findings suggesting that African-American women have a higher than previously reported risk of developing MS.