Epidemiology of Neurological Conditions and Diseases
Epidemiologists define what causes a disease and what puts an individual at risk of getting that disease. Epidemiology is simply the study of disease patterns, which takes into account variations in geography, demographics, socioeconomic status, genetics and infectious causes. This type of study looks for disease patterns to help determine the cause, distribution and prevention. Epidemiologists continue to learn about multiple sclerosis (MS) by studying the relationships between these factors, as well as patterns of migration, in an effort to understand who gets MS and why, and identify and explain areas with high or low rates of MS.

Epidemiological studies do have limitations, but for a complex disease like MS, they can at least rule out some factors and highlight others. They are the first step toward finding a biological mechanism for MS, or possibly a cure. Most epidemiological studies of MS have been observational and retrospective in that researchers have collected information such as ethnicity and age of onset from someone that had been diagnosed with MS earlier. Such studies rely on an individual's ability to accurately recall information from years ago such as any infections that he or she had before the age of five. Other sources of data that can be used, such as death certificates, can contain incorrect information. Despite these shortcomings, a few factors consistently correlate with MS prevalence. Determining how these factors lead to an increased risk of MS has been the big challenge.

Epidemiological studies have helped to identify factors that may be related to the risk of developing MS, including geography, genetics and infectious processes, but with all that we still have few answers. It's difficult to conduct these types of studies in any disease that is not easy to diagnose.

Since there is no single test for MS, the diagnosis can be missed, delayed, or even incorrect. The use of magnetic resonance imaging (MRI) technology is helping to address this problem, but it remains difficult to determine how accurately data from earlier epidemiological studies truly represent the MS population, especially since investigators have used different methods for identifying and counting people with MS, as well as different strategies for analyzing their data.

Studies have also given a better look into when MS strikes for the first time or its onset. In general terms, most people are diagnosed with MS between the ages of 20 to 50 years old. Now we are able to better identify the age of onset in specific countries, thus giving a more targeted view that could provide additional clues as to why or how it happens.


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Incidence and Prevalence of MS

People often want to know the number of people that have MS in various parts of the country or the world, and whether those numbers are increasing. The act of simply counting them is easier said than done due to the diagnostic challenges described above. Not every country has the resources to the same medical tests such as MRI's, and without this type of diagnostic equipment, it's difficult to diagnose.

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. Most epidemiologists have chosen instead to focus on the prevalence of MS which is the number of people with MS at a particular point in time, in a particular place.

While prevalence is a bit easier to determine than incidence, the 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. The MS prevalence studies to date that have been conducted worldwide; the data from the northern hemisphere have been the most reliable so far.


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These studies have contributed in identifying factors that could be related to the risk of developing MS but haven't made any conclusive findings. It has been found that there is no evidence that the rate of MS cases is increasing, but rather awareness and better diagnostic abilities are most likely the reason that the number of cases increased in recent times.

Race and Multiple Sclerosis

Compared with white Americans, African Americans with MS have worse disease on MRI. In a recent study published in Neurology, 2010 Feb 16; 74:538 by Robert T. Naismith, MD, an Assistant Professor of Neurology at Washington University School of Medicine in St. Louis, showed increased tissue damage and lesion volumes in African Americans with MS.

Previous studies have suggested that, despite receiving prompt diagnosis and early treatment, African Americans with MS have greater disability than white Americans. The implication is that genetics, and not socioeconomic factors, are a major determinant in this more severe disease phenotype. To compare brain image findings in African Americans and white Americans with MS, researchers performed MRI on 567 patients (including 79 African Americans) seen at one center. Imaging was part of routine clinical care, negating a bias toward scanning those with worse disease.

Compared with whites, African Americans had similar proportions of MS subtypes, younger age, similar age at disease onset, a trend toward a slightly younger age at MS diagnosis, a significantly shorter disease duration, and no difference in treatment duration. On the basis of these factors alone, African Americans would be expected to have less disease evident on MRI than white Americans. However, African Americans had more T2 lesions and more T1 "black holes." Magnetization transfer ratios were also significantly lower in African Americans, consistent with increased tissue injury. Adjusted rates of brain parenchymal fraction did not differ between the two groups. African Americans showed trends toward lower gray-matter fraction and slightly higher white-matter fraction. African Americans also had greater clinical disability, based on both the Expanded Disability Status Scale (EDSS) score and the MS Severity Scale (MSSS) score.

Comments are that this study adds to the literature about MS in African Americans because it establishes that the more-severe disease phenotype in African American patients is largely inflammatory. A minor limitation of this study (or any such study of race) is that, if African Americans are less likely than white Americans to seek medical attention for mild MS, the authors may have recruited African Americans with more-severe MS than are found in the general population (i.e., selection bias). In addition, African Americans with severe disease may be more likely to seek care at an academic medical center than from a community neurologist (i.e., referral bias). Nonetheless, multiple studies now suggest that a true disease difference between these racial groups may exist.

Clinicians should assess MS activity and treatment response in all patients, but perhaps with increased vigilance in their African American patients. Unfortunately, no clinical studies have determined whether certain MS therapies are more effective than others in African Americans or whether a more aggressive approach would yield better outcomes. With many new MS therapies on the horizon, including some highly effective ones, the issue of early and aggressive treatment will become even more relevant to patient care. Exactly how race should contribute to clinical decisions about treatment remains to be established.

Epidemiological Observations
● In the United States, there are estimated to be 400,000 to 500,000 people with MS depending on the study. Although more people are being diagnosed with MS today than in the past, that reason is probably due to a greater awareness of the disease, better access to medical care, and improved diagnostic capabilities. There is no solid evidence that the rate of MS is generally on the increase.
● Most people are diagnosed between the ages of 20 and 50, although MS can occur in young children and significantly older adults.
● Worldwide, MS occurs with much greater frequency above 40° latitude than closer to the equator. Prevalence rates may differ significantly, however, even within a geographic area, where latitude and climate are fairly consistent. These variations demonstrate that geographical factors are not the only ones involved.
● MS is more common among Caucasians with northern European ancestry than other ethnic groups, but people of African, Asian, and Hispanic ancestry also develop the disease. There are some populations, in spite of the latitude at which they live, where MS is almost unheard of such as Inuit, Yakutes, Hutterites, Hungarian Romani, Norwegian Lapps, Australian Aborigines and New Zealand Maoris - indicating that ethnicity and geography interact in some complex way to impact prevalence figures in different parts of the world.
● Many different viruses have been suggested as a trigger for MS - including rabies, herpes simplex virus, measles, corona virus, canine distemper virus, HTLV-1, Epstein-Barr virus and others, yet none has been confirmed. The most promising candidate appears to be the Epstein-Barr virus which provides us with Mononucleosis. Chlamydia pneumoniae, a bacterial agent, has also been suggested but never proven. Although no trigger has yet been confirmed, most MS experts believe that some infectious agent is involved in initiating the disease process.
● Migration from one geographic area to another seems to alter a person's risk of developing MS. Studies indicate that immigrants and their descendents tend to take on the risk level - either higher or lower - of the area to which they move.
● 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.
● MS is two to three times more common in women than in men, leading researchers to believe that hormones may also play a significant role in determining susceptibility to MS. Recent studies have suggested that the female to male ratio may be as high as three or four to one.
● Genetic factors are thought to play a significant role in determining who develops MS. The average person in the United States has about one chance in 750 of developing MS. But first-degree relatives of people with MS, such as children, siblings or non-identical twins, have a higher chance - ranging from one in 100 to one in 40. The identical twin of someone with MS, who shares all the same genes, has a one in four chance of developing the disease. So 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 twins demonstrates that other factors are involved as well.
● Certain outbreaks or clusters of MS have been identified, but the cause and significance of these outbreaks are not known.

Atlas of MS

September 2008 brought the introduction of the Atlas of MS which contains information and data from 112 countries worldwide. To date, it's the most comprehensive study of its kind ever undertaken. This study was launched at the World Congress on Treatment and Research in MS in Montreal, Canada.

From 2012 to 2013, the MS International Federation carried out a second survey in order to update the information in the Atlas. This was published as an Atlas of MS, and the 2013 data was added to the 2008 information on their website.

The Atlas of MS has brought together information and data on the epidemiology of MS and the availability and accessibility of resources to diagnose, inform, treat, support, manage and rehabilitate people with MS worldwide are available in one database for analysis and comparison at country, regional, and global levels. The atlas can be seen at www.atlasofms.org or directly at www.msif.org/about-us/advocacy/atlas/.