Epidemiology of ALS and Suspected Clusters
What is Epidemiology?
Epidemiology is the study of factors that influence the frequency and distribution of disease, injury and other health-related events and their causes in a defined human population. These studies are done to establish programs that prevent and control the development and spread of disease and injury.1
Epidemiological studies in ALS have contributed to a better understanding of the incidence, prevalence, mortality rate and signs and symptoms of ALS as well as the patterns of occurrence in relation to age, gender, race and geographic distribution. The association of ALS with certain factors such as genetics, occupations and toxic exposures has been studied by comparing groups of people with ALS with groups who do not have ALS over time.2,6
Careful epidemiologic studies must be conducted to determine and describe the frequency and distribution patterns of disease occurrence. These studies are also used to evaluate whether a certain group or geographic area can be associated with a greater than expected number of ALS cases, or a suspected cluster.
Descriptions of disease characteristics come from studying ALS within populations in certain geographic areas. Such studies make significant effort to identify all the people living with ALS within that particular geographic area. Results from careful research can be applied with confidence to the general population.
When studying whether the number of new ALS cases is higher than expected in any certain group or locale, it is often important to include a comparison group. The effort to identify ALS cases must be as intensive in this comparison group as it is in the group which is suspected to have given rise to the cluster report.
If the number of new cases is two times more than expected, scientists may conclude that this is a finding that is not explained by chance alone.
If a higher than expected number of new ALS cases is found, additional study is necessary to determine why the number of cases is higher in a particular group of people or in a certain geographic area. “To establish risk factors, a statistical association must be demonstrated between a certain event and the subsequent occurrence of the disease." 2
Once scientists confirm an increase in new ALS cases, they can begin to understand what is causing the increase.
Good epidemiologic studies begin with the process of collecting information about ALS cases, which in turn depends on correct disease diagnosis. A number of studies in the U.S. over recent decades all indicate that both the number of new ALS cases and the number of people living with ALS are fairly stable both geographically and over time.
There is some information suggesting a small increase in the number of new ALS cases. However, it is not clear if this information reflects an actual increase in the number of ALS cases, the need for an improved process to identify ALS cases or the need to eliminate misdiagnosis. In general, the number of new ALS cases and the prevalence of ALS worldwide are uniform.
What is the incidence of ALS?
The incidence is defined as the number of new cases per year. Within a population of 100,000 people, there are 2 new ALS cases each year.
The average age of people who are diagnosed with ALS is between 55 and 65.1
The number of new cases of ALS varies with age. For people in their 60’s there are three to four new cases within a population of 100,000 each year.2
Although the number of new ALS cases increases with age until age 80, ALS does sometimes occur in people who are in their 20’s and 30’s. Some physicians report that they are “seeing increasing numbers of younger ALS patients”. These reports need further study.
What is the prevalence of ALS?
The prevalence, or number of people living with ALS, varies among geographic regions worldwide. Between October 19, 2010 and December 31, 2011, the National ALS Registry identified 12,187 people living with ALS in the United States. This means they found the prevalence, or number of people living with ALS at any one time, to be about 4 people out of every 100,000.7 Data continues to be collected through the National ALS Registry. Information will be updated as additional data becomes available.
Current information suggests that there is no consistent prevalence rate across the United States. However, the prevalence is believed to be between 5-7 cases within a population of 100,000 which equates to 15,000-21,000 Americans living with ALS at any one time. (Using U.S. Census bureau population data from 2014)
Is ALS more prevalent in men or women?
The disease is more common in men than women and occurs about 1.56 times more frequently in men than women.7
However, for people who are over the age of 70 when the disease begins, the number of men compared to women with ALS is nearly the same. 1 ALS is a disease of aging; as the U.S. population increases and ages, an increase in the prevalence can be anticipated.
Is there a racial predisposition in ALS?
White men and women are about twice as likely to develop the disease as black men and women. The disease is more common among non-Hispanics.
Are there known risk factors?
A risk factor increases a person’s chance of developing a certain disease. Despite many studies, only age and gender have been constantly found to be risk factors in cases of ALS that are not inherited from a family member. Several studies have suggested a connection between certain environmental factors and ALS. The nature of these relationships is unproven, and no environmental factor has been proven to be a risk factor for developing ALS. More research is needed in this area.
Are physicians required to report ALS?
No. There is no requirement for physicians to report cases of ALS. Without mandatory reporting, it is difficult and costly to investigate and describe the incidence, prevalence, risk factors, and clinical course of ALS. A number of epidemiologic studies have and are currently being conducted in the United States. Despite lack of mandatory reporting for ALS, mortality rates by state and local communities can be calculated using data from death certificates. “Studies have shown that 70% to 90% of patients diagnosed as having ALS had this condition recorded on their death certificate.” 2
The National ALS Registry collects information about patients’ family, medical, occupational and community histories. This information could lead to the cause, treatment and cure for the disease. To learn more about the National ALS Registry, including how to enroll, please visit the registry page of The ALS Association’s website, http://registry.alsa.org/ or the Agency for Toxic Substances and Disease Registry’s website at www.cdc.gov/als .
What is The Association’s response to reports of suspected clusters?
Despite a number of studies investigating suspected clusters in the U.S., the only confirmed cluster of ALS was described in the Western Pacific in the 1940’s. Although history has not provided clues to the cause of ALS through study of suspected clusters, this is a potentially significant area of inquiry if it can be shown that an ALS cluster results from an epidemiologic factor and not chance alone. There must be confidence that the cluster occurred due to a certain cause rather than chance alone, in order for clinics and public health agencies to spend resources.1 Community studies take into account the geographic location of each individual with ALS, when the disease was diagnosed, age at time of diagnosis, population of the geographic area and cases in which people inherited the disease from family members
From time to time The Association is notified of a number of ALS patients from the same community or work environment that may be higher than the expected number of cases. When there is a question of a higher than expected number of people with ALS in one geographic or occupational area, The Association recommends that people raise their questions of an increased incidence of ALS to one or more of the following:
- Local, community and state public health agencies, community and academic medical center neuromuscular neurologists, epidemiologists and toxicologists, university school of public health or medicine.
- Epidemiology staff at one or more of these agencies is best equipped to conduct an initial exploration to determine if the number of new cases reaches a number that requires more information about the ALS cases and a full study.5
The ALS Association has been actively involved for several years with a number of ALS environmental studies and has established relationships with the country’s leading ALS epidemiologists. The Association’s Research Program, TREAT ALS, includes exploration of the possible role environmental pollution can play in learning more about the cause(s), frequency and distribution of ALS. The Association welcomes the submission of abstracts for epidemiological studies that would be considered for funding.
Although in the past, studies of ALS clusters that are reported by the community have not been fruitful in identifying the cause of ALS, each new report should be evaluated and studied by scientists, if necessary.
For more information, please visit the following websites.
- Dorland’s Illustrated Medical Dictionary, W.B. Saunders Co., Philadelphia, 1988, p 565.
- Mitsumoto, H., Chad, D., Pioro, E., Amyotrophic Lateral Sclerosis, Reinhardt, R., F.A. Davis Company, Philadelphia, PA, 1998, pp 19, 21, 22, 26.
- Amyotrophic Lateral Sclerosis - Diagnosis and Management for the Clinician, Belsh, J., Schiffman, P., Futura Publishing Company, Armonk, NY, 1995, p 3.
- Francis, K., Bach, J., DeLisa, J., Evaluation and Rehabilitation of Patients with Adult Motor Neuron Disease, Arch Phys Med Rehabil, Vol 80:951, 1999.
- Armon, C., Daube, J., O’Brien, P., Kurland, L., Mulder, D., When is an Apparent Excess of Neurologic Cases Epidemiologically Significant? Neurology Vol 41, No 11, pp1713-1718, November, 1999.
- Batchelor, T., Cudkowicz, M., Principles of Neuroepidemiology, Butterworth-Heinemann, 2001, p 3.
- Mehta, P., Anteo, V., Kaye, W., Sanchez, M., Williamson, D., Bryan, L., Muravov, O., Horton, K., Prevalence of Amyotrophic Lateral Sclerosis- United States, 2010-2011. Centers for Disease Control and Prevention, Weekly Morbidity and Mortality Report, Surveillance Summaries, Vol 63, 7, July, 2014.
The ALS Association thanks and acknowledges Page Jones, Ph.D., for sharing her time and expertise on this factsheet.