While risk increases with age, arthritis - a broad set of disorders - is not limited to the elderly. More than one-half of the people who reported having arthritis were under age 65. Osteoarthritis is the most common form of arthritis, affecting 12.1 percent of U.S. adults or 20.7 million people. Also known as degenerative joint disease, osteoarthritis was the second most common diagnosis, after chronic heart disease, leading to Social Security disability payments due to long-term absence from work.

"Arthritis is a leading cause of disability. With the aging of the population, it will increasingly burden individuals as well as the economy. This report should draw attention to the escalating impact of arthritis and will help us plan future research and prevention efforts," says Stephen I. Katz, M.D., Ph.D., director of the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), the component of the National Institutes of Health (NIH) that led the study.

Published in the May issue of Arthritis & Rheumatism, the report provides the best available prevalence estimates for a number of important rheumatic and musculoskeletal disorders in the U.S. population. It updates a 1989 report and includes estimates for disorders not included in the earlier report, such as fibromyalgia, low back pain, polymyalgia rheumatica, and giant cell arteritis.

The report was developed by the National Arthritis Data Workgroup. The Workgroup, organized by NIAMS to provide a single source of national data on the prevalence and socioeconomic impact of the rheumatic disorders, comprises 14 experts in rheumatology and epidemiology from U.S. universities and medical centers; NIAMS; the National Center for Chronic Disease Prevention and Health Promotion, of the CDC; the national voluntary health organization, the Arthritis Foundation; and the national professional association, the American College of Rheumatology.

The report focuses on estimates of prevalence or existing cases of each disorder rather than incidence, which counts only the number of new cases within a given period of time. National prevalence estimates were calculated by applying data from several studies to the 1990 Census, the most recent population estimates available.

The data in the report are conservative estimates based on carefully collected national data, such as the National Health and Nutrition Examination Surveys (NHANES) conducted by the CDC's National Center for Health Statistics (NCHS), or uniformly collected data from well-defined small populations. In the report, the authors are careful to point out the limitations of each data source. For example, some of the rheumatic diseases are difficult to classify, and some of the data from national surveys are based on self-reported conditions rather than physical examination. In a few cases, the data extrapolated from small population-based studies may not reflect the racial and ethnic profile of the United States.

"It is very difficult to get reliable estimates for relatively rare disorders," explains NIAMS Epidemiology Program Officer Reva C. Lawrence, MPH, lead author of the report. For many of the rheumatic disorders, such as scleroderma and juvenile rheumatoid arthritis, cases are not regularly reported to state health departments. NIAMS is supporting registries on rare diseases to improve disease monitoring and understanding of these conditions. NIAMS is also working to ensure appropriate emphasis on arthritis in the Healthy People 2010 objectives for the nation, which are currently being developed by the Department of Health and Human Services, of which NIH and CDC are a part.

Additional Estimates:

• Although low back pain is a symptom, not a disorder, estimates of its prevalence are included in the report because of its pervasiveness. The authors estimate that about one-half of adults have low back pain in any given year. Approximately 15 percent of U.S. adults report frequent back pain or pain lasting more than 2 weeks annually. Overall, more than 26 million Americans ages 20 to 64 and almost 6 million ages 65 and older have frequent low back pain.

• Fibromyalgia prevalence rates were estimated at 3.7 million Americans ages 18 and older. Prevalence is much lower in men than in women. A chronic condition, fibromyalgia is characterized by widespread pain, greater sensitivity to pain, sleep disturbances, fatigue, and multiple tender points. The authors note that the fibromyalgia data are extrapolated from a study that may not reflect the prevalence of the United States as a whole. "We recognize the importance of fibromyalgia and wanted to be able to provide some estimate. Additional population-based studies are needed," Lawrence says.

• Approximately 1 percent of U.S. adults have definite rheumatoid arthritis, an autoimmune disease that causes inflammation (with pain, stiffness, swelling, and deformity) in the joints. Rheumatoid arthritis occurs more frequently in women than in men. The prevalence of rheumatoid arthritis is approximately 2.1 million people: 600,000 men and 1.5 million women.

• Based on self-reported data from the 1992 National Health Interview Survey by the NCHS, the authors estimate U.S. prevalence of gout at 2.1 million people: 1.56 million men and 550,000 women. They note that these figures may be an overestimate. Gout is a painful rheumatic disorder that occurs when uric acid builds up in the body and accumulates as needle-like crystals in the joints.

• Polymyalgia rheumatica, a disorder involving moderate to severe muscle pain and stiffness in the neck, shoulder, and hip area, is estimated to be prevalent in 450,000 Americans. An estimated 110,000 have the closely related disorder giant cell arteritis, which is characterized by inflammation of blood vessels, which reduces blood flow, usually in the head, neck, and arms.

• The authors estimate that between 354,000 and 412,000 Americans ages 15 and older have some form of spondylarthropathies, a family of musculoskeletal disorders that includes ankylosing spondylitis (spinal arthritis) and psoriatic arthritis (a form of arthritis that occurs with the skin disease psoriasis). The authors consider these estimates conservative.

• Systemic lupus erythematosus (SLE or lupus) is an autoimmune disease in which the body harms its own tissues and can lead to inflammation and damage to joints, skin, kidneys, heart, lungs, blood vessels, and the brain. The report estimates SLE to affect at least 239,000 Americans: 4,000 white males, 41,000 white females, 31,000 black males, and 163,000 black females. Once a disease with high mortality, SLE is now a chronic disease. In 1954, survival was 50 percent after 4 years; today it is 97 percent at 5 years and 90 percent at 10 years. SLE is an area for which the authors recognize the uncertainty of the available data and call for the collection of more recent data.

• The estimated U.S. prevalence of juvenile rheumatoid arthritis (JRA), which occurs in children ages 16 and younger, is between 30,000 and 50,000. Active cases may constitute only one-half of this number, according to the report. These numbers are specific to JRA and do not include other forms of arthritis in children. These estimates are lower than earlier reported figures, which were based on small population studies that do not represent the diversity of the U.S. population.

• Very few data exist to enable reliable estimates of the prevalence of scleroderma, an autoimmune disease that causes hardening of the skin and, sometimes, internal organs. Estimates of the prevalence of scleroderma vary markedly. The prevalence is clearly higher in women than in men, with the highest rates occurring in middle-aged women.

References:

Lawrence RC, Helmick CG, Arnett FC, Deyo RA, Felson DT, Giannini EH, Heyse SP, Hirsch R, Hochberg MC, Hunder GG, Liang MH, Pillemer SR, Steen VD, Wolfe F. Estimates of the prevalence of arthritis and selected musculoskeletal disorders in the United States. Arthritis Rheum 1998;41:778-99.

CDC. Prevalence and impact of arthritis by race and ethnicity - United States, 1989-1991. MMWR Morb Mortal Wkly Rep 1996;45:373-8.

National Arthritis Data Workgroup. Arthritis prevalence and activity limitation -- United States, 1990. MMWR Morb Mortal Wkly Rep 1994;43:433-8.

Lawrence RC, Hochberg MC, Kelsey JL, McDuffie FC, Medsger TAJr, Felts WR, Shulman LE. Estimates of the prevalence of selected arthritis and musculoskeletal diseases in the United States. J Rheumatol 1989;16:427-41.

The National Institute of Arthritis and Musculoskeletal and Skin Diseases, a component of the National Institutes of Health, leads the Federal medical research effort in arthritis and other rheumatic diseases, and in musculoskeletal and skin diseases. The NIAMS supports research and training throughout the United States as well as on the NIH campus in Bethesda, Maryland, and disseminates health and research information. Press contact: Connie Raab/Ray Fleming (301) 496-8190.

The Centers for Disease Control and Prevention is the agency of the Federal Government charged with promoting health and quality of life by preventing and controlling disease, injury, and disability. Press contact: CDC Office of Communications (404) 639-3286.

The Arthritis Foundation is the national voluntary health organization devoted to arthritis. Its mission is to support research to find the cure for and prevention of arthritis and to improve the quality of life for those affected by arthritis. Press contact: Carol Galbreath (404) 872-7100 x6365.

The American College of Rheumatology is an organization of physicians, health professionals, and scientists that provides education, research, and advocacy to foster excellence in the care of people with arthritis and rheumatic and musculoskeletal diseases. Press contact: Kelly Sheehan (404) 633-3777.