Myalgic Encephalomyelitis

National Organization for Rare Disorders, Inc.

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It is possible that the main title of the report Myalgic Encephalomyelitis is not the name you expected.

Disorder Subdivisions

  • None

General Discussion


Myalgic encephalomyelitis (ME) is an acquired complex disorder characterized by a variety of symptoms and physical findings potentially affecting multiple systems of the body. Many cases are preceded by a viral infection, usually a flu-like or upper respiratory illness, although ME can also be preceded by a non-viral illness or other trauma such as chemical exposure. Onset is usually rapid (acute) but gradual onsets are also reported. Affected individuals do not recover from the infection and instead experience a wide variety of symptoms including an inability to produce sufficient energy to meet daily demands. Marked fatigue, sickness, and symptom flare-up follows physical and cognitive exertion. Additional symptoms that may occur include headaches, pain, muscle weakness, neck pain, vision abnormalities, a sensation of tingling, burning or numbness of the extremities (paresthesia), and sleep dysfunction. Cardiovascular abnormalities have also been reported. Myalgic encephalomyelitis is a chronic and disabling disorder. Severe cases can leave affected individuals bedridden or housebound. Myalgic encephalomyelitis may occur as an outbreak that affects a large group of people (epidemically) or may only affect an individual (non-epidemically).


There is significant controversy and debate in the medical literature about the relationship between myalgic encephalomyelitis and chronic fatigue syndrome (CFS). The first outbreak of myalgic encephalomyelitis was in 1934 and the term myalgic encephalomyelitis first appeared in the medical literature in 1956. Myalgic encephalomyelitis is recognized as a distinct disorder and has been classified as a specific neurological disorder by the World Health Organization (WHO) since 1969.

The term CFS was first used in the medical literature during the 1980s in the United States. The criteria focused more on fatigue than the encephalitic (inflammation of the brain) features of the disorder. Consequently, the emphasis on fatigue opened the door to defining the disorder as a psychiatric illness. Because little was known about the cause or physiology of CFS, a wide range of patients were diagnosed with CFS even though they may have had a variety of conditions and experienced different symptoms. CFS eventually evolved into a larger disease designation that overlapped with myalgic encephalomyelitis. Consequently, some researchers, patients, government organizations, and other organizations began to use the terms interchangeably or with the combined acronym ME/CFS, creating a broad disease category.

Further, some researchers, physicians, and patient advocacy groups have pushed to abandon the illness label of CFS as they argue it is inaccurate and trivializes affected individuals. They want to reclassify these individuals as having myalgic encephalomyelitis. Other researchers, physicians, and many ME patient advocacy groups have argued against this change, noting that myalgic encephalomyelitis should retain a strict definition as a distinct neurological disease that includes measurable abnormal changes in the brain and central nervous system. Individuals who meet the stricter criteria would be diagnosed with myalgic encephalomyelitis. The term CFS should be reserved for individuals who fail to meet the more stringent criteria for myalgic encephalomyelitis and in whom no other underlying disorder or condition can be identified. Many patients who have been diagnosed with CFS would meet the more stringent criteria for myalgic encephalomyelitis. Individuals who fail to meet the criteria should be retested for an underlying condition as many individuals initially diagnosed with CFS are eventually diagnosed with an underlying condition such as cancer, multiple sclerosis, lupus, brucellosis, or another condition.

Three of the more common case definitions include the Fukuda et al. (1994) case criteria for CFS, the Canadian Consensus Criteria for ME/CFS (Carruthers et al., 2003) and the Myalgic Encephalomyelitis International Consensus Criteria (ME-ICC). The Fukuda case definition was adopted by the Centers for Disease Control and Prevention (CDC), and stresses fatigue as a primary symptom. It also requires the presence of at least four of eight symptoms including: memory and concentration impairment, sore throat, tender lymph nodes, muscle pain, joint pain, headaches, unrefreshing sleep, and post-exertional malaise). Research has indicated that individuals with a primary psychiatric illness (e.g. primary Major Depressive Disorder) may be misdiagnosed under the Fukuda criteria due to many overlapping symptoms including fatigue and sleep difficulties. The Canadian Consensus Criteria defines ME/CFS as an acquired, organic, pathophysiological multi-systemic illness that occurs in both sporadic and epidemic forms and requires core symptoms including post-exertional malaise (PEM) and neurocognitive dysfunction, in contrast to the polythetic approach of the Fukuda case definition. Lastly, the Myalgic Encephalomyelitis – International Consensus Criteria (ME-ICC) advocates for removing fatigue as a characteristic symptom and defines the disorder as an acquired neurological disease with complex global dysfunctions. The ME-ICC also defines specific symptom requirements: post-exertional neuroimmune exhaustion, neurological impairments, immune, gastrointestinal, and genitourinary impairments, and energy metabolism impairments. The Nightingale Research Foundation, a Canadian charity dedicated to myalgic encephalomyelitis, uses a strict definition that states myalgic encephalomyelitis is an acute onset biphasic epidemic or endemic infectious disease process, where there is always a measureable and persistent diffuse vascular injury of the central nervous system in both the acute and chronic phases. For more information on specific case definitions, see the Resources and References sections of this report.

Unfortunately there is no consensus on nomenclature or classification for these disorders, and different countries, organizations, and researchers continue to use different names to describe these conditions. Until a global consensus is reached on how to name and classify these disorders, confusion will persist.

Supporting Organizations

Centers for Disease Control and Prevention

1600 Clifton Road NE
Atlanta, GA 30333
Tel: (404)639-3534
Tel: (800)232-4636

Genetic and Rare Diseases (GARD) Information Center

PO Box 8126
Gaithersburg, MD 20898-8126
Tel: (301)251-4925
Fax: (301)251-4911
Tel: (888)205-2311

MAME, Inc. (Mothers Against Myalgic Encephalomyelitis)

1 Orne Square
Salem, MA 1970
Tel: (978)744-8293
Fax: (978)744-2027

ME Association

7 Apollo Office Court
Radclive Road
Bucks, MK18 4DF
United Kingdom
Tel: 1280818964

ME Research UK

The Gateway
North Methven Street
Perth, PH1 5PP
United Kingdom
Tel: 1738451234

NIH/National Institute of Allergy and Infectious Diseases

NIAID Office of Communications and Government Relations
5601 Fishers Lane, MSC 9806
Bethesda, MD 20892-9806
Tel: (301)496-5717
Fax: (301)402-3573
Tel: (866)284-4107

Wisconsin ME/CFS Association, Inc.

733 Lois Drive
Sun Prairie, WI 53590
Tel: (608)834-1001
Tel: (855)878-5555

World Health Organization (WHO)

Avenue Appia 20
Geneva 27, 1211
Tel: 41227912111
Fax: 41227913111

For a Complete Report

This is an abstract of a report from the National Organization for Rare Disorders (NORD). For a full-text version of this report, go to and click on Rare Disease Database under "Rare Disease Information".

The information provided in this report is not intended for diagnostic purposes. It is provided for informational purposes only.

It is possible that the title of this topic is not the name you selected. Please check the Synonyms listing to find the alternate name(s) and Disorder Subdivision(s) covered by this report.

This disease entry is based upon medical information available through the date at the end of the topic. Since NORD's resources are limited, it is not possible to keep every entry in the Rare Disease Database completely current and accurate. Please check with the agencies listed in the Resources section for the most current information about this disorder.

Last Updated:  3/24/2016
Copyright  2013 National Organization for Rare Disorders, Inc.