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Myalgic Encephalomyelitis


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NORD is very grateful to Leonard Jason, PhD, and his DePaul research team, for assistance in the preparation of this report.

Synonyms of Myalgic Encephalomyelitis

  • CFS/ME
  • chronic fatigue syndrome/myalgic encephalomyelitis
  • ME

Disorder Subdivisions

  • No subdivisions found.

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.

Myalgic Encephalomyelitis Resources

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