Persons with CFS most often function at a substantially lower level of activity than they were capable of before the onset of illness. In addition to these key defining characteristics, patients report various nonspecific symptoms, including weakness, muscle pain, impaired memory and/or mental concentration, insomnia, and post-exertional fatigue lasting more than 24 hours. In some cases, CFS can persist for years. The cause or causes of CFS have not been identified and no specific diagnostic tests are available. Moreover, since many illnesses have incapacitating fatigue as a symptom, care must be taken to exclude other known and often treatable conditions before a diagnosis of CFS is made.

Chronic fatigue syndrome (CFS) is one of several names given to a poorly understood, highly debilitating disorder of uncertain cause/causes, which is thought to affect approximately 4 per 1,000 adults in the United States and other countries, and a smaller fraction of children.

The disorder is marked by chronic mental and physical exhaustion, often severe, and by other specific symptoms, arising in previously healthy and active persons. Despite promising avenues of research, there remains no objective assay or pathological finding which is widely accepted to be diagnostic of CFS. It remains largely a diagnosis of exclusion, made on the basis of patient history and symptomatic criteria, although a number of tests exist which can help aid diagnosis. Although there is agreement on the genuine threat to health, happiness, and productivity posed by CFS, various physicians' groups, researchers, and patient activists champion very different nomenclature, diagnostic criteria, etiologic hypotheses, and favored treatments, resulting in ongoing controversy about nearly all aspects of the disorder.

The name chronic fatigue syndrome is itself controversial, with some patient advocates and other authorities preferring terms such as myalgic encephalomyelitis ("ME" or "ME/CFS") and post-viral fatigue syndrome ("PVFS"), which imply specific underlying etiologies or pathologic processes.

Chronic fatigue syndrome is not the same as "chronic fatigue”. While fatigue is a common symptom in many illnesses, CFS is a multi-symptom disease and is relatively rare by comparison. Definitions (other than the 1991 UK Oxford criteria) require a number of features, the most common being severe mental and physical exhaustion which is "unrelieved by rest" (according to the 1994 Fukuda definition), and may be worsened by even trivial exertion (a mandatory diagnostic criterion according to some systems). Most diagnostic criteria insist that the symptoms must be present for at least six months, and all insist on there being no other cause for them: i.e. the symptoms must be idiopathic, not caused by other medical conditions such as diabetes, hypothyroidism or anemia.

CFS patients may report many other symptoms which are not included in all diagnostic criteria, including muscle weakness, cognitive dysfunction, hypersensitivity, orthostatic intolerance, digestive disturbances, depression, poor immune response, and cardiac and respiratory problems.

It is unclear if these symptoms represent co-morbid conditions or are produced by the same underlying etiology as CFS itself. Some cases improve over time, and treatments (though none are universally accepted) bring a degree of improvement to many others, though resolution is rare.

CFS occurs more often, but not exclusively, in women, for unknown reasons. CFS is most easily diagnosed when formerly active adults become ill, and is most commonly diagnosed in young to middle aged adults, although it is also reported in children, adolescents and the elderly.

Diagnosis

At this time, there is no accepted conclusive test or series of tests of chronic fatigue syndrome. CFS is therefore largely an exclusionary diagnosis. If a doctor suspects a patient may have CFS they should begin the diagnostic process by eliminating other potential causes of the patient's symptoms, as "chronic fatigue" and related symptoms can be caused by a wide variety of conditions which should be investigated and managed.

CDC 1994 criteria (aka "Fukuda")

According to the 1994 CDC, a diagnosis of CFS requires that the following conditions be met (otherwise, the diagnosis is idiopathic chronic fatigue).

Primary symptom:

Incapacitating fatigue that is:

Additional symptoms:

The fatigue must be accompanied by a minimum of 4 of the following eight symptoms:

1. Impairment of short-term memory and concentration
2. Sore throat
3. Tender lymph nodes
4. Muscle pain
5. Multi-joint pain
6. Headaches of a new type, pattern, or severity
7. Unrefreshing sleep or insomnia
8. Post-exertional malaise or fatigue lasting more than 24 hours after exertion.

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