Chronic fatigue syndrome, or CFS, is a debilitating and complex disorder characterized by profound fatigue that is not improved by bed rest and that may be worsened by physical or mental activity. 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. 

A. Definition of CFS 

A great deal of debate has surrounded the issue of how best to define CFS. In an effort to resolve these issues, an international panel of CFS research experts convened in 1994 to draft a definition of CFS that would be useful both to researchers studying the illness and to clinicians diagnosing it. In essence, in order to receive a diagnosis of chronic fatigue syndrome, a patient must satisfy two criteria: 

1) Have severe chronic fatigue of six months or longer duration with other known medical conditions excluded by clinical diagnosis; and 2)  concurrently have four or more of the following symptoms: substantial impairment in short-term memory or concentration; sore throat; tender lymph nodes; muscle pain; multi-joint pain without swelling or redness; headaches of a new type, pattern or severity; unrefreshing sleep; and post-exertional malaise lasting more than 24 hours. The symptoms must have persisted or recurred during six or more consecutive months of illness and must not have predated the fatigue. 

B. Similar Medical Conditions 

A number of illnesses have been described that have a similar spectrum of symptoms to CFS. These include fibromyalgia syndrome, myalgic encephalomyelitis, neurasthenia, multiple chemical sensitivities, and chronic mononucleosis. Although these illnesses may present with a primary symptom other than fatigue, chronic fatigue is commonly associated with all of them. 

C. Other Conditions That May Cause Similar Symptoms 

In addition, there are a large number of clinically defined, frequently treatable illnesses that can result in fatigue. Diagnosis of any of these conditions would exclude a definition of CFS unless the condition has been treated sufficiently and no longer explains the fatigue and other symptoms. These include hypothyroidism, sleep apnea and narcolepsy, major depressive disorders, chronic mononucleosis, bipolar affective disorders, schizophrenia, eating disorders, cancer, autoimmune disease, hormonal disorders, subacute infections, obesity, alcohol or substance abuse, and reactions to prescribed medications.