Chronic Fatigue Syndrome, or Myalgic Encephalomyelitis (ME), Page 1

Chronic Fatigue Syndrome (CFS), or Myalgic Encephalomyelitis (ME), is characterized by profound fatigue that is not improved by bed rest and that may be worsened by physical or mental activity.

What is CFS?

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; and2) 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.

D. Other Commonly Observed Symptoms in CFS

In addition to the eight primary defining symptoms of CFS, a number of other symptoms have been reported by some CFS patients. The frequencies of occurrence of these symptoms vary from 20% to 50% among CFS patients. They include abdominal pain, alcohol intolerance, bloating, chest pain, chronic cough, diarrhea, dizziness, dry eyes or mouth, earaches, irregular heartbeat, jaw pain, morning stiffness, nausea, night sweats, psychological problems (depression, irritability, anxiety, panic attacks), shortness of breath, skin sensations, tingling sensations, and weight loss.

* Not all hormonal aberrations necessarily exclude a diagnosis of CFS. See Section 3C.


Several studies have helped to establish the distribution and frequency of occurrence of CFS. While no single study can be considered definitive ó each approach has inherent strengths and weaknesses ó epidemiologic studies have greatly improved our understanding of how common the disease is, which individuals are the most susceptible to developing it, whether it can be transmitted to others, and how the illness typically progresses in individuals.

A. How Common Is CFS?

One of the earliest attempts to estimate the prevalence of CFS was conducted by the Centers for Disease Control and Prevention (CDC) from 1989 to 1993. Physicians in four U.S. cities were asked to refer possible CFS patients for clinical evaluation by medical personnel participating in the study. The study estimated that between 4.0 and 8.7 per 100,000 persons 18 years of age or older have CFS and are under medical care. However, these projections were underestimates and could not be generalized to the U.S. population since the study did not randomly select its sites. A more recent study of the Seattle area has estimated that CFS affects between 75 and 265 people per 100,000 population. This estimate is similar to the prevalence observed in another CDC study conducted in San Francisco, which put the occurrence of CFS-like disease (not clinically diagnosed) at approximately 200 per 100,000 persons. In general, it is estimated that perhaps as many as half a million persons in the United States have a CFS-like condition.

B. Who Gets CFS?

This question is complex and does not have a definitive answer. The CDC four-city surveillance study of CFS identified a population of patients that was 98% Caucasian and 85% female, with an average age at onset of 30 years. More than 80% had advanced education and one-third were from upper income families. However, these data included only patients who were under a physician’s care. There is now evidence that CFS affects all racial and ethnic groups and both sexes. The Seattle study found that 59% of the CFS patients were women. Eighty-three percent were Caucasian, an underrepresentation, since over 90% of the patients in the study were white. CDC’s San Francisco study found that CFS-like disease was most prevalent among women, among persons with household annual incomes of under $40,000, and among blacks, and was least common among Asians and whites. Adolescents can have CFS, but few studies of adolescents have been published. A recently published CDC study documented that adolescents 12 to 18 years of age had CFS significantly less frequently than adults and did not identify CFS in children under 12 years of age. CFS-like illness has been reported in children under 12 by some investigators, although the symptom pattern varies somewhat from that seen in adults and adolescents. The illness in adolescents has many of the same characteristics as it has in adults. However, it is particularly important that the unique problems of chronically ill adolescents (e.g., family social and health interactions, education, social interactions with peers) be considered as a part of their care. Appropriate dissemination of CFS information to patients, their families, and school authorities is also important. CDC and the National Institutes of Health (NIH) are currently pursuing studies of CFS in children and adolescents.

C. Is CFS Contagious?

There is no evidence to support the view that CFS is a contagious disease. Contagious diseases typically occur in well-defined clusters, otherwise known as outbreaks or epidemics. While some earlier studies, such as investigations of fatiguing illness in Incline Village, Nev., and Punta Gorda, Fla., have been cited as evidence for CFS acting as a contagious illness, they did not rigorously document the occurrence of person-to-person transmission. In addition, none of these studies included patients with clinically evaluated fatigue that fit the CFS case definition; therefore, these clusters of cases cannot be construed as outbreaks of CFS. CDC worked with state health departments to investigate a number of reported outbreaks of fatiguing illness and has yet to confirm a cluster of CFS cases. Implicit in any contagious illness is an infectious cause for the disease.

Carefully designed case-control studies involving rigorously classified CFS patients and controls have found no association between CFS and a large number of human disease agents (see Possible Causes of CFS). Finally, none of the behavioral characteristics typically associated with contagious disease, such as intravenous drug use, exposure to animals, occupational or travel history, or sexual behavior, have been associated with CFS in case-control studies. It therefore seems unlikely that CFS is a transmissible disease. Nevertheless, the lack of evidence for clustering of CFS, the absence of associations between specific behavioral characteristics and CFS, and the failure to detect evidence of infection more commonly in CFS patients than in controls do not rule out the possibility that infectious agents are involved in or reflect the development of this illness. For example, important questions remain to be answered concerning possible reactivation of latent viruses (such as human herpesviruses) and a possible role for infectious agents in some cases of CFS.

D. Clinical Course of CFS

It is vital to understand the clinical course of CFS. This knowledge is required to facilitate communication between physicians and patients, to evaluate possible new treatments, and to address insurance and disability issues. The clinical course of CFS varies considerably among persons who have the disorder; the actual percentage of patients who recover is unknown, and even the definition of what should be considered recovery is subject to debate. Some patients recover to the point that they can resume work and other activities, but continue to experience various or periodic CFS symptoms. Some patients recover completely with time, and some grow progressively worse. CFS often follows a cyclical course, alternating between periods of illness and relative well being. CDC continues to monitor the patients enrolled in the four-city surveillance study; recovery is defined by the patient and may not reflect complete symptom-free recovery. Approximately 50% of patients reported “recovery,” and most recovered within the first 5 years after onset of illness. No characteristics were identified that made one patient more likely to recover than another. At illness onset, the most commonly reported CFS symptoms were sore throat, fever, muscle pain, and muscle weakness. As the illness progressed, muscle pain and forgetfulness increased and the reporting of depression decreased.

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