What is chronic fatigue syndrome (CFS)?

Updated: Aug 29, 2018
  • Author: Jefferson R Roberts, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Chronic fatigue syndrome (CFS) is a disorder characterized by unexplained profound fatigue that is worsened by exertion. The fatigue is accompanied by cognitive dysfunction and impairment of daily functioning that persists for more than 6 months.

CFS was originally termed myalgic encephalomyelitis (ME) because British clinicians noted a skeletal muscle component manifesting as chronic fatigue and an encephalitic component manifesting as cognitive difficulties. However, this term is inappropriate because of a lack of encephalomyelitis in laboratory and imaging studies, and myalgia is not a core symptom of the disease. [1]

In 2015, the Institute of Medicine (IOM) proposed that the condition be renamed systemic exertion intolerance disease (SEID) to better reflect the condition's hallmark defining symptom, postexertional malaise. [2]

​The cause of CFS is unknown, and there are no direct tests to diagnose CFS. If the source of the fatigue can be explained, the patient probably does not have CFS. The diagnosis is one of exclusion that meets the clinical criteria below.

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