Chronic fatigue syndrome (CFS) is an illness characterized by prolonged, debilitating fatigue and multiple nonspecific symptoms such as headaches, recurrent sore throats, muscle and joint pains, and cognitive complaints. Profound fatigue, the hallmark of the disorder, can come on suddenly or gradually and persists or recurs throughout the period of illness. Unlike the short-term disability of an acute infection, CFS symptoms by definition linger for at least 6 months and often for years.

An estimated one-quarter of all patients seeing general practitioners complain of prolonged fatigue, a symptom common to many illnesses. Several studies indicate that only a small fraction of these patients meet the criteria for chronic fatigue syndrome.

To identify people with CFS, physicians can evaluate patients with persistent fatigue of undetermined cause using the CFS definition in the appendix as a guide. This description, developed by the International CFS Study Group and published in the Annals of Internal Medicine in December 1994, replaces the first research case definition published 6 years earlier. The revised, more inclusive criteria define CFS and other cases of unexplained prolonged fatigue, suggest how to further subdivide these groups for research studies, and include guidelines for nonresearch clinicians as well. The case criteria will continue to be reviewed and refined as new information warrants.

Despite multidisciplinary investigations into the cause of CFS, its etiology remains unknown. Similarly, no specific diagnostic tests or therapies for CFS exist. A supportive program of patient management--including symptom-based treatment, education about the disease, and regular followup visits to rule out alternative diagnoses--can offer reassurance, dispel unfounded beliefs about CFS or its treatment, and help patients and their families adjust to living with this chronic illness.

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