Introduction and definitions

Fatigue is a very common complaint but is typically transient, self-limiting or explained by other circumstances. Chronic fatigue syndrome (CFS) is characterized by persistent or relapsing unexplained fatigue of new or definite onset lasting for at least six months. It is not a new condition and corresponds very clearly to an illness called neurasthenia, commonly seen in Europe around the turn of the twentieth century (Wessely et al., 1998). The terms ‘myalgic encephalomyelitis’ (ME) and ‘post-viral fatigue syndrome’ have also been used to describe CFS but are misleading and unsatisfactory: ME implies the occurrence of a distinct pathological process whereas post-viral fatigue syndrome wrongly suggests that all cases are preceded by a viral illness.

Operational criteria developed for research purposes by the US Centres for Disease Control and Prevention (CDC) (Fukuda et al., 1994) and from Oxford (Sharpe et al., 1991) are now widely used to define CFS. The American criteria require at least six months of persistent fatigue causing substantial functional impairment and at least four somatic symptoms (from a list of eight) occurring with the fatigue in a 6-month period. The presence of a medical disorder that explains the prolonged fatigue excludes a patient from a diagnosis of CFS, as do a number of psychiatric diagnoses. Although the British definition is similar it differs by requiring both physical and mental fatigue but no physical symptoms.