See also MYALGIC ENCEPHALOMYELITIS (ME). A condition characterised by severe, disabling mental and physical fatigue brought on by mental or physical activity and associated with a range of symptoms including muscle pain, headaches, poor sleep, disturbed moods and impaired concentration. The prevalence of the condition is between 0.2 and 2.6 per cent of the population (depending on how investigators define CFS/ME). It is commoner in women and can also occur in children.

In the 19th century CFS was called neurasthenia. In the UK, myalgic encephalomyelitis (ME) is often used, a term originally introduced to describe a specific outbreak such as the one at the Royal Free Hospital, London in 1955. The term is inaccurate, as there is no evidence of inflammation of the brain and spinal cord (the meaning of encephalomyelitis). Doctors prefer the term CFS, but many patients see this as derogatory, perceiving it to imply that they are merely ‘tired all the time’ rather than having a disabling illness.

The cause (or causes) are unknown, so the condition is classified alongside other ‘medically unexplained syndromes’ such as IRRITABLE BOWEL SYNDROME (IBS) and multiple chemical sensitivity – all of which overlap with CFS. In many patients the illness seems to start immediately after a documented infection, such as that caused by EPSTEIN BARR VIRUS, or after viral MENINGITIS, Q FEVER and TOXOPLASMOSIS. These infections seem to be a trigger rather than a cause: mild immune activation is found in patients, but it is not known if this is cause or effect. The body's endocrine system is disturbed, particularly the hypothalamo-pituitary-adrenal axis, and levels of cortisol are often a little lower than normal – the opposite of what is found in severe depression. Psychiatric disorder, usually depression and/or anxiety, is associated with CFS, with rates too high to be explained solely as a reaction to the disability experienced.

Because the cause is not known, the underlying problem cannot be dealt with effectively, and treatments are directed at the factors leading to persistent symptoms. For example, a slow increase in physical activity can help many, as can COGNITIVE BEHAVIOUR THERAPY. Too much rest can be harmful, as muscles are rapidly weakened, but aggressive attempts at coercing patients into exercising can be counter-productive as their symptoms may worsen. Outcome is influenced by the presence of any pre-existing psychiatric disorder and the sufferer's beliefs about its causes and treatment.