Women are affected more than men (3:1). Gender distribution is equal in childhood. Prevalence is between 2 and 16 %. Predominantly, the middle-age group is affected.

Risk factors are a history of depression, family history of mood disorder, poverty, disability, lower social class and infection with hep C, EB virus, parvovirus and Lyme disease.

It is a polygenetic disorder. Muscle changes contribute to the pathophysiology. The microcirculation is affected leading to decreased muscle tissue oxygenation. Typical “red ragged and moth-eaten” appearance is seen. Malfunction of the hypothalamic-pituitary-adrenal axis is seen. Increased levels of adrenocorticotropic hormone are seen with decreased levels of triiodothyronine, oestrogen and urinary cortisol. Patients exhibit decreased stage 4 non-REM sleep. Patients experience hyperalgesia and allodynia due to increased wind-up of fibres in dorsal horn and central sensitisation.