Fibromyalgia syndrome (FM) is a common chronic pain condition that affects at least 2% of the adult population. Chronic widespread pain is the defining feature of FM, but patients may also exhibit a range of other symptoms, including sleep disturbance, fatigue, irritable bowel syndrome, headaches, and mood disorders. The etiology of FM is not completely understood and the syndrome is influenced by factors such as stress, medical illness, and a variety of pain conditions. Establishing diagnosis may be difficult because of the multifaceted nature of the syndrome and overlap with other chronically painful conditions. A unifying hypothesis is that FM results from sensitization of the central nervous system; this new concept could justify the variety of characteristics of the syndrome. FM symptoms can be musculoskeletal, non-musculoskeletal, or a combination of both; and many patients will also experience a host of associated symptoms or conditions. The ACR classification criteria focus only on pain and disregard other important symptoms; but three key features, pain, fatigue and sleep disturbance, are present in virtually every patient with FM. Several other associated syndromes, including circulatory, nervous, digestive, urinary and reproductive systems are probably a part of the so called central sensitivity or sensitization syndrome. A minority subgroup of patients (30-40%) has a significant psychological disturbance. Psychological factors are an important determinant of any type of pain, and psychological comorbidity is frequent in FM. Psychiatric disorders most commonly described are mood disorders, but psychiatric illness is not a necessary factor in the etiopathogenesis of FM.
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