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Is fibromyalgia a distinct clinical entity? The epidemiologist's evidence.

The key features of fibromyalgia are chronic widespread pain, general distress and enhanced pain sensitivity as revealed by a tender point count. Fibromyalgia could be considered a distinct entity if the key features defined a patient group with any one of the following: a characteristics feature or cluster of features with a distribution clearly demarcating the group from the rest of the population, identifiable risk factors, a characteristic natural course or prognosis, or a specific response to treatment. This should be seen in population studies, not only in selected clinic series. The available epidemiological evidence suggests that the distributions of the key features are continuous and no clear population groups can be defined. Also, difficulties in identifying changes such as incidence and recovery, and unreliable measurements of the key features themselves, detract from the utility of the concept of fibromyalgia as an epidemiological entity.