Hypersensitivity in muscle pain syndromes.

The aim of this review is to present research that has a bearing on the pathogenesis of hypersensitivity in muscle pain syndromes. Allodynia and hyperalgesia in these syndromes can be segmental or generalized and temporary or permanent. Hypersensitivity in muscle pain conditions in the clinic is best diagnosed by determining the pressure pain threshold. In a disorder such as fibromyalgia, decreased pain thresholds also are found at sites where there is no tenderness. Pathogenetic mechanisms for allodynia and hyperalgesia can be identified at several levels of the nociceptive system, from the nociceptors in the muscle to the cortex. Central sensitization of nociceptive neurons in the dorsal horn and a disturbed balance between inhibitory and facilitatory impulses in the descending tracts from the brain stem to the dorsal horn are the main mechanisms for pain hypersensitivity. Changes in function, biochemical make-up, and synaptic connections in the nociceptive neurons in the dorsal horn are considered to be caused by neuronal plasticity.