Qualitatively altered nociception in chronic myofascial pain

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Abstract
The aim of the present study was to investigate the stimulus-response function for pressure versus pain in patients with myofascial pain. Forty patients with chronic tension-type headache and 40 healthy controls were examined. Tenderness in 8 pericranial muscles and tendon insertions was evaluated by manual palpation with a standardized evaluated methodology. Thereafter, a highly tender muscle and a largely normal muscle were palpated with 7 different pressure intensities using a palpometer, and the induced pain was recorded by the subjects on a visual analogue scale blinded for the observer. Pericranial myofascial tenderness was considerably higher in patients than in controls (P < 0.00001). The stimulus-response function recorded from normal muscle was well described by a power function. From highly tender muscle, the stimulus-response function was displaced towards lower pressures and, more importantly, it was linear, i.e., qualitatively different from normal muscle. Our results demonstrate for the first time that nociceptive processes are qualitatively altered in patients with chronic myofascial pain and suggest that myofascial pain may be mediated by low-threshold mechanosensitive afferents projecting to sensitized dorsal horn neurons. Further investigations of these mechanisms may lead to an increased understanding and better treatment of these common and often incapacitation pain disorders.