Stimulus-response functions in areas with experimentally induced referred muscle pain--a psychophysical study

Few clinical or experimental studies have carried out systematic investigations of cutaneous and deep sensibility in areas with referred muscle pain. Therefore, no clear signs of increased or decreased psychophysical responses to various somatosensory stimuli are found in referred pain areas. In the present study, a total of 7.1 ml 5% hypertonic saline was infused over 900 s into the m. tibialis anterior of 11 subjects. This produced local muscle pain and pain referred to the ventral aspect of the ankle. A continuous recording of the ongoing pain intensities of the local and referred pain was carried out on two electronic visual analogue scales (VAS). Before, during and 30 min after the period with referred pain, radiant heat (argon laser) stimuli, single and repeated electrical stimuli, and pressure stimuli were applied to the referred pain area. Stimulus-response (SR) functions were obtained by means of pain intensity ratings of the different stimuli at 75%, 112.5% and 150% of the individual pain threshold (PT) intensity. The pain intensities of contact heat (thermode) stimuli at 40 degrees C, 47 degrees C and 50 degrees C and pin-prick stimuli with von Frey hair were also assessed in the referred pain area. The saline-induced local muscle pain intensity was higher than the intensity of the referred pain (P < 0.05). The referred pain intensity was significantly higher in the 20-460 s interval than in the 460-900 s interval (P < 0.05). This difference was not seen for the local muscle pain. During the period with referred pain, significantly decreased responses to radiant heat and pressure stimuli were found at 112.5% and 150% of PT intensity (P < 0.05). Further, significantly increased responses to single and repeated electrical stimuli at 75% and 112.5% of PT intensity (P < 0.05) were also found. After the period with referred pain, a considerably decreased response to single and repeated, electrical stimuli (P < 0.05) was present together with significantly increased responses to contact heat stimuli at 40 degrees C and radiant heat stimuli at 75% of PT intensity (P < 0.05). The present results suggest that ongoing muscle pain can cause modality-specific (and bi-directional) sensory changes in the referred pain area. This could explain why previous studies have reported both decreased and increased responses in referred pain areas.