Changes in the Spatiotemporal Expression of Local and Referred Pain Following Repeated Intramuscular Injections of Hypertonic Saline: A Longitudinal Study

Intramuscular injection of hypertonic saline produces a dull ache that is felt in the muscle belly but also often refers into distal structures. We have previously observed in 2 subjects that the pattern of pain referral alters during painful stimuli separated by a week. In this investigation, we tested the hypothesis that the intensity and area of pain in the local and referred regions exhibits plasticity when an identical noxious stimulus is delivered to the same site over sequential trials. Bolus 1 mL intramuscular injections of 5% hypertonic saline were made into the same site of the tibialis anterior (TA) muscle on the same day each week for 4 consecutive weeks. Twenty-one subjects mapped the areas of local and referred pain and rated the intensities on a visual analog scale every 30 seconds until the cessation of pain. Over 4 weeks there was a progressive reduction in the area and intensity of local pain and a reciprocal increase in the expression of referred pain. We conclude that the decrease in perceived local pain and increase in perceived referred pain reflects plastic processes occurring centrally.PerspectiveWhat happens to the intensity of pain induced by repeated noxious stimuli over time? Does it stay the same, increase or decrease? Here we show that weekly injections of hypertonic saline into the tibialis anterior cause decreases in local but increases in referred pain, suggesting central changes in processing noxious inputs.