Predicting the spatiotemporal expression of local and referred acute muscle pain in individual subjects.

While current data suggest that all referred pain derives from common mechanisms of central sensitisation, there is a paucity of data directly comparing referral in different limbs. Does a common mechanism result in similar precepts of referral from similar stimuli in different limbs? We tested the hypothesis that, in a given subject, the incidence, intensity and spatiotemporal expression of referred pain are similar during the muscle pain induced by bolus intramuscular injection of hypertonic saline into flexor carpi radialis (FCR) and tibialis anterior (TA). We also tested the hypothesis that an increase in stimulus intensity causes a parallel increase in the incidence and intensity of local and referred pain, by comparing the responses to 5 and 10 % hypertonic saline in two groups of subjects. 29 subjects mapped areas of local and referred pain, rating intensities on a visual analogue scale every 30 s until the cessation of pain. Following an injection of 5 % hypertonic saline into TA, 86 % of subjects who had previously reported referred pain (or its absence) in the hand during FCR pain reported referred pain (or its absence) in the foot. Following an injection of the 10 % solution, 67 % of subjects reported a pattern in the lower limb that was the same as that seen in the upper limb. We conclude that the expression of referred pain is largely consistent in widely separated limb segments in individual subjects and is largely dependent on inter-subject differences. This may have implications for the development of chronic pain following an acute episode of pain.