Local heat effect on sympathetic skin responses after pain of electrical stimulus

Objective: To investigate the analgesic effect of local superficial heating by studying sympathetic skin responses. Design: Randomized trial. Setting: Electromyography laboratory in the department of physical therapy and rehabilitation of a university hospital. Subjects: Twenty healthy volunteers participated with informed consent. Interventions: Sympathetic skin response (SSR) amplitudes following electrical stimulation of the right peroneal nerve and skin temperatures in both hands were recorded simultaneously. All of the recordings were repeated at 5-minute intervals during local heat application over the right palm and within 15 minutes after heat application was stopped. Results: SSR amplitudes in both hands decreased significantly during local heating ( p < .05) and did not return to their initial levels within 15 minutes of the recovery period; the reductions remained statistically significant ( p < .05). Amplitude reductions were statistically more significant on the heated hand compared with those on the contralateral hand ( p < .05). Conclusion: Therapeutic local heat application reduces the sudomotor response to a painful stimulus. This analgesic effect may be due to suppression of cortical pain sensation resulting from increased levels of endorphins, and may also be a result of local inhibition of both afferent and efferent C fibres.