Treatment with either high or low frequency TENS reduces the secondary hyperalgesia observed after injection of kaolin and carrageenan into the knee joint.

For years, physical therapists have been utilizing a variety of modalities, including transcutaneous electrical nerve stimulation (TENS), in an attempt to manage pain associated with inflammation. However, the data on clinical effectiveness is conflicting and the neurophysiological mechanism of action is not known. The purpose of this study was to investigate the effects of high and low frequency TENS on the secondary hyperalgesia that occurs after joint inflammation. Secondary hyperalgesia is thought to reflect changes in central neurons and is thus a measure of activity of central neurons. This study utilized the kaolin and carrageenan model of knee joint inflammation and measured the effects of TENS treatment on paw withdrawal latency to radiant heat (secondary hyperalgesia), spontaneous pain behaviors and joint circumference. Either high (100 Hz) or low (4 Hz) frequency TENS was applied to the knee joint for 20 min after the development of hyperalgesia. Both high and low frequency TENS resulted in a reversal of the hyperalgesia immediately following treatment. The effects of high frequency TENS lasted through at least 24 h while those of low frequency TENS lasted through 12 h. There was no effect of TENS on spontaneous pain behaviors or joint swelling when compared to controls. Thus, TENS appears to be more effective in reducing referred pain (or secondary hyperalgesia) without affecting guarding or splinting of the affected limb. Thus, clinically, the choice to use TENS may depend on patient symptoms; specifically TENS should be effective in reducing referred or radiating pain.