The effect of varying frequency and intensity of transcutaneous electrical nerve stimulation on secondary mechanical hyperalgesia in an animal model of inflammation

For years, transcutaneous electrical nerve stimulation (TENS) has been used clinically for the treatment of many types of pain. Although there have been many studies conducted on the efficacy of TENS in the clinical setting, the results are conflicting. The purpose of our investigation was to determine the effect of varying frequency and intensity of TENS on secondary mechanical hyperalgesia induced by acute joint inflammation. Male Sprague-Dawley rats were injected with a mixture of 3% carrageenan and 3% kaolin (100 muL in 0.9% sterile saline) into the joint cavity of one knee. The response threshold to mechanical stimuli was determined before inflammation of the knee joint; 4 hours after inflammation; immediately after the administration of TENS (approximately 5 hours after inflammation); and at 8, 12, and 24 hours after inflammation. TENS was applied to the inflamed knee joint at either high (100 Hz) or low (4 Hz) frequency and at either sensory or motor intensity. Sensory intensity was just below the threshold for motor contraction, and motor intensity was 2 x threshold for motor contraction. Either low- or high-frequency TENS is equally successful in reducing secondary mechanical hyperalgesia. Similarly, either sensory- or motor-intensity TENS equally reduces secondary mechanical hyperalgesia. Thus, selection of TENS should be based on patient comfort and symptoms for relief of secondary mechanical hyperalgesia.