The effect of varying frequency and intensity of transcutaneous electricalnerve 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 be en many studies conducted on the efficacy of TENS in the clinical setting, the results are conflicting. The purpose of our investigation was to determ ine the effect of varying frequency and intensity of TENS on secondary mech anical hyperalgesia induced by acute joint inflammation. Male Sprague-Dawle y rats were injected with a mixture of 3% carrageenan and 3% kaolin (100 mu L in 0.9% sterile saline) into the joint cavity of one knee. The response t hreshold to mechanical stimuli was determined before inflammation of the kn ee joint; 4 hours after inflammation; immediately after the administration of TENS (approximately 5 hours after inflammation); and at 8, 12, and 24 ho urs after inflammation. TENS was applied to the inflamed knee joint at eith er high (100 Hz) or low (4 Hz) frequency and at either sensory or motor int ensity. Sensory intensity was just below the threshold for motor contractio n, and motor intensity was 2 x threshold for motor contraction. Either low- or high-frequency TENS is equally successful in reducing secondary mechani cal hyperalgesia. Similarly, either sensory- or motor-intensity TENS equall y reduces secondary mechanical hyperalgesia. Thus, selection of TENS should be based on patient comfort and symptoms for relief of secondary mechanica l hyperalgesia.