Transcutaneous electrical nerve stimulation: basic science mechanisms and clinical effectiveness.

Transcutaneous electrical nerve stimulation (TENS) is used clinically by a variety of health care professionals for the reduction of pain. Clinical effectiveness of TENS is controversial, with some studies supporting whereas others refute its clinical use. Although used by health professionals for decades, the mechanisms by which TENS produces analgesia or reduces pain are only recently being elucidated. This article describes the basic science mechanisms behind different frequencies of TENS stimulation. Specifically, we describe the literature that supports the use of different frequencies and intensities of TENS. We further describe theories that support the use of TENS such as the gate control theory and the release of endogenous opioids. The literature that supports or refutes each of these theories is described. We also review the clinical literature on TENS effectiveness and elucidate the problems with clinical research studies to date. In conclusion, TENS is a noninvasive modality that is easy to apply with relatively few contraindications. However, the clinical efficacy of TENS will remain equivocal until the publication of sufficient numbers of high quality, randomized, controlled clinical trials.