Transcutaneous electrical nerve stimulation (TENS or TNS) is the use of electric current produced by a device to stimulate the nerves for therapeutic purposes. TENS, by definition, covers the complete range of transcutaneously applied currents used for nerve excitation although the term is often used with a more restrictive intent, namely to describe the kind of pulses produced by portable stimulators used to treat pain. The unit is usually connected to the skin using two or more electrodes. A typical battery-operated TENS unit is able to modulate pulse width, frequency and intensity. Generally TENS is applied at high frequency (>50 Hz) with an intensity below motor contraction (sensory intensity) or low frequency (<10 Hz) with an intensity that produces motor contraction. While the use of TENS has proved effective in clinical studies, there is controversy over which conditions the device should be used to treat.
TENS devices available to the domestic market are used as a non-invasive nerve stimulation intended to reduce both acute and chronic pain. One review from 2007 felt that the evidence supports a benefit in chronic musculoskeletal pain while another review (from the Cochrane Collaboration in 2008) deemed the evidence of poor quality and thus no conclusions were possible regarding chronic pain. Results from a task force on neck pain in 2008 found no clinically significant benefit to TENS for the treatment of neck pain when compared to a placebo treatment. A 2010 review did not find evidence to support the use of TENS for chronic low back pain. There is tentative evidence that it may be useful for painful diabetic neuropathy. As of 2015, the efficacy of TENS therapy for phantom limb pain is not known as no randomized controlled trials have been performed.
In principle, an adequate intensity of stimulation is necessary to achieve pain relief with TENS. An analysis of treatment fidelity (meaning that the delivery of TENS in a trial was in accordance with current clinical advice, such as using “a strong but comfortable sensation” and suitable, frequent treatment durations) showed that higher fidelity trials tended to have a positive outcome.
A few studies have shown objective evidence that TENS may modulate or suppress pain signals in the brain. One used evoked cortical potentials to show that electric stimulation of peripheral A-beta sensory fibers reliably suppressed A-delta fiber nociceptive processing. Two other studies used functional magnetic resonance imaging (fMRI): one showed that high-frequency TENS produced a decrease in pain-related cortical activations in patients with carpal tunnel syndrome, while the other showed that low-frequency TENS decreased shoulder impingement pain and modulated pain-induced activation in the brain.
For use in Childbirth/Labor the TENS has mixed reviews. A few studies have been conducted on the efficacy of TENS devices for pain management, some found them to be helpful, others did not.