Transcutaneous electrical nerve stimulation, TENS
Transcutaneous electrical nerve stimulation, TENS, is a popular self-help and clinical approach, mainly aiming to deal with pain of all kinds. It is, however, controversial if TENS has any clinically significant effects, and if so what exactly the effects are. Its popularity is likely to originate from the over-the-counter availability of affordable, small battery-powered TENS gadgets, ease of use, a high probability that no (or: no serious) harm is done, and the absence of need for systemic analgesia. TENS has been reported as stand-alone and as supportive treatment modality in pain management.
The idea behind TENS relates to an effect that most people will have experienced themselves. For example, if one bumps one’s head, rubbing the painful area seems to moderate / relieve the pain. This effect suggests that some form of local stimulus can modify the perception of pain. Electrical currents are one such type of stimulus and are known to be easily deliverable to body areas, nerves and other tissues near the surface, through intact skin (transcutaneous). This is achieved by placing small electrodes on the skin in the appropriate area and using a variety of regimens which deliver small intensities of electrical current to the area (different regimens of duration, amplitudes, frequencies, pulse forms and repetition). If there are any benefits to be had from TENS, it may make good sense to discuss the best use and regimens with a health professional with experience in the field instead of going purely on the DIY route.
The most commonly quoted hypothesis to explain TENS effects is the so called gate control theory of pain, developed in the 1960s. This theory suggests that if small peripheral nerve fibres are engaged in transmitting pain-related signals to the central nervous system (brain and spinal cord), a simultaneous activation of nerve fibres which normally transmit simply sensory information from the periphery to the central nervous system can act to reduce the pain experience perceived (‘closing the pain gate’). It was also suggested that this ‘pain gate’ could be closed by distracting parallel activities to pain signals being transmitted from brain / spinal cord to the body. An example of such an effect would be an injury sustained during a sports event where the athlete only notices pain after the event.
The many studies carried out on TENS and related effects over the past 20 years lack consistency. For some of the observations in the laboratory about specific mechanisms at work (for example, GABA being involved in the signalling pathways, or the release of endogenous opioids, or changes in the conduction properties of certain types of nerve fibres engaged in nociception, Aδ fibres) it is not clear if they translate to any clinically significant effects.
Whether TENS is clinically effective or not is an unresolved question. A quote from a textbook on TENS nicely summarises: ‘… You can be certain of uncertainty when evaluating the clinical effectiveness of TENS. …’. It is less for a scarcity of studies than for the poor quality and poor design of many studies that there is no robust evidence. On the basis of existing literature it is not possible to come to valid conclusions about effectiveness, potential harm and adverse effects of TENS. TENS itself comes with a substantial placebo effect, so any valid evaluation beyond the placebo effect is not straightforward, especially if no major effects are to be expected anyway. Unsurprisingly, given the lack of robust evidence, there are no consistent clinical guidelines about TENS. Similar to what we said about biofeedback or acupuncture, if somebody finds that TENS helps them manage pain, why not use it as long as it does not cause harm (other than relatively minor effects on the wallet).