According to a recent evidence review and guidelines in the journal Neurology, the popular pain therapy using a method called TENS — transcutaneous electric nerve stimulation — is largely ineffective for treating chronic low-back pain. However, a closer look suggests that this conclusion may be premature and the research is insufficient to completely dismiss TENS' importance as a pain treatment option.
The TENS unit is a small, portable device delivering mild electrical current to nerves through electrodes connected to the skin at the sites of pain. The approach has been used for various types of pain, although exact mechanisms of its analgesic effects are still under examination. Basic science studies show that TENS activates endogenous pain-control chemicals (eg, endorphins, enkephalins, dynorphins, GABA, serotonin) and their receptors. Along with that, the high and low electrical frequencies produced by the TENS unit inhibit pain signals along affected nerves, blocking the impulses from reaching pain-perception areas in the brain.
Clinical trials on the efficacy of TENS have produced some mixed results; therefore, practice guidelines developers representing the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology analyzed data in TENS studies to reassess its efficacy [Dubinsky and Miyasaki 2009]. Their systematic search of the literature encompassed Medline and the Cochrane Library from inception to April 2009, focusing on TENS for low back pain and in painful diabetic neuropathy(PDN).
The search produced only 7 eligible TENS trials, 5 for low back pain and 2 for PDN. For back pain, 2 moderate quality trials showed pain-relieving benefits of TENS, while 1 moderate quality and 2 high quality trials (randomized, placebo/sham-therapy-controlled) failed to show significant benefits. For PDN, both trials were of moderate quality but demonstrated significant TENS benefits in relieving pain. Based on these findings, particularly the quality of the evidence, the guidelines authors make a strong recommendation against TENS for the treatment of chronic low back pain (Level A – established as ineffective). They note that TENS could be considered in the treatment of PDN, but the recommendation is modest (Level B – probably useful). Furthermore, they note that research into the mechanism of action of TENS is needed, as well as more rigorous efficacy studies.
Commentary: Popular news media widely reported on the conclusions, with headlines touting TENS as being worthless for back pain. CNN Health (12/31/09) quoted lead author of the guidelines article — Richard M. Dubinsky, MD, MPH — as saying, "Physicians are advised against ordering TENS for patients with chronic low back pain since it is proven not to work." Actually, the evidence does not “prove” anything, the conclusions seem rather tenuous at best, and this may qualify as an example of what we have previously called “Misguided Guidelines” [see article here].
First, a literature search of only Medline and Cochrane Reviews may not be considered comprehensive.It is of some interest how reviews and meta-analyses by different investigators using alternate methods can produce remarkably dissimilar results. Johnson and Martinson  searched several databases for randomized controlled trials examining the efficacy of electrical nerve stimulation for chronic musculoskeletal pain and discovered for analysis 38 trials in 29 papers published between 1976 and 2006 that encompassed nearly 900 patients having active therapy compared with about 750 in placebo/sham therapy groups. Using sophisticated methods of analysis to achieve adequate statistical power, they demonstrated highly significant decreases in pain (p < 0.0005) afforded by electrical nerve stimulation. While this analysis encompassed modalities and conditions in addition to TENS and back pain, it appears to be a “confirmation of concept” offering a perspective that is contrary to the strongly negative conclusions of Dubinsky and Miyasaki .
Second, it seems somewhat presumptuous to arrive at a strong clinical recommendation against TENS for low-back pain based on a total of 178 patients treated with TENS in only 5 trials, 3 of which were inadequately powered to produce valid results.
Third, in an editorial accompanying the guidelines article, Andreas Binder, MD, and Ralf Baron, MD , write that the conclusions "may heat up the discussion on the usability of TENS and may be viewed as supporting the critics who questioned the value of TENS in pain therapy”; however, they stress, "absence of evidence is not evidence of absence. The clinical impact of meta-analyses is always limited by the quantity and quality of conducted trials." If anything is ineffective or deficient at this time it would appear to be the quality and quantity of research on TENS for developing definitive practice guidelines.
In their editorial, Binder and Baron  emphasize that TENS has had a long-standing role in pain care, is easy to manage, has a favorable benefit-to-risk ratio, and can be discontinued easily if it is not clinically effective — all "desirable properties when treating pain." They concede that the review by Dubinsky and Miyasaki may be helpful in defining current limitations of the evidence base for TENS; however, they suggest that "it is not unreasonable to take a practical position that, in spite of the relatively weak scientific and clinical evidence, TENS still represents a valuable therapeutic alternative in neurologic pain disorders.”
Disclosure: We have no vested interests in or support from any manufacturers of electrical nerve stimulation devices, including TENS. Our only interest here is in arriving at sound practice decisions in pain management based on appropriately credible, reliable, and valid analyses of clinical evidence. — SBL
> Binder A, Baron R. Utility of transcutaneous electrical nerve stimulation in neurologic pain disorders [editorial]. Neurology 2009 (Dec 30 online ahead of print) [access by subscription here].
> Dubinsky RM, Miyasaki J. Assessment: Efficacy of transcutaneous electric nerve stimulation in the treatment of pain in neurologic disorders (an evidence-based review). Report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology. Neurology. 2009 (Dec 30 online ahead of print) [abstract here].
> Johnson M, Martinson M. Efficacy of electrical nerve stimulation for chronic musculoskeletal pain: A meta-analysis of randomized controlled trials. Pain. 2007;130(1):157-165 [abstract here].