Abstract

Background

Chronic pain, considered to be pain lasting more than three months, is a common and often difficult to treat condition that can significantly impact upon function and quality of life. Treatment typically includes pharmacological and non‐pharmacological approaches. Transcutaneous electrical nerve stimulation (TENS) is an adjunct non‐pharmacological treatment commonly recommended by clinicians and often used by people with pain.

Objectives

To provide an overview of evidence from Cochrane Reviews of the effectiveness of TENS to reduce pain in adults with chronic pain (excluding headache or migraine).

To provide an overview of evidence from Cochrane Reviews of the safety of TENS when used to reduce pain in adults with chronic pain (excluding headache or migraine).

To identify possible sources of inconsistency in the approaches taken to evaluating the evidence related to TENS for chronic pain (excluding headache or migraine) in the Cochrane Library with a view to recommending strategies to improve consistency in methodology and reporting.

To highlight areas of remaining uncertainty regarding the effectiveness of TENS for chronic pain (excluding headache or migraine) with a view to recommending strategies to reduce any uncertainty.

Methods

Search methods

We searched the Cochrane Database of Systematic Reviews (CDSR), in the Cochrane Library, across all years up to Issue 11 of 12, 2018.

Selection of reviews

Two authors independently screened the results of the electronic search by title and abstract against inclusion/exclusion criteria. We included all Cochrane Reviews of randomised controlled trials (RCTs) assessing the effectiveness of TENS in people with chronic pain. We included reviews if they investigated the following: TENS versus sham; TENS versus usual care or no treatment/waiting list control; TENS plus active intervention versus active intervention alone; comparisons between different types of TENS; or TENS delivered using different stimulation parameters.

Data extraction and analysis

Two authors independently extracted relevant data, assessed review quality using the AMSTAR checklist and applied GRADE judgements where required to individual reviews. Our primary outcomes included pain intensity and nature/incidence of adverse effects; our secondary outcomes included disability, health‐related quality of life, analgesic medication use and participant global impression of change.

Main results

We included nine reviews investigating TENS use in people with defined chronic pain or in people with chronic conditions associated with ongoing pain. One review investigating TENS for phantom or stump‐associated pain in people following amputation did not have any included studies. We therefore extracted data from eight reviews which represented 51 TENS‐related RCTs representing 2895 TENS‐comparison participants entered into the studies.

The included reviews followed consistent methods and achieved overall high scores on the AMSTAR checklist. The evidence reported within each review was consistently rated as very low quality. Using review authors' assessment of risk of bias, there were significant methodological limitations in included studies; and for all reviews, sample sizes were consistently small (the majority of studies included fewer than 50 participants per group).

Six of the eight reviews presented a narrative synthesis of included studies. Two reviews reported a pooled analysis.

Primary and secondary outcomes

One review reported a beneficial effect of TENS versus sham therapy at reducing pain intensity on a 0 to 10 scale (MD −1.58, 95% CI −2.08 to −1.09, P < 0.001, I² = 29%, P = 0.22, 5 studies, 207 participants). However the quality of the evidence was very low due to significant methodological limitations and imprecision. A second review investigating pain intensity performed a pooled analysis by combining studies that compared TENS to sham with studies that compared TENS to no intervention (SMD −0.85, 95% CI −1.36 to −0.34, P = 0.001, I² = 83%, P < 0.001). This pooled analysis was judged as offering very low quality evidence due to significant methodological limitations, large between‐trial heterogeneity and imprecision. We considered the approach of combining sham and no intervention data to be problematic since we would predict these different comparisons may be estimating different true effects. All remaining reviews also reported pain intensity as an outcome measure; however the data were presented in narrative review form only.

Due to methodological limitation and lack of useable data, we were unable to offer any meaningful report on the remaining primary outcome regarding nature/incidence of adverse effects, nor for the remaining secondary outcomes: disability, health‐related quality of life, analgesic medication use and participant global impression of change for any comparisons.

We found the included reviews had a number of inconsistencies when evaluating the evidence from TENS studies. Approaches to assessing risk of bias around the participant, personnel and outcome‐assessor blinding were perhaps the most obvious area of difference across included reviews. We also found wide variability in terms of primary and secondary outcome measures, and inclusion/exclusion criteria for studies varied with respect to including studies which assessed immediate effects of single interventions.

Authors' conclusions

We found the methodological quality of the reviews was good, but quality of the evidence within them was very low. We were therefore unable to conclude with any confidence that, in people with chronic pain, TENS is harmful, or beneficial for pain control, disability, health‐related quality of life, use of pain relieving medicines, or global impression of change. We make recommendations with respect to future TENS study designs which may meaningfully reduce the uncertainty relating to the effectiveness of this treatment in people with chronic pain.

Author(s)

William Gibson, Benedict M Wand, Catherine Meads, Mark J Catley, Neil E O'Connell

Abstract

Plain language summary

Transcutaneous electrical nerve stimulation (TENS) for chronic pain ‐ an overview of Cochrane Reviews

Bottom line

For people with chronic pain, this overview of Cochrane Reviews found it was not possible to confidently state whether TENS is effective in relieving pain compared to sham TENS, usual care/no treatment or when TENS is combined with another active treatment versus the active treatment alone. We were unable to find any reliable evidence that the effectiveness of TENS varies when using different delivery modes (e.g. different frequency, intensity or electrode placement).

Background

Chronic pain (pain for longer than three months) is associated with a range of common conditions and can be difficult to treat effectively. TENS is a common treatment for pain conditions and involves using a small battery‐operated unit to apply low‐intensity electrical current to the body using electrodes attached to the skin. This is suggested to relieve pain. TENS has been previously investigated by a number of Cochrane Reviews.

Review question

By identifying relevant Cochrane Reviews on TENS for common chronic pain conditions, we investigated whether TENS is effective in reducing pain in adults with chronic pain (excluding headache or migraine).

Study characteristics

As of November 2018, we found nine reviews eligible for inclusion. Seven reviews specifically investigated TENS for the treatment of pain/function in a variety of chronic conditions in adults. We also included one review investigating a range of electrotherapy modalities for neck pain and one review examining non‐pharmacological interventions in people with spinal cord injury. Both of these reviews included studies investigating TENS. Though the included reviews were of high quality, we found the quality of the evidence presented within the reviews to be very low.

Key findings

We are unable to confidently state whether TENS is effective in relieving pain in people with chronic pain. This is due to the very low quality of the evidence, and the overall small numbers of participants included in studies in the reviews. Issues with quality, study size and lack of data meant we were unable to draw any conclusion on TENS‐associated harms or side‐effects or the effect of TENS on disability, health‐related quality of life, use of pain‐relieving medicines or people's impression of how much TENS changed their condition.

Author(s)

William Gibson, Benedict M Wand, Catherine Meads, Mark J Catley, Neil E O'Connell

Reviewer's Conclusions

Authors' conclusions 

Implications for practice 

For people with chronic pain

This overview offers a summation of very low quality evidence and we cannot confidently make any statement regarding the effectiveness of TENS for people with chronic pain. The very low quality of all reviewed evidence means we have very limited confidence in any suggested estimate of effect for all outcomes and the true effect is likely to be different from that summarised here and within individual reviews. A number of reviews reported minor skin irritation at the site of application, one review included one study in which a participant developed a severe skin rash following sham TENS use. Typically, reviews also included studies reporting either no adverse events or did not report adverse events. We therefore cannot make any meaningful comment on adverse events associated with TENS.

For clinicians

This overview is unable to derive any conclusions regarding the efficacy/effectiveness of 1) TENS versus sham, 2) TENS versus usual care or no treatment or waiting list control, 3) TENS plus active intervention versus active intervention alone or 4) comparisons between different types of TENS or TENS delivered using different stimulation parameters in people with chronic pain for pain intensity, disability, health‐related quality of life, analgesic medication use or participant impression of change. This is due to limited data, methodological limitations (with subsequent risk of bias) and predominantly small sample sizes leading to the evidence within all reviews being assessed as very low quality. This means estimates of effect summarised here and within individual reviews should be viewed with very limited confidence and the true effect is likely to be different from that reported here. A number of reviews reported mainly minor skin irritation (one case of severe rash in one review), while the remainder either reported no adverse events or did not report on adverse events. We were unable to make any statement regarding risk of adverse events with TENS for chronic pain.

For policy makers and funders

This overview provides no evidence to either support or refute the use of TENS in people with chronic pain. The conclusions reported in this overview reflect review results derived from studies that had overall substantial methodological limitations and were predominantly small in size.

Implications for research 

Design of new trials

The overwhelming factors limiting the accurate estimation of effectiveness in TENS for chronic pain are the methodological limitations of studies from all included reviews. Analysis of risk of bias in the reviews reveals a consistent pattern with multiple ratings of high or unclear risk of bias decisions in the domains of allocation concealment, blinding (participants, personnel and outcome assessors), incomplete outcome data, selective reporting and size of study. This speaks to a problem of research waste in TENS research.

Clear published guidelines on reporting of study design for non‐pharmacological treatments are available in the public domain through the CONSORT statement and associated checklist (Boutron 2017). Careful analysis and implementation of the checklist into study design would greatly improve many of the common methodological and reporting problems seen in TENS for chronic pain studies. A key part of this should include clear descriptions of the intervention. TENS delivery encompasses multiple factors (frequency of applied stimulation, intensity of stimulation, duration, frequency of application etc.) which may influence outcome and a critical review of methodological quality in TENS studies has been published which may usefully inform future work (Bennett 2011). TENS may be considered a complex non‐pharmacologic intervention and published checklists of templates for intervention description and replication are available (TIDieR checklist) which are specifically designed to assist in reporting of complex interventions (Hoffmann 2014). Future researchers and systematic review authors would benefit from the implementation of this template into TENS research designs.

Blinding of participants and care providers in physical interventions is an acknowledged difficulty. However, the observed variation in efforts to maintain naivety of participants/personnel to sham TENS in this overview is another source of ambiguity in estimates of effect of TENS for chronic pain. Devices are now available which appear 'live', deliver initial current before fading to zero and are suggested as being viable devices to maintain blinding (Rakel 2010). Efforts to use similar sham devices combined with the good sham TENS practice employed by Buchmuller 2012, and Dailey 2013 are worth considering for future studies.

TENS is a simple‐to‐use, portable, self‐administered and relatively inexpensive treatment intervention. With this in mind, it is recommended that future studies in this area take advantage of the ease of use and cost to scale up to larger trials possibly through multi‐centre designs where the intervention is self‐administered but at doses and stimulation parameters consistent with proposed best practice (Sluka 2013). Further repetition of small sample‐size studies is unlikely to add any clarity to the ambiguity surrounding estimates of effect for TENS in people with chronic pain. We suggest that given the exaggerated effects associated with meta‐analyses of small sample‐size studies (Dechartres 2013), researchers seeking to further investigate this area do not replicate the numerous existing small studies and instead aim for samples of sufficient size to produce robust estimates of effectiveness (Guyatt 2011; Higgins 2011). Self‐administration (as opposed to clinic administration) may address issues around adequate duration and frequency of treatment as well as allowing the participant to monitor/titrate intensity of stimulation, as optimal effects are suggested when the perception is adjusted to maintain continual 'strong but comfortable' sensation (Johnson 2011; Moran 2011; Sluka 2013). Large‐scale self‐administration designs are more likely to provide pragmatic estimates of the effect of TENS in people with chronic pain.

Outcome measures

This overview reviewed evidence from 51 studies across eight reviews. Of these, the majority of interventions were less than 6 weeks' duration and most of the follow‐up assessment time points were either immediately post intervention or within two weeks, rendering these short‐term follow‐up studies. It is worth noting that the nature of conditions included in these reviews means the chronic pain is inherently resistant to change and is by definition persistent. The value of short‐term interventions and follow‐up in TENS studies must be questioned. We recommend future studies should be designed such that interventions are of sufficient duration to assess change and also that follow‐up time points ideally extend to at least three months post‐intervention, as well as capturing effects during use.

We found a lack of detail with respect to timing and the specific parameters of pain assessment in the studies from included reviews. No reviews explicitly stated minimal pain level for study inclusion which may influence sensitivity of studies to detect intervention effects. We suggest this be considered for future studies. With respect to timing of assessment, TENS is purported to have a rapid onset and offset of effect (Moran 2011); we therefore suggest pain (and health‐related quality of life measures) should be assessed during TENS use or, ideally, during TENS use while undertaking normal daily activities as well as via explicitly stated summary pain measures such as average 24‐hour pain or average weekly pain. Additionally, dichotomous categorisation of pain relief as per IMMPACT recommendations (Dworkin 2008), or by assessing the proportion of people who perceive their pain as reduced to 'no worse than mild' may offer outcomes that are directly meaningful to people with pain (Moore 2013b).

Measures of treatment effect are obviously important; however treatment safety is paramount. On balance, the standard of reporting of adverse events across all studies included in reviews was poor. Researchers should consider recording and full reporting of adverse events to be an implicit aspect of good study design.

Design of future systematic reviews

Future reviews of TENS should take a consistent approach to important methodological considerations that affect TENS trials. We recommend this includes taking a clear and consistent approach to assessing blinding of participants and personnel and recognising that, while blinding studies of TENS is challenging, this represents an important risk of bias that must be adequately considered. We would also recommend that studies which deliver TENS at a sub‐perceptual level or in a single dose should not be included in future reviews since it is reasonable to predict that such doses are sub‐optimal. Finally when pooling data review authors should be careful not to include comparisons of TENS versus sham and TENS versus no treatment in the same analysis.

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