Approximately 18% of the adult population of the United States (> 40 million adults) report experiencing pain on most days or every day in the previous three months. Such high prevalence is not limited to the United States. For example, in a random survey of over 2000 Canadian adults chronic non‐cancer pain was reported by 29% of the respondents. Additionally, it is currently estimated that over three million Australians are currently living with chronic pain, costing the economy AUD 73.2 billion each year including AUD 48.3 billion in lost productivity. Beyond this direct economic impact, chronic pain has a detrimental impact on quality of life for the individual, affecting physical, mental, and social health. Chronic pain needs to be considered as a primary disorder, not as a symptom of another disease. In recognition, the World Health Organization International Classification of Diseases now includes chronic pain.
Chronic pain is notoriously difficult to treat. Patients and clinicians have multiple potential treatment options but no clear guidance on comparative effectiveness and safety. Transcutaneous electrical nerve stimulation (TENS) is one such option. TENS entails the application of electrical currents, usually generated by small portable devices to achieve the stimulation of nerves through the skin with the goal of reducing the experience of pain. TENS is far from a recent clinical innovation and as such we might expect that its value and safety would be well established. A new overview of Cochrane Reviews sought to address the therapeutic value and safety of TENS by synthesizing and summarizing the evidence from Cochrane Reviews of TENS for chronic pain.
The overview looked at eight Cochrane Reviews, including 51 discrete TENS‐related randomized controlled trials (RCTs) with 2895 participants. With such a large body of evidence one might expect a precise and reliable estimate of the treatment effect of TENS for chronic pain. Rather, the overview found that it was not possible to conclude with confidence whether TENS was beneficial or safe for pain control, disability, health‐related quality of life, or analgesic use.
The included reviews all scored high on the AMSTAR (A MeaSurement Tool to Assess systematic Reviews) checklist, indicating they were well conducted and reported. But the evidence reported within each review was consistently rated by the overview authors as very low quality with major concerns regarding imprecision of the estimates due to small sample sizes, uncontrolled risks of bias, and important insufficiencies in the interventions delivered. So, while the reviews themselves may be of high quality, the data derived from the existing body of RCTs included in the reviews is of such low quality that they hamper progress. To use a culinary metaphor, it really does not matter how good the chef is, or how creative and skilled she is at making soup, if the ingredients are rotten the soup will ultimately be unpalatable.
It may appear disappointing that after 51 clinical trials, multiple Cochrane Reviews and an overview of these reviews we do not have adequate evidence to recommend (or not) TENS for management of chronic pain. However, to quote Henry Ford: “Failure is only the opportunity to begin again more intelligently”.
And so, rather than generically stating that more research is required, we propose effort to improve the next generation of trials. We offer five recommendations based on the flaws observed in the existing trials, and these recommendations echo those made in other areas of pain research.
Control for known biases. The majority of existing studies were considered to have a high or uncertain risk of bias from the failure to adequately control for selection, performance, and detection bias. In many trials it was not possible to state with any confidence that patients, practitioners, or researchers did not know which treatment had been delivered. This is important as unclear blinding of participants and/or assessors in a trial has an inflationary influence on positive effects. It may be that a trial has accounted for such biases but the reporting is inadequate. The Consolidated Standards of Reporting Trials (CONSORT) statement can help. However, in TENS trials failures of blinding often arise from an inadequate sham or from the failure to compartmentalize the research procedure. Ideally the researchers should isolate the different people involved in each stage of the process. For example, the person who allocates the patient to treatment should be separated from the person who chooses and fits the device (sham or real). Similarly, the person who then administers the treatment should be isolated from the person collecting the data. In terms of patient blinding and performance bias, protocols exist to administer sham TENS using identical machines with the sham version not administering an electrical current. Another option is to use an initial brief period of stimulation and then fade out the stimulus. In these protocols, the information given to participant regarding the intervention may help to enhance blinding. However, the effectiveness of TENS is considered to be related to the intensity of the stimulus and as such existing protocols may not represent a sufficiently credible sham to overcome the patient‐side performance bias in TENS trials.
Invest in more precise estimates. The existing literature largely consists of small trials (n < 50), which are prone to random error and inflation of treatment effects.16, More forensic attempts to find evidence not published in the scientific literature may also be warranted, in particular those residing with device manufacturers, or unpublished trials in university repositories. Small positive trials are more likely to be published. Unfortunately, this adds to the uncertainty in the evidence. Thus, future research efforts should focus on larger (possibly multi‐centre) trials capable of providing robust and precise estimates.
Test the technology in the setting in which it will be used. Most of the existing studies administer TENS in a clinical setting. However, this may limit the generalizability of results as TENS is proposed as a simple, cost‐effective treatment that can be easily and regularly self‐administered at home. To enhance the ecological validity of studies and reduce the knowledge translation gap, clinical trials with patients in the most relevant clinical setting should be the norm.
Employ meaningful pain assessment, consider timing of effect and examine long‐term outcomes. Measurement of pain intensity should use validated scales that are clearly described. The mean difference between interventions on a continuous pain scale is likely the most sensitive to detect an effect, though dichotomous categorization of pain relief such as per the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) recommendations may be valuable. Recommended categories are: less than 15% as no important change; 15% to 29% as a minimally important change; 30% to 49% as a moderately important change; and 50% or more as a substantially important change. Further, one should only enter participants into trial who are experiencing greater than mild pain.
TENS is considered to have its optimal effect during use. Thus, future trials are recommended to examine the effect of TENS on pain and function during use rather than just using summary measures like average weekly pain.
Furthermore, TENS is typically suggested as a long‐term solution for a long‐term condition, casting doubt on the relevance of data collected after short‐term use. Single‐use studies give no information about the long‐term utility of TENS for those with chronic pain and should be discouraged. Similarly, short follow‐up duration (e.g. six weeks) provides limited useful information for patients whose condition is resistant to change. In the 51 studies included in the Cochrane Reviews in the overview, the majority of interventions were of less than six weeks' duration and most of the follow‐up assessment time points were either immediately post‐intervention or within two weeks. Thus, most of the existing data can be classified as short‐term use and short‐term follow‐up studies. Assessment at six weeks, three months, six months, and 12 months should be included as a minimum design criterion to assess long‐term outcomes.
In recognition of under‐reporting of adverse events across many forms of interventional trials we recommend that future trials include formal processes for recording and reporting adverse events alongside pre‐specified analysis of adverse events.
Describe what was done, so other people can do it the same way. Insufficient detail regarding the specifics of how an intervention has been applied in an RCT is a known barrier to uptake of positive findings in the clinical settings. As such, the exact parameters and application of the intervention, including intensity and duration of TENS, should be documented in future trials. Therefore, researchers are pointed to the Template for Intervention Description and Replication (TIDieR) checklist. As many existing RCTs do not adequately report all methodological aspects of the trial, reproducibility is compromised and the trustworthiness of the body of evidence is undermined. Consistent with a recent drive to increase the reproducibility and replicability of pain research, researchers are advised to transparently and clearly report all methodological aspects of their studies.
TENS is fairly simple to use, and is portable, self‐administered, and relatively inexpensive. In that respect, if ultimately found to be effective and safe, it could be a valuable option for people with persistent pain. It is regrettable that after synthesizing studies examining TENS for chronic pain over the past 30 years, uncertainty remains. But without concerted action to improve the quality of trial conduct and reporting, we will never improve on that uncertainty. It is time for the clinical research community to establish a series of large high‐quality trials, building on the strengths of what went before and correcting the known errors. Then usable, relevant, and trusted evidence may inform shared clinical decisions for those who live their lives with a constant shadow of pain and disability.
Cochrane Database of Systematic Reviews
22 April 2020
Copyright © 2020 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd.
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Pain & anaesthesia
Mervyn J Travers
The University of Notre Dame Australia, Australia
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Neil E O'Connell
Declarations of interest
The authors have completed the ICMJE form for disclosure of potential conflicts of interest and declare no conflicts of interest.
Provenance and peer review
This editorial was commissioned based on a proposal by Cochrane Pain, Palliative and Supportive Care, and it was peer reviewed.
Pitcher MH, Von Korff M, Bushnell MC, Porter L. Prevalence and profile of high‐impact chronic pain in the United States. Journal of Pain 2019;20(2):146–60. https://doi.org/10.1016/j.jpain.2018.07.006
Moulin DE, Clark AJ, Speechley M, PK Morley‐Forster. Chronic pain in Canada ‐ prevalence, treatment, impact and the role of opioid analgesia. Pain Research and Management2002;7(4):179–84. https://doi.org/10.1155/2002/323085
Deloitte Access Economics. The cost of pain in Australia. April 2019. www2.deloitte.com/au/en/pages/economics/articles/cost‐pain‐australia.html
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