This review concluded that transcutaneous electric nerve stimulation was probably effective in reducing pain from diabetic peripheral neuropathy and ineffective for chronic low back pain. Given the small number of included studies, types of study designs used and the inconsistent findings, the authors' cautious conclusions appear justified.

To determine if transcutaneous electric nerve stimulation (TENS) is efficacious in the treatment of pain in neurologic disorders.

MEDLINE and The Cochrane Library were searched from inception to April 2007. Search terms were reported. Reference lists of retrieved articles and reviews were checked for further studies.

Clinical trials with more than 10 participants that compared TENS with placebo or an alternative therapy for well-defined painful neurologic disorders were eligible for inclusion in the review.

Most of the included studies assessed TENS in populations of patients with low back pain (various aetiologies included multiple sclerosis); the other studies were in patients with diabetic neuropathy. Almost all of the included studies compared TENS with sham TENS; other intervention groups included exercise and high frequency muscle stimulation and other TENS regimens. TENS regimens varied widely between studies (details reported in the review). Almost all of the studies assessed pain with a visual analogue scale (VAS); other measurement tools varied between studies (details reported in the review). In most studies a decrease of 20mm or a 25% reduction in VAS score from a baseline of 50mm or less was described as clinically significant.

The authors did not state how many reviewers performed the study selection.

No validity assessment was performed. However, studies were assigned a level of evidence (I for highest to IV for lowest) based on study design characteristics (randomisation, blinding, adequate handling of withdrawal and dropouts, clearly defined inclusion/exclusion criteria, clearly defined primary outcome and equivalent relevant baseline characteristics).

More than one reviewer was involved in the assessment process. Disagreements were resolved through consensus.

Data were extracted according to pain definitions and data reported within each individual paper. Extracted data included the number/percentage of participants who experienced a reduction or improvement in pain and the percentage reduction/improvement in pain intensity.

The authors did not state how many reviewers performed data extraction.

Studies were grouped according to pain indication and level of evidence and summarised using a narrative synthesis.

Nine studies (n=565) were included in the review. Seven studies used a randomised controlled trial design (one was crossover study) and two studies used a 2x2 factorial design. Sample sizes ranged from 15 to 155. Three studies were graded as evidence class I, five as class II and one as class III.

Chronic low back pain (five studies): Two class II studies showed that TENS was beneficial in terms of pain and quality of life in comparison with sham TENS. However, two class I studies and another class II study did not show any difference in comparison with sham TENS or alternative TENS regimens.

Painful diabetic neuropathy (four studies): Two class II studies reported that TENS improved pain in comparison with control. One class I and one class III study showed that a larger proportion of participants felt a benefit with high frequency muscle stimulation compared with TENS.

TENS is probably effective in reducing pain from diabetic peripheral neuropathy, but ineffective for chronic low back pain.

This review answered a research question that was defined in terms of study design, population and interventions. Specific outcomes were not clearly defined, although it was apparent that measurement of pain in some form was of interest. Several relevant sources were searched. It was unclear whether language restrictions were applied and there may be the potential for publication bias. Attempts were made to reduce risks of reviewer error and bias during quality assessment; it was unclear whether similar precautions were taken during study selection and data extraction. The assessment of study quality was based on a grading system of evidence levels and did not not assess individual quality criteria. Given the subjective nature of pain as an outcome, a more detailed individual assessment of the study methods and quality would have been more informative. The authors' use of a narrative summary appeared justified given the variation between studies, particularly in terms of design and outcomes. Some details of the review and study data were lacking in the review publication; some further details were available online.

Overall, given the small number of included studies, types of study designs used and the inconsistent findings, the authors' cautious conclusions appear justified.

Practice: The authors stated that transcutaneous electric nerve stimulation (TENS) was not recommended for treatment of chronic low back pain (Level A), but should be considered for the treatment of painful diabetic neuropathy (Level B).

Research: The authors stated that more research was required to investigate the mechanism of action of TENS. More rigorous large randomised controlled studies were required to determine the efficacy of TENS for common painful disorders such as post-traumatic nerve injuries in comparison with sham TENS. Such studies should be performed on well-defined TENS-naive patient populations and should use TENS for chronic therapy rather than single sessions.

The review was carried out by the American Academy of Neurology (AAN), but funding sources were not stated.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.