Abstract: We included eight studies (seven RCTs, one quasi-RCT, 315 adults (299 women), aged 18 to 75 years): six used a parallel-group design and two used a cross-over design. Sample sizes of intervention arms were five to 43 participants. Two studies, one of which was a cross-over design, compared TENS with placebo TENS (82 participants), one study compared TENS with no treatment (43 participants), and four studies compared TENS with other treatments (medication (two studies, 74 participants), electroacupuncture (one study, 44 participants), superficial warmth (one cross-over study, 32 participants), and hydrotherapy (one study, 10 participants)). Two studies compared TENS plus exercise with exercise alone (98 participants, 49 per treatment arm). None of the studies measured participant-reported pain relief of 50% or greater or PGIC. Overall, the studies were at unclear or high risk of bias, and in particular all were at high risk of bias for sample size. Only one study (14 participants) measured the primary outcome participant-reported pain relief of 30% or greater. Thirty percent achieved 30% or greater reduction in pain with TENS and exercise compared with 13% with exercise alone. One study found 10/28 participants reported pain relief of 25% or greater with TENS compared with 10/24 participants using superficial warmth (42 °C). We judged that statistical pooling was not possible because there were insufficient data and outcomes were not homogeneous. There were no data for the primary outcomes participant-reported pain relief from baseline of 50% or greater and PGIC. There was a paucity of data for secondary outcomes. One pilot cross-over study of 43 participants found that the mean (95% confidence intervals (CI)) decrease in pain intensity on movement (100-mm visual analogue scale (VAS)) during one 30-minute treatment was 11.1 mm (95% CI 5.9 to 16.3) for TENS and 2.3 mm (95% CI 2.4 to 7.7) for placebo TENS. There were no significant differences between TENS and placebo for pain at rest. One parallel group study of 39 participants found that mean ± standard deviation (SD) pain intensity (100-mm VAS) decreased from 85 ± 20 mm at baseline to 43 ± 20 mm after one week of dual-site TENS; decreased from 85 ± 10 mm at baseline to 60 ± 10 mm after single-site TENS; and decreased from 82 ± 20 mm at baseline to 80 ± 20 mm after one week of placebo TENS. The authors of seven studies concluded that TENS relieved pain but the findings of single small studies are unlikely to be correct. One study found clinically important improvements in Fibromyalgia Impact Questionnaire (FIQ) subscales for work performance, fatigue, stiffness, anxiety, and depression for TENS with exercise compared with exercise alone. One study found no additional improvements in FIQ scores when TENS was added to the first three weeks of a 12-week supervised exercise programme. No serious adverse events were reported in any of the studies although there were reports of TENS causing minor discomfort in a total of 3 participants. The quality of evidence was very low. We downgraded the GRADE rating mostly due to a lack of data; therefore, we have little confidence in the effect estimates where available. There was insufficient high-quality evidence to support or refute the use of TENS for fibromyalgia. We found a small number of inadequately powered studies with incomplete reporting of methodologies and treatment interventions.

Plain-language summary: In January 2017, we found eight clinical studies that examined 315 people. We included TENS administered to produce a non-painful 'tingling' sensation at the site of pain either as a treatment alone or combined with exercise treatment. All studies used TENS in comparison with 'fake' (called placebo or sham) TENS, no treatment, or other treatments such as medicine or hydrotherapy (treatment in water). We did not find enough high-quality studies to allow us to come to any conclusions about the effectiveness of TENS for fibromyalgia pain. Even though seven studies concluded that TENS relieved pain associated with fibromyalgia, the studies were low quality and the findings for measures of pain were inconsistently reported. Studies did not measure most of our outcomes and it was not always clear what aspects of pain were being reported (e.g. present pain, remembered pain, pain severity, etc.). Only one small pilot study found that one 30-minute treatment of TENS reduced pain on movement during and immediately after treatment; however, there were too few participants observed and it is unknown whether this effect would be maintained over a longer course of TENS treatments. Overall, it is not possible to judge whether TENS reduces pain associated with fibromyalgia. There were no serious side events reported in any of the studies. We rated the quality of the evidence from studies using four levels: very low, low, moderate, or high. Very low-quality evidence means that we are very uncertain about the results. High-quality evidence means that we are very confident in the results. The quality of the evidence was very low overall because of a lack of data.