Low back pain is the world's largest health problem in terms of years lived with disability.1,2 The prevalence increases from 30 to 60 years old, being more prevalent in female. Low back pain can reach up to 65% of the population per year, and 84% of people will experience at least one episode of low back pain in their lifetime.3,4 Low back pain generates high costs with absenteeism5 and disability6 and is considered the health condition that generates the highest medical cost in the world.5,6

Analgesic electrical currents are widely used by physical therapists in their clinical practice.7 Interferential current (IC) is the most commonly used analgesic current in physical therapy services in Canada, Australia, and England.8 IC is a medium-frequency alternating current, with frequencies ranging from 1 to 10 kHz and with a modulated amplitude.9 Because it is a medium-frequency current, the IC has, in theory, the ability to reduce the skin's impedance, thus increasing penetration into the tissues and generating less discomfort to the patient,9,10 which in turn may be associated with greater effectiveness in pain relief.11,12

Some studies have showed that IC seems better than placebo for reducing pain intensity13 in patients with chronic low back pain. Systematic reviews have showed that IC seems to be more effective than control or placebo treatment.8,15 However, these findings were in patients with musculoskeletal pain.8,15

Only one systematic review,16 published in 2008, evaluated the effectiveness of electrophysical interventions, including IC, in patients with low back pain. Although many studies have evaluated analgesic currents in patients with non-specific chronic low back pain, most of these have evaluated low-frequency pulsed currents, often referred to as transcutaneous electrical nerve stimulation (TENS).17–19 To date, there is no specific systematic review assessing the effectiveness of IC in patients with chronic non-specific low back pain.

Therefore, a review summarizing the results of IC treatment (following the recommendations of the Cochrane collaboration20) is necessary to verify the possible effects of this intervention. Thus, the objective of this systematic review is to investigate the effectiveness of IC in patients with chronic non-specific low back pain.

This systematic review is reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-analyses statement (PRISMA Statement),21 and it was prospectively registered in the International Prospective Register of Systematic Reviews - PROSPERO (registration number CRD42013005083).

The searches identified a total of 2061 trials. After applying the inclusion criteria, 13 randomized controlled trials were considered eligible and were included in the review (Fig. 1). The sample sizes ranged from 2838 to 28039 patients, totaling 1367 patients in the review. The characteristics of the selected trials are presented in Table 1. The trials were conducted in four different countries: eight7,38–44 in Brazil, three14,45,46 in Spain, one47 in Poland, and one48 in Nigeria. All included trials evaluated pain intensity as an outcome measure.7,14,38,40–43,45–49 Six trials7,14,38,42,45,47 rated pain using the 0–10 or 0–100 Visual Analogue Scale (VAS) and seven trials40,41,43,44,46,48,49 rated pain using the 0–10 Numeric Rating Scale (NRS). Disability outcome was evaluated in eight trials.7,14,38,40–42,45,48 Seven trials7,14,38,40–42,48 used the 0–24 Roland Morris Disability Questionnaire (RMDQ) and one trial45 used the 0–100 Oswestry Disability Index (ODI). Seven trials had more than two groups for comparisons.7,41–44,48,49

Fig. 1.

Selection of studies for the systematic review.

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Eleven studies used a carrier frequency of 4 kHz,7,14,38,40,41,43–46,48,49,47 three used 2 kHz,42,43,49 one used 1 kHz,41 and one study did not provide the frequency used.47 The amplitude-modulated frequency (AMF) of 2 Hz was used in four studies,42–44,49 20 Hz in two,7,38 65 Hz in two,45,46 80 Hz in one,14 100 Hz in six,40–44,49 150 Hz in one,48 and 90–100 Hz in one study.47 Quadripolar technique was used in most studies with exception of one study that used bipolar technique14 and one study that did not provide any information.47 Regarding the number of sessions, a single session was used in five studies,42–44,46,49 10 sessions in five studies,7,38,45,47,48 12,41 18,40 and 2014 sessions were applied in one study each. The treatment time per session for IC application was of 20,47,48 25,45,46 or 307,14,38,40–44,49 minutes.

The methodological quality of the studies is described in Table 2. Nine studies were identified in the PEDro database.7,14,38,40,41,43,45,47,49 Methodological quality scores ranged from 4 to 9 with a mean score of 6.2 (SD = 1.5) (Table 2). The overall quality of evidence according to GRADE was rated as very low to moderate (Table 3).

Treatment effectsInterferential current versus no interventionPain intensity and disability immediately post-treatment

One study7 compared IC to no intervention. We found very low-quality evidence that the IC reduces pain and disability compared to no treatment. Estimated mean pain intensity after treatment (VAS): IC group:10 mm, and control group: 55 mm. Additional data could not be obtained as the authors did not respond to our emails.

Interferential current versus placeboPain intensity immediately post-treatment

Seven trials40–44,46,49 compared IC to placebo for pain intensity. We found moderate-quality evidence (downgraded for inconsistency) that IC probably reduces pain intensity compared to placebo (MD = −1.57 points in a 0–10 scale; 95% CI: −2.17, −0.98, n = 865), resulting in a moderate effect that is potentially clinically meaningful. A subgroup analysis showed that IC may reduce pain intensity with a single session (MD = −1.83 points in a 0–10 scale; 95% CI: −2.42, −1.24, n = 567) or after 12 to 18 sessions (MD = −0.60 points in a 0–10 scale; 95% CI: −1.13, −0.07, n = 298) compared to placebo (Fig. 2).

Fig. 2.

Meta-analysis of pain intensity immediately post-treatment comparing IC versus Placebo.

IC, interferential current; SD, standard deviation.

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Disability immediately post-treatment

Three trials40–42 compared IC to placebo for disability. We found moderate-quality evidence (downgraded for imprecision) that IC probably reduces disability compared to placebo (MD = −1.51 points on a 0–10 scale; 95% CI: −2.57, −0.46, n = 403), resulting in a moderate effect that is potentially clinically meaningful (Supplementary material – Fig. S1).

Pain intensity and disability at intermediate-term follow-up

Two trials40,41 compared IC to placebo for pain intensity and disability at intermediate-term follow-up. We found moderate-quality evidence (downgraded for imprecision) that IC probably does not reduce pain intensity (MD = −0.02 points; 95% CI: −0.70, 0.65, n = 298) (Supplementary material – Fig. S2) and disability (MD = −0.72 points; 95% CI: −2.27, 0.83, n = 298) (Supplementary material – Fig. S3).

Interferential current versus other intervention (TENS and manual therapy)Pain intensity immediately post-treatment - Interferential Current versus tens

Five trials7,38,47–49 compared IC to TENS. We found low-quality evidence (downgraded for risk of bias and inconsistency) that IC may reduce pain intensity compared to TENS (SMD = −0.32; 95% CI: −0.61, −0.03, n = 420) (Supplementary material – Fig. S4), resulting in an uncertain clinical meaningfulness effect given the lower boundary of the 95% CI near 0.

Pain intensity immediately post-treatment - interferential current versus manual therapy

One trial45 compared IC to manual therapy. We found very low-quality evidence that IC may reduce pain intensity compared to manual therapy (MD = −15.52 mm in a 0–40 mm scale; 95% CI: −24.89, −6.15, n = 64), resulting in a small effect of uncertain clinical significance given the width of the confidence interval.

Disability immediately post-treatment - interferential current versus tens

Three trials6,7,38,48 compared IC to TENS. We found very low-quality evidence that IC may not reduce disability compared to TENS (MD = −0.67 points; 95% CI: −3.72, 2.38; n = 50) (Supplementary material – Fig. S5).36,46 One trial6 reported the data only in graphs. We found very low-quality evidence that IC may not reduce disability compared to TENS. Additional data could not be obtained as the authors did not respond to our emails.

Disability immediately post-treatment - Interferential Current versus manual therapy

One trial45 compared IC to manual therapy. We found very low-quality evidence that IC may reduce disability compared to manual therapy (MD = 13.38 points in a 0 – 100 scale; 95% CI: 4.97, 21.78, n = 64).

Interferential current plus other intervention versus other interventionPain intensity and disability immediately post-treatment

One trial compared the additional effect of IC to massage.14 We found very low-quality evidence that IC plus massage may reduce pain intensity compared to massage alone (MD = −1.05 points; 95% CI: −1.87, −0.23, n = 62). We also found very low-quality evidence (downgraded for imprecision and inconsistency) that IC plus massage may improve disability compared to massage alone (MD = −3.01 points; 95% CI: −4.62, −1.40, n = 62). However, these results are small and probably not clinically meaningful.

One trial compared the additional effect of IC to exercises48 for pain and disability. We found very low-quality evidence that IC plus exercise does not reduce pain intensity (MD = −0.09 points; 95% CI −1.30, 0.48, n = 22) and disability (MD = −1.72 points; 95% CI −2.71, 1.65, n = 22) more than exercise alone.

To our knowledge, this is the first systematic review to evaluate the effectiveness of IC in patients with non-specific chronic low back pain, comparing IC versus: no intervention, placebo, other intervention (TENS and manual therapy), and the additional effect of IC when combined with exercises or massage. The most relevant findings of this systematic review showed evidence of moderate quality that IC probably reduces pain intensity and disability compared to placebo immediately post-treatment with moderate effect size. These potentially clinically meaningful results were found in studies that used the following parameters: a carrier frequency of 1, 2, or 4 kHz, an AMF of 2, 65, or 100 Hz, quadripolar technique, and from 1 to 18 sessions for 30 min per session. But, moderate quality evidence also showed that these findings were not sustained at intermediate-term follow-up. Then, similar to TENS,50 IC seems to be potentially effective for pain relief only immediately after the treatment.

To date, the physiological effects are not well-established.51 IC is thought to decrease the stimulation of cutaneous sensory nerves close to the electrodes while increasing the effect on deep tissues.11 In addition, some theories have been proposed to explain the analgesic effect, such as the gate control theory, descending pain suppression pathway, and physiological block.51 Another theory that should be considered is that medium-frequency currents, such as IC, are better for reducing skin impedance and increasing tissue penetration, consequently providing greater patient comfort9,10 possibly associated with greater effectiveness of pain relief.11 There is no experimental research investigating the mechanism of action of IC for pain relief, with claims based on the mechanism of action of TENS. Thus, studies are needed to investigate the analgesic mechanism of action of IC.

Our systematic review also showed low quality evidence that IC is better than other intervention in decreasing pain intensity immediately post-treatment in patients with chronic non-specific low back pain. However, this result should be interpreted with caution as the quality of the evidence was low, the effect size was small, and the heterogeneity was 56%. In contrast, a systematic review suggested that TENS and IC had similar positive effects on pain and disability.52 However, their systematic review was performed in patients with acute and chronic pain, and the methodological quality was considered very heterogenic.52

There were no significant results in the additional effect of IC for improving pain intensity (IC plus massage versus massage; IC plus exercises versus exercises)14 and disability (IC plus exercises versus exercises).47 The evidence was of low quality and the meta-analyses were performed with only two trials. We believe that more pragmatic studies, with larger samples and high methodological quality, that compare or add IC to a guideline-based treatment are necessary.

This review has a number of strengths. There was no restriction of language for the included studies, thus minimizing the risk of language bias.53 This review followed all recommendations of the Cochrane Handbook for systematic reviews of interventions.54 The quality of the evidence was carefully assessed according to GRADE recommendations,55 in which the quality of evidence is assessed and consequently the quality of confidence in the results. A highly sensitive search strategy was used to identify studies in the main databases and clinical trial registries, complemented by a manual search in studies relevant to the topic. However, it is possible that some studies were published in local databases and consequently were not found and included in this review, which may be considered a potential limitation of the review.

Evidence of moderate-quality shows that interferential current seems to be more effective than placebo for improving pain intensity and disability in patients with chronic non-specific low back pain. Future trials are needed to investigate the IC efficacy comparing to other interventions and when combined to other interventions.