Neck pain is a common public health problem leading to disability and resulting in high economic costs.1 Its incidence is higher in women and generally increases with age.1 The condition is considered chronic when it persists for >3 months,2 and nonspecific when the etiological factor is unknown. Photobiomodulation (PBM) and transcutaneous electrical nerve stimulation (TENS) are electrophysical agents commonly used to treat chronic neck pain.2–5 A systematic review from Chow et al.6 showed that PBM using low-level laser therapy reduced chronic neck pain up to 22 weeks post-treatment.6 Another systematic review by Kadhim-Saleh et al.7 demonstrated inconclusive evidence due to the heterogeneity of the studies and high risk of bias, in addition to the results, albeit significant, showing no minimally important clinical difference.7 These inconsistent findings on the effects of PBM in reducing chronic neck pain highlight the need for further research. Furthermore, there is a significant lack of studies comparing PBM to analgesic electrical currents. Therefore, investigations that both compare and combine PBM with TENS are particularly important and necessary.

In terms of TENS, the latest systematic reviews reported very low quality evidence for the use of TENS when compared to a sham treatment in individuals with neck pain.3,8 This finding was mainly due to the high heterogeneity of studies and high risk of bias. Hence, clinical trials with larger sample sizes with low risk of bias and adequate TENS application9 are still needed in this field.

PBM and TENS are both commonly used in clinical practice to reduce neck pain. However, to date, no studies have investigated whether PBM is superior to TENS, or if combining these electrophysical agents in the same session may enhance the analgesic effect. Given their different action pathways for analgesic production, it is possible that combining both interventions may result in a faster and/or longer-lasting analgesic effect. Therefore, this study investigated whether the effect of PBM and TENS alone or the combined effect of PBM and TENS on pain intensity, central sensitization (i.e., pressure pain threshold [PPT], temporal summation [TS] of pain, conditioned pain modulation [CPM]), cervical range of motion (ROM), psychosocial factors (i.e., depressive symptoms, neck disability, catastrophizing of pain, and quality of life), drug intake for neck pain, and global perceived effect compared with sham in individuals with nonspecific chronic neck pain.

A total of 144 individuals with neck pain were considered eligible to participate in the study (Fig. 1). Clinical and demographic characteristics for each group are described in Table 2. A total of 360 treatment sessions per group were offered. There were 27 absences in the PBM + TENS group (92.5 % present); 69 in the PBM group (80.8 % present); 36 in the TENS group (90 % present), and 43 in the Sham group (88 % present). In the PBM group, four participants were lost at the post-treatment time point of assessment and three participants at the 1-month follow-up. The reasons for loss to follow-up were: moved to another city (n = 1), health problems not related to neck pain (n = 4), unable to contact (n = 1), and personal reasons (n = 2). Average PBM energy (J) and average current amplitude (mA) applied in the groups were 1836 J and 38.7 mA in the PBM + TENS group, 1692 J and 0 mA in the PBM group, 0 J and 40.1 mA in the TENS group, and 0 J and 0 mA in the Sham group. The laser diodes emitted 170, 172, 171, and 170 mW, respectively, prior to the beginning of the study, and 170, 164, 170, and 169 mW, respectively, at the end. A digital oscilloscope (TDS 430A, Tektronix Inc, Beaverton, OR) was used during the study to calculate the current amplitude (mA). Antares IBRAMED® device was calibrated by the IBRAMED® to verify diode output power before and at the end of the study.

Fig. 1.

Flowchart of the study.

ITT, Intention-to-treat; NDI, Neck Disability Index; PBM, Photobiomodulation; TENS, Transcutaneous electrical nerve stimulation.

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Table 3 summarizes median (interquartile range) of pain intensity at rest and during movement, TS of pain, neck disability, depressive symptoms, and GPE overtime according to group. There were no significant between-groups differences for changes in pain intensity at rest from baseline to end of intervention, and from baseline to one-month follow-up (Table 4). Pain intensity during movement declined significantly more in the PBM+TENS compared to the PBM (MD: 1.0 points; 95 % CI: 0.0, 2.0) and Sham (MD: 2.0 points; 95 %CI: 1.0, 3.0) groups (Table 4).

PBM + TENS increased PPT significantly at most points in the neck and shoulder girdle areas compared to PBM (MD: 33.3 kPa; 95 %CI: 10.5, 56.0); and Sham (MD: 34.5 kPa; 95 %CI: 11.7, 57.3) groups (Tables 5 and 6 - Supplementary material online). TENS increased local PPT compared to PBM (MD: 35.7 kPa; 95 %CI: 12.9, 58.5); and Sham (MD: 36.9 kPa; 95 %CI: 14.2, 59.7) groups. TENS increased distant PPT compared to PBM+TENS (MD: 41.8 kPa; 95 %CI: 4.5, 79.1), PBM (MD: 57.1 kPa; 95 %CI: 19.7, 94.4), and SHAM (MD: 73.4 kPa; 95 %CI: 36.1, 110.8) groups (Tables 5 and 6 – Supplementary material online). Fig. 2 shows the topographical maps of individuals with neck pain of each group pre- and post-treatment.

Fig. 2.

Pressure pain sensitivity maps of the neck and shoulder girdles areas for each group pre- and post-treatment. The maps show higher PPTs in PBM+TENS and TENS groups compared to the PBM and Sham groups post treatment.

PBM, Photobiomodulation; PPT, Pressure Pain Threshold; TENS, Transcutaneous electrical nerve stimulation.

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No significant between-group differences were observed for TS of pain, neck disability, depressive symptoms (Table 4), CPM, ROM, pain catastrophizing, and quality of life (Tables 7 and 8 - Supplementary material online).

No significant between-group difference was observed for drug intake for neck pain (Morphine equivalence, acetaminophen equivalence). The drugs that did not allow morphine or acetaminophen equivalence were segmented according to each component for analysis. No significant between-group difference was observed (carisoprodol, ketoprofen, lysine clonixinate and meloxicam (Mobic), adiphenine hydrochloride, promethazine hydrochloride, nimesulide, ketorolac trometamol, diclofenac, dipyrone, orphenadrine, isometheptene, and cyclobenzaprine).

The difference in GPE post-treatment in relation to treatment onset was significantly higher in the PBM+TENS compared to the PBM (MD: 2.0 points; 95 %CI: 1.0, 3.0) and Sham (MD: 2.0 points; 95 %CI: 1.0, 3.0) groups (Table 4). At 1 month follow-up the improvement was kept in the PBM+TENS compared to the Sham (median difference: 2.0 points; 95 %CI: 1.0, 4.0) (Table 4).

The examiner correctly identified the subjects who received active PBM treatment in 47.2 % of the cases (34 of 72). In the group that received sham PBM, the examiner correctly identified the treatment in 11.1 % of the cases (8 of 72). In other words, the examiner was blind 70.9 % of the time, which indicated successful blinding (p < 0.001). The examiner correctly identified that the subjects received active TENS treatment in 47.2 % of the cases (34 of 72). In the group that received sham TENS, the examiner correctly identified treatment in 30.6 % of the cases (22 of 72). In other words, the examiner was blind 61.1 % of the time, demonstrating successful blinding (p = 0.007).

The individuals were blind to the treatment 63.9 % (46 of 72) of the time in the active PBM group and 86.1 % (62 of 72) of the time in the sham PBM showing successful blinding (p < 0.001). The active TENS individuals were blind to treatment 6.9 % (5 of 72) of the time, and the sham TENS group 61.1 % (44 of 72) of the time. In the former group, the treatment was more frequently identified as correct than chance guesses (p < 0.001), showing that the individuals were not blind to the treatment. By contrast, the sham TENS group was not different from randomness (50:50 random probability), suggesting adequate blinding (p = 0.059). No severe adverse effects were observed, and more details can be accessed in the Supplementary material online.

To the best of our knowledge, this is the first randomized controlled trial to investigate the efficacy of PBM and TENS in chronic neck pain. Our findings show that there was no significant intergroup difference for self-reported pain intensity at rest. Interestingly, for pain intensity during movement, the combined use of PBM and TENS was found more effective than PBM alone and Sham. In addition, the combined use of PBM and TENS improved local hyperalgesia and GPE compared to PBM alone and Sham post-treatment and compared to Sham at one month follow-up. TENS reduced local hyperalgesia compared to PBM and Sham, and distant hyperalgesia compared to all groups.

The hypothesis of this study was confirmed for pain intensity during movement and GPE. We believe that this additional analgesic effect is related to the different mechanism of action to achieve analgesia between these resources. The analgesic efficacy of TENS, primarily high-frequency TENS (> 50 Hz) ,28 is considered related to: δ-opioid receptor activation in the rostroventromedial medulla,29,30 M1M3 muscarinic receptor activation,31 α2A-adrenergic receptor activation,32 increased GABA neurotransmitter release and GABA receptor activation,33 increased dynorphin A concentration34 and β-endorphins,35,36 a decline in aspartate and glutamate release,37 and a decrease in substance P in the dorsal root ganglion of the spinal cord.38

Unlike TENS, the analgesic efficacy of PBM seems to have a relation with opioid release,39 decreasing oxidative stress and fatigue,40,41; increasing serotonin42,43 and inhibiting transmission in the neuromuscular junction.44 The fact that no studies that investigated the efficacy of these resources in combination or the superiority of one were found, precluded making comparisons.

According to our results, isolated PBM with the protocol used, was not effective for patients with chronic neck pain. Our findings corroborate those of studies that reported no difference between PBM and Sham for the following outcomes: pain intensity at rest,45–47 local hyperalgesia,46 active ROM,45,46 quality of life,48,49 and drug use for neck pain.47 By contrast, some studies have observed benefits of PBM compared to Sham in improving pain intensity at rest5,17,49–51 or during movement,17,51 local hyperalgesia, active neck ROM51 and neck disability,50,51 depressive symptoms,17 and quality of life.5,17

With respect to TENS, our findings agree with other studies that reported no differences between TENS and Sham for pain intensity at rest.48,52 Moreover, our findings agree with those of other studies that found TENS to be superior to Sham in decreasing local hyperalgesia.52–54 However, the literature is inconsistent, given that some studies reported differences for pain intensity at rest53 or during movement,52 and no differences for local hyperalgesia48,54 between TENS and Sham. Although TENS alone was better than PBM in decreasing local hyperalgesia, it was not possible to compare these findings with those of other studies, because none have compared these therapies. A significant decrease in distant hyperalgesia was also observed after TENS application compared to the other groups. This suggests that TENS could have a central effect on pressure pain sensitivity.

It is important to emphasize that in this study, over 60 % of the time the examiner was blind to the type of treatment (active or sham) that patients received during PBM and TENS application. Individuals with neck pain were also blind to PBM (active or sham) and for TENS (sham) 60 % of the time. These data show the blinding success of the study, consistent with earlier investigations.18,55–57

The present study exhibited strong points, such as being the first to our knowledge to investigate the efficacy of combined use of PBM and TENS in individuals with nonspecific chronic neck pain, and to compare these electrophysical agents. The adherence of individuals with chronic pain was high in relation to the proposed treatment. Limiting factors were the lack of blinding for therapy due to the type of interventions applied, the lack of evaluating psychosocial factors in the follow-up period, and PBM and TENS are not in the first line of recommendations for the treatment of neck pain. Although, we have found few statistically significant results, we believe that this well-designed clinical trial and according to the proposed protocol, makes an important contribution to science-based rehabilitation practice.

PBM and TENS were not effective to alter resting pain. However, the combination of PBM and TENS improved pain during movement, local hyperalgesia, and GPE. In addition, TENS reduced local and distant hyperalgesia. Future studies using different protocols are recommended, mainly for PBM and the additional effect of these therapies on exercises or pain education.