Table 1 Characteristics of all included studies
Author, publication year | country | design | Active group (n) | Sham group (n) | active electrode location | Cathode location | tDCS protocol | Associated interventions/ form of tDCS | Side effects | Pain outcome used | Notes (aim, result, duration of effects last) |
|---|---|---|---|---|---|---|---|---|---|---|---|
qualitative | |||||||||||
 Roizenblatt et al., 2007 [32] | Brazil | RCT | 11 (f) |  | M1 | Right SO | Anodal, 2 mA, 20 min, 5 sessions in consecutive days | None | None | VAS | Aim: to investigate the effect of tDCS-induced pain reduction on sleep structure in FM Result: tDCS stimulation can decrease pain only in M1 condition, and tDCS can change sleep structure, specific to the site of stimulation. Duration of effect: NA |
11 (f) | Â | Left DLPFC | |||||||||
| Â | 10 (f) | sham | Turned off after 30Â s of stimulation | ||||||||
 Silva et al., 2017 [35] | Brazil | RCT, cross-over design | 17/20 (f) | 18/20 (f) | Left DLPFC | Right SO | Anodal, 1 mA, 20 min, Single session | Go/No-go task | Minor: tingling, burning, and itching. In both conditions | HPTh, HPTo, | Aim: Anodal tDCS over the left DLPFC modulates attention and pain in fibromyalgia Result: active stimulation increased HPTh and HPTo Duration: NA |
 Mendonca et al., 2016 [20] | Brazil | RCT | 30 - f/m (tDCS/AE group (n = 15), tDCS group (n = 15)) | 15 (AE group) | Left M1 | Right SO | Anodal, 2 mA, 20 min, 5 sessions, consecutive days over the first week. | AE on a treadmill, 40 min, 9 sessions over 4 weeks | Mild adverse effects, not different between groups. | VNS | Aim: to assess the effect of combined intervention of tDCS and AE on pain in FM Result: the combination intervention is superior in pain reduction compared to individual treatments. Duration of effect: after one month of intervention. |
 De Ridder et al., 2017 [24] | Belgium | RCT, crossover design | 19 | 19 matched healthy control (Crossover design, 2 weeks washout period between conditions) | Right Occipital nerve field (OCF) | Left OCF (C2 dermatome) | Anodal, 1.5 mA, 20 min, 3 sessions, every two days, over a week | None | None | NRS, PCS | Aim: to investigate the mechanisms behind the effect of OCF on pain in FM. Result: active tDCS shows significant pain reduction compared to sham and baseline. Duration of effects: NA |
Sham | |||||||||||
 Brietzke et al., 2020 [31] | Brazil | RCT | 10 (f) | 10 (f) | Left DLPFC | Right DLPFC | Anodal, 2 mA, 30 min, 60 sessions over 12 weeks (5 consecutive days minimum interval of 16 h) | None/ bifrontal HB-tDCS | Mild and transient: headache, itching, tingling, and local redness | VAS (global pain in last 24 h), B-PCP:S, PPT, HPTo, | Aim: To test the effectiveness of HB-tDCS over DLPFC in multiple sessions on daily pain scores. Result: tDCS was superior to the sham group alone in reducing pain intensity. Levels of BDNF predicted better response Duration of effect: NA |
 Kang et al., 2020 [29] | South Korea | RCT | 46 | No sham group | Left M1 | Right SO | Anodal, 2 mA, 20 min, 5 sessions, consecutive days. | pharmacotherapy | No serious adverse effect was reported. | VAS | Aim: to investigate the effects of add-on tDCS stimulation on pain in FM. Result: tDCS is effective in pain reduction and other features in FM. Duration of effects: after one month post-stimulation. |
 Forogh et al., 2021 [27] | Iran | RCT | 15 (f) | No sham group | Left DLPFC | Right SO | Anodal, 2 mA, 20 min, 3 sessions, over one week (every other day) | None | None | VAS | Aim: to compare the effects of rTMS and tDCS on pain and quality of life in FM Result: pain intensity was significantly reduced in both groups, however, rTMS was more effective. Duration of effect: just after course termination |
 EL-Badawy et al., 2021 [25] | Egypt | RCT | 15 | No sham group – but the TMS group with n = 15 was examined | Left M1 | Right SO | Anodal, 2 mA, 20 min, 8 sessions, | None | Local tingling, well-tolerated complaint of headache and dizziness | VAS | Aim: to compare the efficacy of rTMS and tDCS in pain reduction in FM Result: both interventions were significantly efficient in pain reduction significantly, however, rTMS resulted in better improvement. Duration of effects: |
 Caumo et al., 2022 [23] | Brazil | RCT | 24 (f) | 24 (f) | Left DLPFC | Right DLPFC | Anodal, 2 mA, 20 min | None/ bifrontal HB- tDCS | Severe: burning sensation, Mild: tingling, redness, headache, neck pain, mood swings, concentration difficulties | VAS, PCS, FIQ | Aim: to evaluate the efficacy and safety of home-based bifrontal tDCS in reducing pain and disability due to pain in FM Result: positive effect Duration of effects: NA |
quantitative | |||||||||||
 Fregni et al., 2006 [28] | Brazil | RCT |  |  |  |  |  | None | Mild, similar to sham: Sleepiness and headache were the most frequent | VAS | Same as study subjects in the Roizenblatt et al. study Result: tDCS stimulation of M1 had significant pain reduction in FM compared to DLPF and sham group Duration of effect: at least 3 weeks after stimulation |
 Valle et al., 2009 [21] | Brazil | RCT | 14 (f) |  | M1 | Contralateral SO | 2 mA, 20 min, 10 sessions, consecutive days | None | Minor and uncommon: tingling, skin redness. Same with the sham group | VAS | Aim: to investigate the effect of tDCS stimulation of M1 and DLPFC on FM Result: M1 stimulation markedly decreases pain in FM but no evidence for the efficacy of DLPFC stimulation Duration of effect: M1 stimulation reduces pain that persists for up to 2 months. |
13 (f) | Â | DLPFC | |||||||||
| Â | 14 (f) | Sham (M1) | |||||||||
 Mendonca et al., 2011 [19] | Brazil, | RCT | 6 |  | Right SO | Left M1 | Cathodal, 2 mA, 20 min | None | Mild tingling at the beginning, no side effects | VNS, PPT, total body area of pain | Aim: to determine the efficacy of tDCS with different active electrode positions on pain reduction in FM Result: SO tDCS resulted in a significant pain reduction both as cathode and anode Duration of effects: NA |
6 | Left M1 | Right SO | |||||||||
6 | Left M1 | Right SO | Anodal, 2 mA, 20Â min | ||||||||
6 | Right SO | Left M1 | |||||||||
| Â | 6 | Left M1 | Right SO | Sham, current on only for the initial 30Â s | |||||||
 Villamar et al., 2013 [22] | USA | RCT, crossover design | 16/18 | Crossover design, participants have 3 different interventions with 7 days interval | Left M1 |  | Anodal, 2 mA, 20 min, 1 session | None /HD-tDCS | Mild to moderate tingling or itching during both active and sham stimulation, which resolved over a few minutes | VNS, PPT, and others | Aim: short term effects of HD-tDCS on pain reduction in FM Results: Immediately after stimulation only cathodal HD-tDCS was effective and 30 min after stimulation both active interventions resulted in better pain reduction than the sham. Duration of effects: NA |
Cathodal, 2 mA, 20Â min, 3 sessions | |||||||||||
sham | |||||||||||
 Foerster et al., 2015 [26] | USA | RCT, crossover design | 12 (f) | Crossover design, 7 days washout period between conditions | Left M1 | Right SO | Active Anodal, 2mA, 20 min, 5 consecutive days | None | None | VAS | Aim: To investigate the effects of tDCS on brain metabolites and the predictive value of treatment efficacy in FM. Result: tDCS reduced pain intensity. tDCS also have effects on the brain metabolites. Baseline levels of these metabolites predicted pain reduction after tDCS. Duration of effects: NA |
sham | |||||||||||
 Fagerlund et al., 2015 [18] | Norway | RCT | 24 | 24 | Left M1 | Right SO | Anodal, 2 mA, 20 min, 5 sessions, consecutive days | None | Skin redness, sleepiness, and tingling were reported same in the active and sham group. Acute mood changes were more reported in the sham group | VAS, PPT | Aim: to investigate the effect of tDCS stimulation on pain in FM Result: tDCS has a small but significant effect on pain reduction in FM.Duration of effects: NA |
 Junior et al., 2015 [34] | Brazil | RCT | 10 (f) | 10 (f) | Left M1 | Right SO | Anodal, 1 mA, 20 min, 10 sessions, consecutive days. | None | None | VAS | Aim: to evaluate the effect of tDCS on pain and quality of life in FM. Result: tDCS is effective in pain control of FM. patients Duration of effects: NA |
 Yoo et al., 2018 [6] | Belgium | RCT | 20 |  | Left DLPFC + ONS | Right SO | 1.5 mA, 20 min, each intervention, 8 sessions over 4 weeks, sessions were 3 days apart. | ONS | Tingling and itching | NRS | Aim: to investigate the effect of adding prefrontal tDCS before ONS on pain and quality of life in FM. Results: prefrontal tDCS did not change the pain compared to ONS-only group. Duration of effects: NA |
20 | Â | ONS alone | Â | ||||||||
| Â | 20 | Â | Â | ||||||||
 To et al., 2017 [33] | Belgium | RCT | 11 |  | Left DLPFC | Right DLPFC | Anodal, 1.5 mA, 20 min, 8 sessions, in 4 weeks | None | None | NRS, PCS | Aim: to compare the effects of bifrontal and occipital tDCS on pain and fatigue in FM Result: both bifrontal and occipital tDCS reduced pain scores, DLPFC also improved fatigue and provided more general relief than C2 stimulation Duration of effects: NA |
15 | Â | Left occipital | Right occipital | ||||||||
| Â | 16 | Â | Â | Initial 10Â s of active stimulation and then inactive for 20Â min, same number of sessions. | |||||||
 Khedr et al., 2017 [30] | Egypt | RCT | 18 | 18 | Left M1 | Right SO | Anodal, 2 mA, 20 min, 10 sessions, 5 consecutive days over two weeks. | None | Itching and redness of skin in only 3 cases from the active group. | VAS | Aim: to evaluate the effects of tDCS on pain, mood, and serum endorphin levels in the treatment of FM. Results: M1-tDCS was able to improve pain in FM significantly. This effect is related to changes in serum endorphin levels Duration of effects: at least one month after stimulation |
 Melo et al., 2020 [8] | Brazil | RCT | 11 (f) |  | Left M1 | Right SO | Anodal, 2 mA, 20 min, 1 week (5 consecutive days) | None | None | VAS | Aim of study: To compare the effects of two tDCS protocols on pain and EEG alpha-2 oscillations in FM Result: Both protocols reduced pain intensity without significant difference, but only those received for 5 consecutive days, showed a significant reduction in alpha-2 power in the frontal and parietal region Duration of effects: NA |
9 (f) | Anodal, 2 mA, 20Â min, 2 weeks (10 consecutive days excluding weekends) | ||||||||||
|  | 11 (f) − 5 consecutive days |  |  |  | |||||||