Table 1.
Characteristics of included studies investigating the effects of tDCS in ADHD.
| Reference | Study type (Blinding) | n (mean age ± SD); gender (m/f) | ADHD type (inattentive/hyperactive/combined) | Medication intake (n) | Task (measure) | Expected outcome | Current intensity; duration | Repetition rate | Polarity/electrode size | Control | Results |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Prehn-Kristensen et al. [45] | RCT (double blind) | ADHD children n = 12 (12 ± 1.4, 10–14 years)/Healthy control n = 12; 24/0 | 4/0/8 | Off stimulants 48 h (n = 12) | Declarative memory | Change in score in the retrieval phase | So-tDCS (0–250 µA, 0.75 Hz); 5 × 5 min | Single session | Anodal F3- cathodal M1 & anodal F4 - cathodal M2/0.503 cm2 (Ag/AgCl sintered skin electrodes) | Sham | Enhanced memory consolidation and retrieval following active tDCS vs sham tDCS |
| Munz et al. [41] | RCT (double blind) successful blinding | n = 14 (12.3 ± 1.39, 10–14 years); 14/0 | 6/0/8 | Off stimulants 48 h (n = 10) | Go/No-Go (response inhibition), motor memory task | Change in the RT and accuracy of tasks | So-tDCS (0–250 µA, 0.75 Hz); 5 × 5 min | Single session | Anodal F3- cathodal M1 & anodal F4 – cathodal M2/0.503 cm2 (Ag/AgCl sintered skin electrodes) | Sham | Significantly improved RT after active stimulation vs sham in Go/No-Go and motor memory tasks but no significant improvement in the correct responses. |
| Cosmo et al. [43] | RCT (double blind) successful blinding | n = 60 (32.25 ± 10.96,18–65 years); 35/25 | 13/3/44 | On stimulants (n = 11) | Go/No-Go (response inhibition) | Change in the Go/No-Go task performance | 1 mA; 20 min | Single session | Anodal F3-cathodal F4/5 × 7 cm | Sham | No significant differences in inhibitory control between active tDCS vs sham tDCS |
| Soltaninejad et al. [62] | RCT (single-blind) | n = 20 (16.40 ± 1.09, 15–17 years); 20/0 | NR | Off medication | Go/No-Go and Stroop (response inhibition, attention, interference inhibition) | Change in Go/No-Go & Stroop tasks performance | 1.5 mA; 15 min | Single session |
(1) anodal F3 - cathodal Fp2 (2) cathodal F3, anodal Fp2 /5 × 7 cm |
Sham | Anodal left dlPFC tDCS vs sham, did not improve response inhibition in the Go-No/Go task. Cathodal left dlPFC Tdcs vs sham, improved inhibition accuracy in the Go-No/Go task. Noprotocols improved selective attention |
| Bandeira et al. [44] | Open-label study | n = 9 (11.11 ± 2.08, 6–16 years); 8/1 | NR | Off medication 1 week before | TAVIS 3, Corsi Cube test, NEPSY II & Digit Span (selective attention, WM, response inhibition) ADHD symptoms via SNAP-IV | Changes in selective attention, WM, IC tasks, and SNAP-IV | 2 mA; 30 min | Five sessions | Anodal F3 cathodal Fp2/5 × 7 cm | Baseline Control | Significantly reduced omission errors in selective attention task, shorter RT and reduced errors in the switching but not inhibition phase. Clinical symptoms slightly improved according to the parent’s response in the SNAP-IV |
| Breitling et al. [42] | RCT (single blind) unsuccessful blinding | ADHD children n = 21 (14.33)/Healthy control n = 21 (14.24); 21/0 | 5/0/6 | Off stimulants 24 h (n = 11) | Flanker task (interference control, response inhibition) (online) | Changes in the Flanker task | 1 mA; 20 min | Three sessions | Anodal-cathodal-shamF8, reference electrode over left mastoid/5 × 7 cm | Sham | No significantly improved interference control in ADHD patients. Only anodal tDCS, if applied in the first session, significantly diminished commission errors in ADHD group vs healthy controls (learning effect). |
| Cachoeira et al. [48] | RCT (double blind) successful blinding | n = 17 (32.37 ± 4.91, 18–45 years); 8/9 | 7/0/10 | Off stimulants | ASRS (ADHD symptoms) & SDS | Change in the ASRS and SDS scores | 2 mA; 20 min | Five sessions | Anodal F4- cathodal F3/5 × 7 cm | Sham | Bilateral dlPFC tDCS (anodal right) improved symptoms in adult ADHD. This improvement persisted after the end of the stimulation |
| Nejati et al. [46] (Exp 1) | RCT (double-blind) | n = 15 (10 ± 2.3, 8–15 years); 15/0 | 6/0/9 | Off stimulants | Go/No-Go (response inhibition)/1-back (WM)/Stroop (interference control)/WCST (cognitive flexibility) | Change in the Go/No-Go, 1-back, Stroop & WSCT tasks performance | 1 mA; 15 min | Two sessions | Anodal F3- cathodal F4/5 × 5 cm | Sham | Bilateral dlPFC tDCS (anodal left) improved executive control function (WM, interference control) but not prepotent response inhibition and cognitive flexibility |
| Nejati et al. [46] (Exp 2) | RCT (double-blind) | n = 10 (9 ± 1.8, 7–12 years); 5/5 | 5/5 | Off stimulants | Go/No-Go (response inhibition)/1-back (WM)/WCST (cognitive flexibility) | Change in the Go/No-Go, 1-back, & WSCT tasks performance | 1 mA; 15 min | Three sessions | (1) Anodal F3-cathodal Fp2 (2) cathodal F3-anodal Fp2/ 5 × 5 cm | Sham | Anodal left dlPFC tDCS improved WM accuracy and RT. Cathodal left dlPFC tDCS improved response inhibition. Both anodal and cathodal left dlPFC tDCS improved cognitive flexibility. |
| Soff et al. [47] | RCT (double-blind) successful blinding |
n = 15 (14.20 ± 1.2, 12–16 years); 12/3 |
patients were combined or hyperactive type | Off stimulants 96 h (n = 5) | FBB-ADHD &QbTest | Change in the inattention, hyperactive and impulsivity scales | 1 mA; 20 min | Five sessions | Anodal F3, cathodal Cz/5 × 7 cm | Sham | Anodal tDCS significantly reduced symptoms of inattention (measured by FBB-ADHD), and hyperactivity/inattention(measured by the Qbtest) in adolescents with ADHD compared to sham stimulation |
| Sotnikova et al. [40] | RCT (double-blind) successful blinding |
n = 13 (14.33 ± 1.32, 12–16 years); 11/2 |
patients were combined or hyperactive type | Off stimulants 96 h (n = 5) | Q-b Test (WM) | Changes in the Qb test scales | 1 mA; 20 min | Two sessions | Anodal F3, cathodal Cz/5 × 7 cm | Sham | Moderate effect of tDCS. Anodal tDCS led to significantly less increased RT and RT variability but also more errors. Increased neuronal activation and connectivity in the left dlPFC and other remote brain regions |
| Jacoby et al. [63] | RCT (single-blind) | ADHD group n = 21/Healthy control n = 16 (23.03 ± 2.54, 19–29 years); 23/12 | NR | On stimulants | MOXO-CPT (Attention, hyperactivity) | Changes in the CPT performance | 1.8 mA; 20 min | Two sessions | Double anodal bilateral tDCS (anodal 1 F3, anodal 2 F4, cathodal cerebellar cortex) | Sham | tDCS did not improve attention, timing, and impulsivity. The only measure which was improved by tDCS was hyperactivity |
| Allenby et al. [49] | RCT (double-blind) unsuccessful blinding | n = 37 (18–65); 26/11 | 21/0/16 | On stimulants (n = 17) | CPT, SST | Change in the CPT false error and SST accuracy and RT | 2 mA; 20 min | Three sessions | Anodal F3-cathodal Fp2/5 × 5 | Sham | Anodal left dlPFC tDCS improved impulsivity symptoms in ADHD compared to sham tDCS |
| Breitling et al. [64] | RCT (double-blind) | ADHD children n = 14 (13.3 ± 1.9)/Healthy control n = 15 (13.3 ± 1.8); 25/4 | 4/0/10 | Off stimulants 24 h (n = 5) | 2-back (WM) (online) and right after tDCS during EEG/K-SADS-PL (symptoms) | Change in 2-back task performance |
1 mA (conventional) 0.5 mA (4 × 1 montage); 20 min |
Three sessions | (1) Anodal F8, cathodal Fp1/5 × 7 cm; (2) 1 cm diameter electrodes in 4 × 1 montage (HD-tDCS) | Sham | No effect of conventional or HD-tDCS on WM performance. Numerically higher rate of responders for 4 × 1 (50%) than conventional (35%) tDCS. Higher N200 and P300 amplitudes after both protocols |
atDCS, transcranial direct current stimulation; So-tDCS, Slow-oscillating tDCS; ADHD, attention-deficit hyperactivity disorder; RCT, randomized control study; SD, standard deviation; RT, response time; m, male; f, female; F3, left dorsolateral prefrontal cortex (dlPFC); F4, right dlPFC; F8, right inferior frontal gyrus (rIFG); M1, A1 in 10-20- EEG system (left mastoid); M2, A2 in 10-20 EEG system (right mastoid); IFG, inferior frontal gyrus; Fp1, left supraorbital area; Fp2, right supraorbital area; NR, not reported; TAVIS 3, computerized test of visual attention; WM, working memory; ASRS, Adult ADHD Self-Report Scale Symptom Checklist 3; SDS, Sheehan Disability Scale; CPT, Conners Continuous Performance Task; SST, Stop Signal Task; FBB-ADHD, German adaptive ADHD Diagnostic Checklist; QbTest, Quantified Behavior Test; SNAP-IV, Swanson, Nolan, and Pelham–IV; K-SADS, Schedule for affective disorders and schizophrenia for school-age children; WSCT, Wisconsin Card Sorting Test.