TABLE 1.

Characteristics of included studies.

References	Study design	N	Age (years), mean (SD)	Time since injury	Sex (ratio F:M)	Design	Blinding	Stimulation site	Sham location	Parameters	Results
Estraneo et al., 2017	RCT	23		≥3 months after brain injury	06:07:00	Crossover	Double-blind	Anode in L-DLPFC. Cathode in the superior margin of the right orbit.	Anode in L-DLPFC. Cathode in the superior margin of the right orbit. Electrodes: 35-cm2 (7 × 5 cm)	10 sessions (5 active and 5 sham); 20 min of anodal tDCS, 2 mA; or sham tDCS. One-week washout	Negative. Substantial clinical and EEG changes were observed in 5/13 patients (3 in MCS and 2 in VS). No baseline features distinguished patients who improved from patients who did not improve.
Martens et al., 2018	RCT	22	41.86	Chronic MCS	06:16:00	Crossover	Double-blind	Anode in L-DLPFC. Cathode in the superior margin of the right orbit.	Anode in L-DLPFC. Cathode in the superior margin of the right orbit. Electrodes: 35-cm2 (7 × 5 cm)	20 sessions in two periods with an interval of 8 weeks; 20 min of anodal tDCS, 2 mA or tDCS sham.	Positive. A moderate effect size (0.47 and 0.53, for modified intention to treat and per protocol analysis, respectively) was observed at the end of the 4 weeks of tDCS in favor of the active treatment. Electrodes:
Rushby et al., 2020	RCT	30	50 (15.09)	13.9 ± 12.12 years since	09:21:00	Crossover	Single-blind	Anode in left parietal cortex.	Anode in left parietal cortex.	One session; 2 mA for 20 min.	Negative. tDCS led to no improvements in accuracy on the working memory tasks. A slight increase in variability and reaction time with tDCS was related to decreased task activated arousal. electrodes
Sacco et al., 2016	RCT	32	Experimental group: 37.7 (10.4) control group: 35.2 (12.9)	8.73 ± 4.45 years	06:26:00	Parallel groups	Double-blind	Bi-montage: anode in DLPFC right or left (the lesioned hemisphere. Cathode on the other hemisphere.	Bi-montage. (7 × 5 cm, 35 cm2)	10 sessions (twice a day); 20 min; 2 mA.	Positive. The results showed that the experimental group significantly improved in DA performance between pre- and post-treatment, showing faster reaction times (RTs), and fewer omissions.
Thibaut et al., 2014	RCT	55		1 week after acute traumatic or non-traumatic insult		Crossover	Double-blind	Anode in L-DLPFC. Cathode in the superior margin of the right orbit. the superior margin of the right orbit.	Anode in L-DLPFC. Cathode in the superior margin of the right orbit. Electrodes: 35-cm2 (7 × 5 cm)	2 sessions; 20 min of anodal tDCS, 2 mA; or sham tDCS; 48-h washout.	Negative. Patients in MCS (n = 30; interval 43 ± 63 months; 19 traumatic, 11 non-traumatic) showed a significant treatment effect (p = 0.003) as measured by CRS-R total scores. In patients with VS/UWS (n = 25; interval 24 ± 48 mo; 6 traumatic, 19 non-traumatic), no treatment effect was observed (p = 0.952). Thirteen (43%) patients in MCS and 2 (8%) patients in VS/UWS further showed post-anodal tDCS-related signs of consciousness, which were observed neither during the pre-tDCS evaluation nor during the pre- or post-sham evaluation.
Thibaut et al., 2014	RCT	21	47 (17–74)	Chronic MCS	05:09:00	Crossover	Double-blind	Anode in L-DLPFC. Cathode in the superior margin of the right orbit.	Anode in L-DLPFC. Cathode in the superior margin of the right orbit. 35-cm2 (7 × 5 cm)	10 sessions (5 active and 5 sham); 20 min of anodal tDCS, 2 mA; or sham tDCS. One-week washout sham); 20 min of anodal tDCS, 2 mA; or sham tDCS. One-week washout	Positive. A treatment effect (p = 0.013; effect size = 0.43) was observed at the end of the active tDCS session (day 5) as well as 1 week after the end of the active tDCS session (day 12; p = 0.002; effect size = 0.57).
Ulam et al., 2015	RCT	26	31.4 (35.70)	Hospitalized in the acute and subacute phase of brain injury.	04:22:00	Parallel group	Double-blind	Anode in L-DLPFC. Cathode in the superior margin of the right orbit.	Anode in L-DLPFC. Cathode in the superior margin of the right orbit. Electrodes were 3.8 cm × 4.4 cm	10 consecutive sessions; 1 mA; 20 min.	Positive. Theta was significantly reduced for active tDCS patients following the first tDCS session. Delta decreased and alpha increased, both significantly, for the active tDCS group after 10 consecutive tDCS sessions. Decreases in delta were significantly correlated with improved performance on neuropsychological tests for the active tDCS group to far greater degree than for the sham group.

L-DLPFC, left dorsolateral prefrontal cortex; L-TC, left temporal cortex; DOC, disorder of consciousness; MCS, minimally conscious state; mTBI, mild traumatic brain injury.