Tolerability and blinding of 4x1 high-definition transcranial direct current stimulation (HD-tDCS) at two and three milliamps

Background: Transcranial direct current stimulation (tDCS) is an in-demand form of neuromodulation generally regarded as safe and well tolerated. However, few studies have examined the safety, tolerability, or blinding of High Definition (HD-) tDCS, especially in older adults and at stimulation intensities of 2 milliamps (mA) or greater. Objective: We examined the rates of serious adverse events and common side effects to establish safety and tolerability, respectively, in HD-tDCS. Blinding was evaluated using participants' accuracy in correctly stating their condition (i.e., active or sham). Methods: The sample included 101 older adults (M-age = 69.69, SD = 8.33; M-educ = 16.27, SD = 2.42) who participated in our double blind randomized controlled studies or in case studies that used HD-tDCS for 20-30 min at 2 mA (n = 66, 31 active) or 3 mA (n = 35, 20 active). Participants completed a standardized side effect questionnaire and were asked whether they received active or sham stimulation at the end of each session. Results: There were no serious adverse events and no participants withdrew, suggesting that HD-tDCS meets basic safety parameters. Tolerability was comparable between active and sham HD-tDCS regardless of intensity (2 mA and 3 mA) in first session (all p>.09). Tingling was the most commonly endorsed item (59% active; 56% sham) followed by burning sensation (51% active; 50% sham), the majority of which were mild in nature. Severe ratings were reported in fewer than 4% of sessions. Blinding appeared adequate since there were no significant group differences between individuals correctly stating their stimulation condition (chi 2 = 0.689, p = .679). The above tolerability and blinding findings generally persisted when multiple session data (i.e., 186 total sessions) were considered. Conclusions: HD-tDCS appears well-tolerated and safe with effective sham-control in older adults, even at 3 mA. These data support the use of HD-tDCS in randomized controlled trials and clinical translation efforts. Published by Elsevier Inc.