Adaptive current tDCS up to 4 mA

Abstract

Background: Higher tDCS current may putatively enhance efficacy, with tolerability the perceived limiting factor. Objective: We designed and validated electrodes and an adaptive controller to provide tDCS up to 4 mA, while managing tolerability. The adaptive 4 mA controller included incremental ramp up, impedancebased current limits, and a Relax-mode where current is transiently decreased. Relax-mode was automatically activated by self-report VAS-pain score >5 and in some conditions by a Relax-button available to participants. Methods: In a parallel-group participant-blind design with 50 healthy subjects, we used specialized electrodes to administer 3 daily session of tDCS for 11 min, with a lexical decision task as a distractor, in 5 study conditions: adaptive 4 mA, adaptive 4 mA with Relax-button, adaptive 4 mA with historical-Relaxbutton, 2 mA, and sham. A tablet-based stimulator with a participant interface regularly queried VAS pain score and also limited current based on impedance and tolerability. An Abort-button provided in all conditions stopped stimulation. In the adaptive 4 mA with Relax-button and adaptive 4 mA with historical-Relax-button conditions, participants could trigger a Relax-mode ad libitum, in the latter case with incrementally longer current reductions. Primary outcome was the average current delivered during each session, VAS pain score, and adverse event questionnaires. Current delivered was analyzed either excluding or including dropouts who activated Abort (scored as 0 current). Results: There were two dropouts each in the adaptive 4 mA and sham conditions. Resistance based current attenuation was rarely activated, with few automatic VAS pain score triggered relax-modes. In conditions with Relax-button option, there were significant activations often irrespective of VAS pain score. Including dropouts, current across conditions were significantly different from each other with maximum current delivered during adaptive 4 mA with Relax-button. Excluding dropouts, maximum current was delivered with adaptive 4 mA. VAS pain score and adverse events for the sham was only significantly lower than the adaptive 4 mA with Relax-button and adaptive 4 mA with historical-Relaxbutton. There was no difference in VAS pain score or adverse events between 2 mA and adaptive 4 mA. Conclusions: Provided specific electrodes and controllers, adaptive 4 mA tDCS is tolerated and effectively blinded, with acceptability likely higher in a clinical population and absence of regular querying. Indeed, presenting participants with overt controls increases rumination on sensation.

Cerebellar Transcranial Direct Current Stimulation in People with Parkinson’s Disease: A Pilot Study

“…This is particularly noteworthy because many of the responders in this study likely would have been classified as non-responders if either of the 'standard' configurations (unilateral) or intensities (2 mA) were used (Figure 2). Fourth, this study adds to the early, but growing, evidence of the tolerability of 4 mA stimulation intensities [29][30][31][52][53][54].…”

“…Thus, the effects of higher intensity (4 mA) tDCS on motor function has received little attention [28,29]. Nevertheless, the tolerability of 4 mA tDCS has been shown to be comparable to sham [30,31] or 2 mA [29] and a recent review concluded that there was no evidence that tDCS up to 4 mA for 30 min had any serious negative side effects [27]. Studies exploring the differences in performance between lower (2 mA) and higher (4 mA) intensities are necessary to determine if increasing intensity enhances the desired outcome.…”

The Tolerability and Efficacy of 4 mA Transcranial Direct Current Stimulation on Leg Muscle Fatigability

“…A few limitations are noted for this study. First, although this study recruited more subjects (n = 31) than other tolerability reports ([16] n = 18), it was not as large as other investigations ( [19] n = 50 young, healthy; [1] n = 77 young, healthy; [15] n = 512 young, healthy). Therefore, the relatively small number of subjects in this study may caution generalization.…”

“…There are a few that have used intensities >2 mA and have found that 4 mA was safe and well tolerated in stroke survivors [14,16], patients with movement disorders [17], and patients with major depression [18], but each had a very small number of subjects (n < 3). Additionally, a recent report testing an adaptive stimulation controller found that 4 mA tDCS was well tolerated by young adults [19]; however, they did not explore the effects of higher intensity stimulation on a motor task. Thus, investigation of the effects and tolerability of intensities >2 mA is still necessary.…”

Different Effects of 2 mA and 4 mA Transcranial Direct Current Stimulation on Muscle Activity and Torque in a Maximal Isokinetic Fatigue Task

“…Early studies examining the safety of tDCS for human subjects used careful and moderate stimulation approaches (Bikson et al, 2016 ). However, recent studies have shown that intensities up to 4 mA are safe, tolerable, and do not elicit any serious adverse effects (Bikson et al, 2016 ; Workman et al, 2019 , 2020b ; Khadka et al, 2020 ). Now that the safety and tolerability of tDCS at higher intensities is better established, work exploring the performance differences between moderate (i.e., 2 mA) and higher (i.e., 4 mA) intensities is necessary to determine if increasing intensity further enhances outcomes.…”