Safety and effects on motor cortex excitability of five cathodal transcranial direct current stimulation sessions in 25 hours

Objective. -To assess the safety and effects on motor cortex excitability of five cathodal-tDCS sessions (charge density 342.9 C/m(2)) delivered over the dominant motor cortex with a return electrode over the ipsilateral shoulder at increasing time intervals in 25 hours. Methods. -Safety was operatively defined as absence of serious adverse events related to tDCS including brain tissue alterations documentable by magnetic resonance imaging and spec- troscopy. Effects on motor cortex excitability were evaluated by motor evoked potential (MEP) amplitude. Results. -Thirty-two healthy subjects were enrolled. No serious adverse events occurred. Magnetic resonance imaging and spectroscopy did not show alterations. Inter-individual MEP variability was assessed by the standard error of mean at baseline and subjects were classi- fled on the basis of the ratio between normalized MEPs after the first stimulation compared to baseline. Fifty-six percent of subjects responded with reduction of MEP amplitude, 25% were non-responders and 19% were inverse responders. In responders, MEP suppression was 32% one hour after the end of first cathodal-tDCS, 21% three hours after the second, no longer present with increasing stimulation intervals and 38% two and half hours after the fifth stimulation. Intra-individual inter-sessional reliability in response was high (88-92%). Conclusions. - Five cathodal-tDCS sessions in 25 hours are safe. Inter-individual variability in MEP suppression is considerable but response to one cathodal-tDCS highly predicts the response to other sessions. Duration of MEP suppression is limited to three hours. These findings should be considered in trials utilizing repeated cathodal-tDCS. (C) 2017 Elsevier Masson SAS. All rights reserved.