Safety and effects on motor cortex excitability of five anodal transcranial direct current stimulation sessions in 24 hours

Background and objective. - Application parameters of transcranial direct current stimulation (tDCS) for therapeutic purposes are relatively restricted. The aim of this study was to assess safety and effects on motor cortex excitability of an intensive anodal-tDCS protocol. Methods. - In 26 healthy subjects, five 15-minute anodal-tDCS sessions were delivered, at increasing time intervals, over 24 hours. Safety was defined as absence of serious adverse events including brain tissue alterations on magnetic resonance imaging. Effect on motor cortex excitability was evaluated by motor evoked potential (MEP) amplitude, measured eight times. Results. - No serious adverse events occurred. Mild adverse events, such as reversible scalp erythema or transient metallic taste, were observed in 27% of subjects. MEP amplitudes did not change in any of the recording periods. When inter-individual variability was taken into account and threshold values defined, 50% of subjects were classified as responders, 15% were inverse responders, and 35% non-responders. In the responders, normalized MEP was increased by 57% 1 hour after the first anodal-tDCS and increased by 50% three hours after two stimulations delivered 1 hour apart. Intra-individual, inter-sessional consistency of MEP response over four measurements was 61-77%. Discussion. - Five anodal-tDCS delivered in 24 hours are safe and well tolerated, expanding the safety standard of tDCS. However, only half of subjects respond to anodal-tDCS with a robust and durable MEP augmentation. On the other hand, the response to a single anodal-tDCS predicts fairly well the response to other sessions in the same subject. Conclusions. -These findings should be considered in clinical trials utilizing repeated anodal-tDCS. (