While polarity-specific after-effects of monopolar transcranial direct current stimulation (tDCS) on corticospinal excitability are well-documented, modulation of vital parameters due to current spread through the brainstem is still a matter of debate, raising potential concerns about its use through the general public, as well as for neurorehabilitation purposes. We monitored online and after-effects of monopolar tDCS (primary motor cortex) in 10 healthy subjects by adopting a neuronavigated transcranial magnetic stimulation (TMS)/tDCS combined protocol. Motor evoked potentials (MEPs) together with vital parameters [e.g., blood pressure, heart-rate variability (HRV), and sympathovagal balance] were recorded and monitored before, during, and after anodal, cathodal, or sham tDCS. Ten MEPs, every 2.5-min time windows, were recorded from the right first dorsal interosseous (FDI), while 5-min epochs were used to record vital parameters. The protocol included 15 min of pre-tDCS and of online tDCS (anodal, cathodal, or sham). After-effects were recorded for 30 min. We showed a polarity-independent stabilization of cortical excitability level, a polarity-specific after-effect for cathodal and anodal stimulation, and an absence of persistent excitability changes during online stimulation. No significant effects on vital parameters emerged both during and after tDCS, while a linear increase in systolic/diastolic blood pressure and HRV was observed during each tDCS condition, as a possible unspecific response to experimental demands. Taken together, current findings provide new insights on the safety of monopolar tDCS, promoting its application both in research and clinical settings.
Keywords: neuromodulation; safety; transcranial direct current stimulation; transcranial magnetic stimulation; vital parameters.