Introduction

Since the first report of transcutaneous spinal direct current stimulation (tsDCS) on humans in 2008¹, more than 50 experimental studies have been published. However, systematic reports of adverse effects (AEs) and unspecific effects (UEs) of tsDCS are scarce. In this study, we aimed to systematically record tsDCS AEs via a structured questionnaire and also record UEs via tsDCS – concurrent monitoring of skin conductance, electrocardiographic and respiratory activity.

Materials / Methods

Twenty healthy participants (10 females, 20-40 years old) were recruited for this study. All of them underwent three consecutive sessions (at least 1 week apart) with active (anodal/cathodal) or sham stimulation in double-blinded way. A pair of rubber-electrodes was placed over the twelfth thoracic vertebra (anode) and the suprascapular region (cathode). The active stimulation was applied at 2.5 mA for 20 min with 15 sec for fade-in/fade-out, the sham stimulation only lasted for 45 sec with same intensity and fade-in/fade-out setting. Spontaneous skin conductance responses (SCR) were recorded from the right hand and electrocardiographic (ECG: heart-rate and its variability) and respiratory activity (breathing-rate and its variability) was measured from the chest. A tsDCS adverse effects questionnaire (Figure 1, adapted from Brunoni and colleagues2) was filled in by participants directly after tsDCS termination.

Results

Several potential AEs and whether they were deemed to be associated with tsDCS were recorded (Figure 2). Skin redness (60.66%), burning (40%), tinging (26.67%), and itching (20%) were the most reported AEs (with skin redness being reported by the experimenter). Most reported AEs occurred within the first minute of tsDCS onset, lasted for 1-2 minutes and solely occurred at the skin area beneath the electrodes. In some sessions, sleepiness (6.67%) and trouble concentrating (1.67%) were reported, but instead of being strongly associated with tsDSC, they were likely due to no task during tsDCS according to the participants' feedback. For all the AEs separately reported by participants, as well as their aggregation into one overall score, Bayesian Wilcoxon Signed-rank tests were employed to compare active (anodal or cathodal) and sham tsDCS. These were more indicative for an absence of differences between active and sham conditions (Figure 3, not a single Bayes Factor [BF] above 3, with several below 1/3 [thus providing moderate evidence against differences]). Similar results were obtained for UEs (Figure 4), which were analysed with a Bayesian repeated-measures ANOVA: no BF was above 3, with nearly all BFs being below 1 and often close to 1/3.

Discussion

Skin-related changes at the site of electrode contact were the main reported AEs. Bayesian testing provided moderate evidence for an absence of differences in several AEs and in no case was there evidence for a difference. Such a pattern of results is reassuring from a safety and blinding perspective, as it indicates that both active and sham interventions have a similar profile in terms of AEs. Somewhat weaker, but qualitatively similar results for autonomic responses between active and sham tsDCS suggests it possibly has minimal impact on the autonomic nervous system. However, further research is needed to validate these results in larger samples as well as for other locations (e.g. cervical tsDCS) and explore potential individual variations that may influence tsDCS related AEs.

Conclusions

In this pioneering study, we present a systematic recording of AEs and UEs associated with tsDCS in healthy participants. As research in this area expands, there is an increasing demand for safety-focused investigations, and our study offers the first comprehensive assessment, contributing valuable insights to the field.

References

1. Cogiamanian F, Vergari M, Pulecchi F, Marceglia S, Priori A. Effect of spinal transcutaneous direct current stimulation on somatosensory evoked potentials in humans. Clin Neurophysiol. 2008;119(11):2636-2640.

2. Brunoni AR, Amadera J, Berbel B, Volz MS, Rizzerio BG, Fregni F. A systematic review on reporting and assessment of adverse effects associated with transcranial direct current stimulation. Int J Neuropsychopharmacol. 2011;14(8):1133-1145.

Learning Objectives

1. Discuss with colleagues and experts in non-invasive neuromodulation field and look forward to geeting constructive advice.

2. Practice my presenting skills to clearly convey the idea of my research.

3. Establish a professional network and look forward to future collaborations that can be initiated.