Recent Progress and Future Directions of Transcranial Electrical Stimulation for Analgesia

Transcranial electrical stimulation (tES) is a non-invasive neural modulation technique known for its high safety, patient compliance, and portability. It holds promise as a potential non-pharmacological method for analgesia. However, challenges persist in utilizing tES for pain management, including inconsistent research findings and limited understanding of its analgesic mechanisms. Therefore, by summarizing the advances in the analgesic researches employing the 3 primary tES techniques, transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS), and transcranial random noise stimulation (tRNS), we reviewed the analgesic effects on both acute and chronic pain, as well as the neural mechanisms underlying the analgesic effect of each technique. Accumulating evidence suggests that the analgesic effects of tDCS are significant, but studies on analgesic effects of tACS and tRNS remain limited. And the exact mechanisms of pain relief through tES turned out to be not yet well established. Furthermore, we systematically discussed the limitations of analgesia-related studies employing tES techniques across various aspects, involving research design, stimulation protocol formulation, neural response observation, analgesic effect assessment, and safety considerations. To address these limitations and advance clinical translation, we emphasized utilizing promising stimulation techniques and offered practical suggestions for future research endeavors. Specifically, employing numerical simulation of electric field guided by magnetic resonance imaging (MRI) would reduce variability of outcomes due to individual differences in head anatomy. For this purpose, it is advisable to establish standardized head models based on MRI data from the Chinese populations and validate simulated electric field results in tES research to diminish confounding factors concerning anatomy. Meanwhile, novel techniques like multi-site brain stimulation and interferential stimulation (IFS) could broaden the range of stimulation sites in both scope and depth. Multi-site brain stimulation facilitates modulation of entire neural networks, enabling more sophisticated investigations into the complexity of pain. IFS can reach deep brain tissues without invasive surgical procedures, achieving more comprehensive modulation. Regarding neural response observations, establishing a tES-neuroimaging synchronized platform would enable revealing its mechanisms and personalizing protocols based on inter-subject neural response variability detected through recordings. By integrating tES with various neuroimaging techniques, such as functional MRI, electroencephalography (EEG) and magnetoencephalography, into one unified platform, researchers could examine brain activities in baseline before stimulation, dynamic changes in brain activities during stimulation, and sustained brain responses after stimulation. Additionally, collecting finer-grained data on participant characteristics and pain intensity would enhance the sensitivity of future studies. In designing clinical trials to evaluate chronic pain treatments and reporting the results, adopting the six core outcome domain measures recommended by the Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials (IMMPACT) could prove beneficial. Lastly, safety considerations can never be overemphasized in future tES studies especially when combining tES with MRI and EEG techniques. These efforts may help to broaden the research scope, reconcile inconsistencies in findings and elucidate the analgesic mechanisms of tES, thus facilitating the development of pragmatic pain management strategies such as combination therapies and home therapies. Ultimately, these suggestions will maximize the clinical application value of tES in pain treatment to achieve pain relief for patients.