The Effects of High Definition Transcranial Direct Current Stimulation Targeting the Primary Motor Cortex on Experimental Capsaicin-Induced Pain in Healthy Volunteers

Abstract
Background: Chronic pain is one of the most common health problems in the world and a significant challenge to clinical practice. Which has severe effects on patient's social and daily work that costs a lot of the governmental support. Traditionally, the focus on alleviating chronic pain has relied mainly on pharmacotherapy which has limited efficacy and significant side-effects, especially with of prolonged use. Especially with opioids which is the most widely prescribed drugs for pain.
Transcranial Direct Current Stimulation (tDCS) which deliver a weak current up to (2mA) through electrodes on the scalp has attracted much interest as it is safe, inexpensive and easy to implement and can be used as an alternative or adjuvant to pharmacotherapy.
Aim of Study: In this study, we aimed to test the effects of the newly developed and more focally targeted anodal High Definition transcranial Direct Current Stimulation (HD-tCDS) targeting the PMC on a capsaicin-induced pain model in healthy subjects. We set out to test the hypothesis that it is possible to use HD-tDCS to stimulate the PMC and to compare this with sham stimulation.
Patients and Methods: After obtaining informed consent, 36 healthy volunteers (20 male). All volunteers subjected to the following experimental paradigm: Application of capsaicin cream (0.075%) on a 9cm2 area of skin for 30 minutes on the volar surface of both forearms. The volunteers were blinded and then divided into two groups of 18 with each subjected to 20 minutes of the following: Group 1 received Sham HD tDCS targeting PMC, group 2 anodal HD tDCS (2mA) targets the PMC. Pain severity was collected using Numerical Rating Scale (NRS) score at regular intervals before and after the session. The subjects also asked whether they were able to differentiate between sham and active stimulation and any complications of stimulation recorded. The NRS scores also compared between the two groups. Statistical significance was calculated using suitable tests and pResults: Compared to sham stimulation, the PMC stimu-lation by HD-tDCS associated with a faster reduction in NRS pain scores after application of capsaicin to forearm skin. Also, it was well tolerated without documented adverse effect that necessitates discontinuation of the session.
Conclusion: Anodal HD-tDCS of the primary motor cortex was associated with a faster reduction in reported pain score when compared with sham stimulation. Which can be used as a new treatment option for different chronic pain conditions.