Modelling the current density generated by transcutaneous spinal Direct Current Stimulation (tsDCS)

Objective: Non-invasive transcutaneous spinal direct current stimulation (tsDCS) induces changes in spinal cord function in humans. Nonetheless, the current density (J) spatial distributions generated by tsDCS are unknown. This work aimed to estimate the J distributions in the spinal cord during tsDCS. Methods: Computational electromagnetics techniques were applied to realistic human models, based on high-resolution MRI of healthy volunteers (a 26-years-old female adult model "Ella"; a 14 years-old male adolescent model "Louis"; an 11 years old female adolescent model "Billie"). Three electrode montages were modelled. In all cases, the anode was always over the spinal process of the tenth thoracic vertebra and the cathode was placed: A) above the right arm; B) over the umbilicus; C) over Cz. The injected current was 3 mA. The electrodes were conductors within rectangular sponges. Results: Despite inter-individual differences, the J tends to be primarily directed longitudinally along the spinal cord and cauda equina with the region of higher amplitude influenced by the reference electrode position; on transversal sections, the J amplitude distributions were quite uniform. Conclusions: Our modeling approach reveals that the J generated by tsDCS reaches the spinal cord, with a current spread also to the muscle on the back and the spinal nerve. Significance: This study is a first step in better understanding the mechanisms underlying tsDCS.