Influence of electrode configuration on the electric field distribution during transcutaneous spinal direct current stimulation of the cervical spine

Transcutaneous spinal direct current stimulation (tsDCS) is a recent technique with promising neuromodulatory effects on spinal neuronal circuitry. The main objective of the present study was to perform a finite element analysis of the electric field distribution in tsDCS in the cervical spine region, with varying electrode configurations and geometry. A computational model of a human trunk was generated with nine tissue meshes. Three electrode configurations were tested: A) rectangular saline-soaked sponge target and return electrodes placed over C3 and T3 spinous processes, respectively; B1) circular saline-soaked sponge target and return electrodes placed over C7 spinous process and right deltoid muscle, respectively; B2) same configuration as B1, considering circular shaped electrodes with sponge and rubber layers and a small circular connector on the top surface. The electric field distribution for cervical tsDCS predicted higher magnitude in configurations B1 and B2, reaching a maximum of 0.71 V/m in the spinal white matter and 0.43 V/m in the spinal grey matter, with values above 0.15 V/m in the region of the spinal circuits related with upper limb innervation. In configuration A, the values were found to be <; 0.15 V/m through the entire spinal cord. Electric fields with magnitude above 0.15 V/m are thought to be effective in neuromodulation of the human cerebral cortex, so the configurations B1 and B2 could be an optimal choice for cervical tsDCS protocols. Computational studies using realistic models may be a powerful tool to predict physical effects of tsDCS on the cervical spinal cord and to optimize electrode placement focused on specific neurologic patient needs related with upper limb function.