The effect of inter-electrode distance on the electric field distribution during transcutaneous lumbar spinal cord direct current stimulation

Previous studies have indicated potential neuromodulation of the spinal circuitry by transcutaneous spinal direct current stimulation (tsDCS), such as changes in motor unit recruitment, shortening of the peripheral silent period and interference with supraspinal input to lower motor neurons. All of these effects were dependent on the polarity of the electrodes. The present study investigates how the distance between the electrodes during tsDCS influences the electric field's (E-field) spatial distribution in the lumbar and sacral spinal cord (SC). The electrodes were placed longitudinally along the SC, with the target electrode over the lumbar spine, and the return electrode above the former, considering four different distances (4, 8, 12 and 16 cm from the target). A fifth configuration was also tested with the return electrode over the right deltoid muscle. Peak values of the E-field's magnitude are found in the lumbo-sacral region of the SC for all tested configurations. Increasing the distance between the electrodes results in a wider spread of the E-field magnitude distribution along the SC, with larger maximum peak values and a smoother variation. The fifth configuration does not present the highest maximum values when compared to the other configurations. The results indicate that the choice of the return electrode position relative to the target can influence the distribution and the range of values of the E-field magnitude in the SC. Possible clinical significance of the observed effects will be discussed.