Intensity Dependent Effects of Transcranial Direct Current Stimulation on Corticospinal Excitability in Chronic Spinal Cord Injury

Abstract Objective To investigate the effects of anodal transcranial direct current stimulation (a-tDCS) intensity on corticospinal excitability and affected muscle activation in individuals with chronic spinal cord injury (SCI). Design Single-blind, randomized, sham-controlled, crossover study. Setting Medical research institute and rehabilitation hospital. Participants Volunteers (N=9) with chronic SCI and motor dysfunction in wrist extensor muscles. Interventions Three single session exposures to 20 minutes of a-tDCS (anode over the extensor carpi radialis [ECR] muscle representation on the left primary motor cortex, cathode over the right supraorbital area) using 1mA, 2mA, or sham stimulation, delivered at rest, with at least 1 week between sessions. Main Outcome Measures Corticospinal excitability was assessed with motor-evoked potentials (MEPs) from the ECR muscle using surface electromyography after transcranial magnetic stimulation. Changes in spinal excitability, sensory threshold, and muscle strength were also investigated. Results Mean MEP amplitude significantly increased by approximately 40% immediately after 2mA a-tDCS (pre: 0.36±0.1mV; post: 0.47±0.11mV; P =.001), but not with 1mA or sham. Maximal voluntary contraction measures remained unaltered across all conditions. Sensory threshold significantly decreased over time after 1mA ( P =.002) and 2mA ( P =.039) a-tDCS and did not change with sham. F-wave persistence showed a nonsignificant trend for increase (pre: 32%±12%; post: 41%±10%; follow-up: 46%±12%) after 2mA stimulation. No adverse effects were reported with any of the experimental conditions. Conclusions The a-tDCS can transiently raise corticospinal excitability to affected muscles in patients with chronic SCI after 2mA stimulation. Sensory perception can improve with both 1 and 2mA stimulation. This study gives support to the safe and effective use of a-tDCS using small electrodes in patients with SCI and highlights the importance of stimulation intensity.