Case studies in neuroscience: movement-related cortical stimulation to enhance corticospinal transmission in chronic incomplete spinal cord injury.

Abstract

Incomplete spinal cord injury (iSCI) disrupts signal transmission at the level of injury (LOI) and in higher brain structures, weakening intracortical circuits and impairing movement initiation. A potential approach to target intracortical circuits is to deliver transcranial magnetic stimulation (TMS) during motor intention, known as movement-related cortical stimulation (MRCS). We hypothesize that delivering TMS during motor intention will enhance corticospinal excitability (CE) and improve muscle activation below the LOI. One participant with chronic severe iSCI participated in a crossover study followed by five consecutive treatment sessions. First, we investigated the impact of TMS intensity (subthreshold vs. suprathreshold) on CE when delivered 50 ms before movement. The participant then received five consecutive days of MRCS with active subthreshold TMS for 15-20 min. Experiment 1: CE was assessed before and after sham, suprathreshold, and subthreshold MRCS (1-wk washout), targeting the abductor hallucis muscle. Experiment 2: CE and volitional motor unit recruitment were measured at baseline, the start of each session and 3- and 7-day follow-up. Corticomotor maps were assessed at baseline and post 3- and 7-day follow-up. Subthreshold MRCS increased CE compared with sham and suprathreshold MRCS. Five days of subthreshold MRCS increased CE, motor maps, and volitional motor unit recruitment, with improvements lasting up to the 3-day follow-up and remaining above baseline at day 7. These findings suggest that timed cortical stimulation with movement intention may enhance signal transmission in iSCI below the LOI. Future research is needed to determine if MRCS can prime intracortical circuitry before therapy to improve motor function.NEW & NOTEWORTHY We demonstrate that five consecutive days of movement-related cortical stimulation can enhance corticospinal excitability, expand motor maps, and improve volitional motor unit recruitment in a person with severe incomplete spinal cord injury. These results support the brain's adaptive capacity following spinal cord injury, despite limited motor drive to the muscle, and corroborate the potential to improve motor function by targeting higher-order networks during volitional motor intention with noninvasive brain stimulation.