Neuromodulatory Effects of Non-invasive Transspinal Stimulation on Spinal Excitability After Incomplete Spinal Cord Injury: A Longitudinal Pilot Study

Research Objectives: To study the changes in spinal excitability following non-invasive Transspinal stimulation (TSS) after incomplete spinal cord injury (SCI).

Design: MEPs were captured at three different time points over a period of 21 weeks as a part of a clinical trial. Participants were assigned to a telehealth-home training sessions to undergo cervical TSS, fore-arm electrically evoked blood flow restricted exercise and task-specific training three times weekly for 16 weeks.

Setting: Home-based training

Participants: Four participants with chronic incomplete SCI [AIS B-D], with an age range of 37 ± 14.6 years and time since injury of 2-23 years, were recruited as a part of a clinical trial.

Interventions: Two cathodal electrodes (25 mm) were placed between interspinous processes of C3-C4 and C6-C7. Two rectangular electrodes (5 x 10 cm) were placed bilaterally over the clavicles. Testing was done at baseline 1 (BL1; before the study), baseline 2 (BL2; 4 weeks after BL1), and post-intervention 1 (P1; 16 weeks after BL2). The initial 4 weeks were used to educate the participants and their caregivers to safely use the stimulation unit (Ex4, Richmar) at home setting. Blood pressure was monitored before, during and after the training session. MEP was measured using a stimulation frequency of 1 Hz, a biphasic pulse of 2 msec, and stimulation intensity increased from 1 to 40 mA. MEP data were collected using wireless TRINGO EMG sensors that were placed bilaterally on triceps, biceps, wrist flexor and wrist extensor muscles.

Main Outcome Measures: Recruitment curves were plotted for all muscles at BL1, BL2 and P1 after being normalized to the maximum MEP amplitude vs stimulation intensity. The Boltzmann sigmoidal function was used to calculate the slope of the recruitment curves.

Results: Compared to BL1 and BL2, recruitment curves plotted at P1 for the triceps, biceps, and wrist flexor muscles revealed a noticeable left shift with a trend towards plateauing. Conversely, the recruitment curve of the wrist extensors displayed a non-plateau rise with increasing the stimulation intensity. Finally, the stimulation intensity required to achieve motor threshold decreased over the course of the study.

Conclusions: The findings highlight the neuromodulation effects of TSS and increased excitability of the cervical neural circuitries, which may subsequently enhance neuroplasticity and restoration of motor control in persons with incomplete SCI. The behavior of the wrist extensors curve over time may represent reciprocation of the tenodesis pattern to develop a more natural grasp.

Author(s) Disclosures: We do not have any COI to declare.