Spinal motor evoked responses elicited by transcutaneous spinal cord stimulation in chronic stroke: Correlation between spinal cord excitability, demographic characteristics, and functional outcomes

Transcutaneous spinal cord stimulation (tSCS) is becoming a promising neuromodulation technique to promote motor recovery in various neurological conditions, including stroke. As this intervention moves forward into clinical practice, it is important to understand how the elicited neurophysiological measures are related to the functional and neuromuscular deficits of the population of interest in order to personalize tSCS interventions and assess its effectiveness. Specifically, neurophysiological measurements of spinal cord excitability can be achieved by recording with EMG spinal motor evoked responses (sMERs) in muscles after applying single pulses of tSCS to the spinal cord. The objective of this study was to investigate potential correlations between baseline spinal cord excitability, as measured by resting motor threshold (RMT) and peak-to-peak (P2P) amplitude of the sMERs, and various factors including demographic characteristics, severity of spasticity, muscle strength, and gait speed in individuals post-stroke. Additionally, the study sought to explore disparities in excitability between the paretic and non-paretic sides. Fifteen participants with chronic stroke underwent sMER assessments. We observed a strong positive correlation between RMT and body weight, indicating weight as a potential confounding variable when comparing RMTs from sMERs between individuals. Furthermore, paretic muscles exhibited lower RMTs and higher P2P amplitudes compared to non-paretic muscles. The results demonstrate that sMERs hold promise in uncovering disparities in spinal excitability in stroke participants. Furthermore, careful interpretation and analysis of sMERs is advised, particularly as higher RMTs were associated with higher body weight and could impact the clinical feasibility of tSCS for some participants. These results should be considered in future tSCS protocols that aim to develop more personalized interventions across different neurological populations and optimize sMERs' utility as an outcome measure.