Motor Evoked Potential Input-Output Curves Indicate Neuroplasticity After Cervical Spinal Cord Injury

Abstract

Motor evoked potential (MEP) input-output curves generated using transcranial magnetic stimulation applied to the motor cortex are useful to evaluate neuromuscular function. The slope of the curve indicates the excitability of the targeted neuromuscular circuit across multiple neuronal populations, such as direct corticospinal projections and inter-cortical neurons. The effect of spinal cord injury (SCI) on input-output curves is understudied and methods to analyze MEP input-output curves remain non-standardized. Our primary aim was to compare MEP input-output curves of the biceps brachii between individuals with SCI-induced tetraplegia and nonimpaired individuals to investigate differences in corticomotor excitability between groups. We also aimed to test the reliability of input-output curve measures across three sessions, and the effect of linear and sigmoidal curve fitting on reliability. MEP input-output curves were generated using single pulse, monophasic TMS applied to the biceps brachii motor hotspot across a range of stimulator intensities in ten individuals with cervical level SCI and ten nonimpaired participants. In our cohort, the SCI group had increased slopes of the MEP input-output curve compared to nonimpaired controls indicating injury-induced corticomotor reorganization. In the SCI group, no differences in quality of fit between curve-fitting methods were found. In the nonimpaired group, quality of fit was greater using the sigmoidal function compared to linear regression. Reliability across three sessions of the slope extracted from linear regression was moderate and good in the SCI and nonimpaired groups, respectively. These findings support the use of MEP input-output measures for monitoring SCI neurorehabilitation. Future work is needed to investigate relationships between MEP input-output curve measures and changes in neuromuscular function after SCI.