Multipoint Stimulation Motor Unit Number Estimation of the Extensor Indicis and Anconeus after Cervical Spinal Cord Injury

ABSTRACT Introduction The health of infralesional lower motor neurons (LMNs) after a cervical spinal cord injury is frequently overlooked, despite its critical role in mediating effective clinical interventions for improving arm and hand function. Prior studies suggest high frequencies of infralesional lower motor abnormalities in muscles that are potential targets for nerve transfer surgery, a procedure that has the potential to restore upper limb function. Methods In this prospective, two-center cohort study, we used multipoint stimulation motor unit number estimation (MPS-MUNE) to evaluate the number of motor units in clinically relevant infralesional muscles, including the predominantly C7-innervated anconeus and the predominantly C8-innervated extensor indicis (EI) in 15 individuals with cervical spinal cord injury (26 limbs) and 17 healthy controls. Results Both CMAP and MUNE values were significantly lower (p < 0.05) for those with cervical spinal cord injury (EI CMAP: 2.0 mV±1.57, EI MUNE: 33±30.5, Anconeus CMAP:2.7 mV±1.9, Anconeus MUNE: 39±50.6) versus controls (EI CMAP: 6.6mV±1.0, EI MUNE:137±33.9, Anconeus CMAP:6.6 mV±1.3, Anconeus MUNE: 146 ±42.3). The test-retest reliability as measured by intraclass correlation coefficient and confidence interval (CI) for the EI and anconeus were 0.84 (CI: 0.45-0.95) and 0.78 (CI: 0.36-0.93), respectively. Discussion This study shows significant loss of infralesional motor units after cervical spinal cord injury. We demonstrate the potential utility of MPS-MUNE for evaluating the health of LMNs. The LMN abnormalities observed underscore the significance of this approach to evaluating potential targets for nerve transfer surgery for the restoration of upper limb function.