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

Cervical spinal cord injury (SCI) significantly impairs upper limb function, affecting patients' quality of life. Nerve transfer surgery can restore arm and hand function, but its success depends on the health of infralesional lower motor neurons (LMNs). LMN abnormalities are prevalent in muscles targeted for nerve transfer, particularly those innervated by the posterior interosseous nerve (PIN) and radial nerve, essential for wrist extension and hand opening. This study evaluates the health of infralesional LMNs in cervical SCI using multipoint stimulation motor unit number estimation (MPS-MUNE). We assessed motor unit counts in the C7-innervated anconeus and the predominantly C8-innervated extensor indicis (EI) to determine their viability as targets for nerve transfer surgery.

We conducted a prospective, two-center cohort study using MPS-MUNE to evaluate 15 individuals with cervical SCI (26 limbs) and 17 healthy controls.

Compound muscle action potential (CMAP) and MUNE values were significantly lower in SCI patients compared to controls (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: 6.6 mV ± 1.0, EI MUNE: 137 ± 33.9; anconeus CMAP: 6.6 mV ± 1.3, anconeus MUNE: 146 ± 42.3). Test-retest reliability for EI and anconeus were 0.84 (CI: 0.45-0.95) and 0.78 (CI: 0.36-0.93), respectively.

Significant LMN loss was observed after cervical SCI. MPS-MUNE shows potential for evaluating LMN health, highlighting its importance for assessing nerve transfer targets and planning interventions.