Toward the validation of a new method (MUNIX) for motor unit number assessment

Introduction: This prospectively designed study analyzed the correlation of a new, non-invasive neurophysiological method (Motor Unit Number Index – MUNIX) with two established Motor Unit Number Estimation (MUNE) methods. Methods: MUNIX and incremental stimulation MUNE (IS-MUNE) were done in the abductor digiti minimi muscle (ADM), while MUNIX and spike-triggered averaging MUNE (STA-MUNE) were tested in the trapezius muscle. Twenty healthy subjects and 17 patients with amyotrophic lateral sclerosis (ALS) were examined. Results: MUNIX and MUNE values correlated significantly (ADM: n = 108; Spearman-Rho; r = 0.88; p < 0.01; trapezius muscle: n = 49; Spearman-Rho; r = 0.46; p < 0.01). Discussion: MUNIX indeed reflects the number of motor units in a muscle, and may sensibly be recorded from the trapezius muscle. With MUNIX being both much more patient friendly and much more rapid to assess than MUNE, the results support the use of MUNIX when motor unit number assessment is desired.