Motor Unit Number Index (MUNIX) in Control Children: Reference Values and Reliability

The motor unit number index (MUNIX) is recognized as a reliable electrophysiological biomarker, and reference values are available for healthy adults but not for children. The aim of this study was to determine reference MUNIX values in healthy children.

Sixty control children (28 females) were grouped as under 5 years (n₁ = 16), 5-10 years (n₂ = 15), 10-15 years (n₃ = 20), and 15-17 years (n₄ = 9). MUNIX measurements were performed by an experienced and an inexperienced investigator in the abductor digiti minimi (ADM) and abductor pollicis brevis (APB) on both sides when possible.

The APB MUNIX increased by 6.5 [95% confidence interval, 3.2-9.7] per year of age, 1.0 [0.5-1.5] per cm, and 1.7 [0.4-4.0] per kg (p < 0.001 in each case), but was not independently associated with either of these three variables in multivariable analysis. The APB compound muscle action potential (CMAP) increased by 0.42 [0.30-0.52] per year of age, 0.06 [0.04-0.08] per cm, and 0.11 [0.07-0.14] per kg (p < 0.001 in each case), and increased with age independently of height and weight at borderline significance (p = 0.065). Measurement reproducibility was high for the experienced investigator, but lower between the experienced and the inexperienced investigator.

The high intra-investigator reproducibility for the experienced investigator indicates that these parameters are useful electrophysiological biomarkers for disease progression and potential treatment response.