Motor unit number index (MUNIX) in myasthenia gravis: A cross-sectional pilot sudy.

OBJECTIVE: To evaluate the Motor Unit Number Index (MUNIX) as a biomarker of motor unit function and disease progression in Myasthenia Gravis (MG), by comparing MG patients with healthy controls (HCs) and analyzing correlations with clinical and neurophysiological measures. DESIGN: MUNIX, Compound Motor Action Potential (CMAP), and Motor Unit Size Index (MUSIX) were assessed in the abductor pollicis brevis (APB) and orbicularis oculi (OO) muscles. Clinical severity was measured using MG Composite Score (MGCS), MG-Activities of Daily Living (MG-ADL), and MGFA classification. Statistical analyses included ANOVA, Spearman correlations, and multivariate models(p < 0.05). RESULTS: Forty-two MG patients (63.1 ± 12.7 years) and 21 age-matched HCs (59.7 ± 8.7 years) were enrolled. MG patients showed lower MUNIX and higher MUSIX values p = 0.009, p < 0.001) compared with HCs. Age correlated negatively with CMAP and MUNIX, positively with MUSIX. Disease duration correlated negatively with all indices. MuSK-positive patients had lower CMAP in OO (p = 0.001). CMAP and MUSIX in OO predicted disease duration. CONCLUSIONS: Reduced MUNIX and CMAP may suggest impaired motor unit function loss and neuromuscular transmission, whereas increased MUSIX is compatible with compensatory motor unit remodeling. SIGNIFICANCE: MUNIX emerges as a promising, non-invasive biomarker to monitor disease severity and progression in MG, with potential clinical and research applications.