Impaired motor unit profile in Fabry disease: A quantitative electrophysiological study.

OBJECTIVE: To investigate the relationship between routine electrophysiological findings, sympathetic function, and the number and size of functioning motor units in Fabry disease. METHODS: Thirteen patients with Fabry disease and thirteen healthy individuals were evaluated. Nerve conduction studies and sympathetic skin responses were recorded from upper and lower limbs. Motor unit function was analyzed using the Motor Unit Number Index (MUNIX) technique by recording the first dorsal interosseous (FDI) muscle following ulnar nerve stimulation and the tibialis anterior (TA) muscle following peroneal nerve stimulation. RESULTS: The MUNIX protocol demonstrated excellent reproducibility. MUNIX had excellent reproducibility (coefficient of variation: 20.7% in FDI; 14.7% in TA). FDI-MUNIX and MUSIX values were comparable between groups, whereas TA-CMAP amplitude and MUSIX were significantly lower in patients than in healthy controls, with no significant difference in TA-MUNIX. Correlation analysis revealed a positive association between Lyso-Gb3 and MUNIX-FDI (ρ = 0.55, p = 0.049) and inverse correlations between neuropathy scores and MUSIX-TA (ρ = -0.72, p = 0.013; ρ = -0.76, p = 0.004). CONCLUSIONS: The observed reductions in CMAP and MUSIX, particularly in the TA, indicate subtle length-dependent motor unit involvement despite preserved strength and normal MUNIX values. SIGNIFICANCE: This study provides the first evidence that MUNIX can detect early or subclinical motor unit alterations in Fabry disease.