The reproducibility and usefulness of motor unit number index (MUNIX) using abductor digiti minimi and tibialis anterior muscles in ALS patients

Abstract

Introduction: Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive loss of motor neuron. The motor unit number index (MUNIX) is a novel diagnostic technique developed to quantify axonal loss in ALS patients. The objective of this study was to establish the reproducibility and usefulness of the MUNIX using the abductor digiti minimi (ADM) and tibialis anterior (TA) muscles in ALS patients and normal controls. Methods: The MUNIX values were obtained from bilateral ADM and TA muscles in 30 ALS patients and 27 normal controls. The MUNIX, compound muscle action potential (CMAP), Medical Research Council (MRC) sum score, and ALS functional rating scale (ALSFRS) were evaluated, and their correlations were examined by using Pearson correlation analysis. All muscles were recorded twice to assess the reproducibility and coefficient of variation (COV). The mean values of all 4 (bilateral ADM and TA) muscles were obtained, and their correlations with functional status were analyzed. Results: The mean values of CMAP amplitude and MUNIX showed no significant difference. The COVs for CMAP and MUNIX were within the acceptable range of less than 20%. There was a significant correlation between MUNIX and CMAP amplitude in both ADM (r = 0.918, P < 0.01) and TA (r = 0.850, P < 0.01) muscles in ALS patients. In normal controls, there was also a significant correlation between MUNIX and CMAP amplitude in both ADM (r = 0.629, P < 0.01) and TA (r = 0.894, P < 0.01) muscles. In ALS patients, MUNIX values significantly correlated with ALSFRS scores in the ADM (r = 0.439, P = 0.015), but not in the TA (r = 0.357, P= 0.053). There was a significant correlation between MUNIX and MRC sum score in the TA (r = 0.474, P < 0.01), but not in ADM (r = 0.349, P = 0.058). Using the mean values of all 4 muscles, there was a more significant correlation between CMAP and MRS sum score (r = 0.480, P < 0.01), and between CMAP and ALSFRS (r = 0.517, P < 0.01). There was also a more significant correlation between MUNIX and MRS sum score (r = 0.493, P < 0.01) and between MUNIX and ALSFRS (r = 0.481, P < 0.01). Conclusions: This study demonstrated the reproducibility of the MUNIX using ADM and TA muscles. Both ADM and TA muscles showed good correlations, but their patterns of correlations in the MUNIX were slightly different. The overall mean value of the 4 muscles was more useful for assessing functional status and disease progression. Further prospective follow-up studies with a larger sample size are needed to confirm our results.