Reinnervation as measured by the motor unit size index is associated with preservation of muscle strength in amyotrophic lateral sclerosis but not all muscles reinnervate

Introduction

The motor unit size index (MUSIX) may provide insight into reinnervation patterns in diseases such as amyotrophic lateral sclerosis (ALS). However, it is not known if MUSIX detects clinically relevant changes in reinnervation, or if all muscles manifest changes in MUSIX in response reinnervation following motor unit loss.

Methods

57 patients with ALS were assessed at 3-monthly intervals for 12 months in 4 centres. Muscles examined were abductor pollicis brevis, abductor digiti minimi, biceps brachii and tibialis anterior. Results were split into two groups: muscles with increases in MUSIX and those without. Longitudinal changes in MUSIX, motor unit number index (MUNIX), compound muscle action potential (CMAP) amplitude and medical research council (MRC) strength score were investigated.

Results

133 muscles were examined. 59% of muscles exhibited an increase in MUSIX during the study. Muscles with MUSIX increases lost more motor units (MUNIX decline at 12 months -58%, p<0.001) than muscles that did not increase MUSIX (MUNIX decline at 12 months -34.6%, P<0.001). Despite this, longitudinal changes in muscle strength were similar. When motor unit loss was similar, an absence of MUSIX increase was associated with a significantly greater loss of muscle strength (P = 0.002).

Discussion

MUSIX increases are associated with greater motor unit loss but relative preservation of muscle strength. Thus, MUSIX appears to be measuring a clinically relevant response that can provide a quantitative outcome measure of reinnervation in clinical trials. Furthermore, MUSIX suggests that reinnervation may play an important role in determining the progression of weakness.