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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

Young Chan, James J P Alix, Christoph Neuwirth, Paul E Barkhaus, José Castro, Thomas M Jenkins, Christopher J McDermott, Pamela J Shaw, Mamede De Carvalho, Sanjeev Nandedkar, Erik Stålberg & Markus Weber

abstract

INTRODUCTION/AIMS
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 whether MUSIX detects clinically relevant changes in reinnervation, or if all muscles manifest changes in MUSIX in response to reinnervation after motor unit loss.

METHODS
Fifty-seven patients with ALS were assessed at 3-month intervals for 12 months in four centers. 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 increases. Longitudinal changes in MUSIX, motor unit number index (MUNIX), compound muscle action potential amplitude, and Medical Research Council strength score were investigated.

RESULTS
One hundred thirty-three muscles were examined. Fifty-nine percent of the muscles exhibited an increase in MUSIX during the study. Muscles with MUSIX increases lost more motor units (58% decline in MUNIX at 12 months, P < .001) than muscles that did not increase MUSIX (34.6% decline in MUNIX at 12 months, P < .001). However, longitudinal changes in muscle strength were similar. When motor unit loss was similar, the absence of a MUSIX increase was associated with a significantly greater loss of muscle strength (P = .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 a major role in determining the progression of weakness.
   
citation Chan Y, Alix J J P, Neuwirth C, Barkhaus P E, Castro J, Jenkins T M, McDermott C J, Shaw P J, de Carvalho M, Nandedkar S, Stålberg E, Weber M. 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. Muscle Nerve 2021;.
   
type journal paper/review (English)
date of publishing 23-10-2021
journal title Muscle Nerve
ISSN electronic 1097-4598
PubMed 34687220
DOI 10.1002/mus.27444