Motor Unit Number Index as a Biomarker for Spinal Muscular Atrophy

Major challenges in the clinical care of those with spinal muscular atrophy (SMA) are determining patients’ clinical stages, identifying exactly how the disease is progressing, and distinguishing SMA from other, muscular disorders. To help overcome these difficulties, researchers are actively seeking improved diagnostic tools as well as disease biomarkers.1 A new study has shown that a method known as the motor unit number index (MUNIX) may offer a way to both diagnosis SMA and evaluate disease severity.

MUNIX is a noninvasive, electrophysiological method that identifies the loss of motor neurons, which are the cells that are primarily affected in SMA. The tool, which has shown both reliability and validity, works through a combination of neurography and surface electromyography.2,3 

Proximal muscles tend to be affected more than distal muscles in SMA, and hand muscles are largely spared. These muscles have therefore been of interest to clinicians and researchers who study SMA. This new study investigated hand muscles in SMA patients using MUNIX for the first time, and it led to two important results.

First, MUNIX could be used to identify SMA patients regardless of their disease severity. Interestingly, while SMA is known for affecting proximal muscles significantly more than distal muscles, MUNIX identified the loss of motor units in the hand even before hand muscles appeared clinically affected. When used on less affected muscles, MUNIX measurements did not correlate with disease progression. 

In addition, MUNIX in SMA patients revealed a pattern of muscle innervation in the hand that is distinct from the pattern observed in amyotrophic lateral sclerosis (ALS) patients and can therefore distinguish these patients, who often have overlapping symptoms and pathology, with a high degree of sensitivity and specificity. Given these findings, the authors conclude that MUNIX could be a used as a diagnostic tool for SMA. 

A second major finding was that MUNIX could differentiate patients with different degrees of SMA severity when used on certain individual hand muscles. The researchers interpreted these results to mean that MUNIX could provide a valuable biomarker for SMA. 

Given that MUNIX has a higher degree of sensitivity than other disease scores and that it is a noninvasive tool that can be used easily and painlessly with SMA patients across the severity spectrum in a few minutes, the researchers suggest that it is worth pursuing this option for diagnostic and biomarker purposes. More research is needed, however, to determine the details of how MUNIX could best be used to help patients with SMA.


1. Gunther R, Neuwirth C, Koch JC, et al. Motor Unit Number Index (MUNIX) of hand muscles is a disease biomarker for adult  spinal muscular atrophy. Clin Neurophysiol. November 2018. doi:10.1016/j.clinph.2018.11.009

2. Neuwirth C, Nandedkar S, Stalberg E, et al. Motor Unit Number Index (MUNIX): a novel neurophysiological marker for neuromuscular disorders; test-retest reliability in healthy volunteers. Clin Neurophysiol. 2011;122(9):1867-1872. doi:10.1016/j.clinph.2011.02.017

3. Neuwirth C, Barkhaus PE, Burkhardt C, et al. Motor Unit Number Index (MUNIX) detects motor neuron loss in pre-symptomatic muscles in Amyotrophic Lateral Sclerosis. Clin Neurophysiol. 2017;128(3):495-500. doi:10.1016/j.clinph.2016.11.026