Motor unit number index (MUNIX): principle, method, and findings in healthy subjects and in patients with motor neuron disease

TLDR

MUNIX is a method that estimates motor unit number and size by combining the compound muscle action potential and surface electromyographic interference patterns. The study aimed to establish normal MUNIX values for the hypothenar muscle and evaluate its reproducibility in healthy adults. MUNIX was applied to the hypothenar muscle of 34 healthy volunteers to define normative ranges, and serial measurements over one year were performed in 4 healthy subjects and 13 amyotrophic lateral sclerosis patients. MUNIX was reproducible and unchanged over one year in healthy subjects, but in ALS patients it declined despite minimal CMAP change, indicating motor unit loss compensated by reinnervation, and when CMAP fell >30% both CMAP and MUNIX decreased together, demonstrating its usefulness for monitoring motor neuron degeneration. Muscle Nerve 42: 798–807, 2010.

Abstract

Abstract The motor unit number index (MUNIX) is a method for assessment of number and size (MUSIX) of motor units (MUs) using the compound muscle action potential (CMAP) and surface electromyographic interference pattern (SIP). This method was used to study the hypothenar muscle in 34 healthy subjects to define normal range, and to study reproducibility. Four healthy subjects and 13 patients with amyotrophic lateral sclerosis (ALS) were studied serially over a 1‐year period. In healthy subjects, MUNIX showed good reproducibility. In serial studies, healthy subjects showed no change in the CMAP amplitude and MUNIX. ALS patients with minimal change in CMAP amplitude had a significant drop in MUNIX and increase in MUSIX, indicating MU loss compensated by reinnervation. When the CMAP changed significantly (>30%) in 1 year, the CMAP and MUNIX decreased in parallel. MUNIX would be useful to study MU loss in degenerative diseases of motor neurons. Muscle Nerve 42: 798–807, 2010