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

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.

Assessment of Motor Units in Neuromuscular Disease

“…This method, developed by Nandedkar et al [47], is noninvasive with standard nerve conduction studies (NCS) electrodes typically placed over the median or ulnar innervated hand muscles. The maximal CMAP is carefully determined.…”

Correlations between MUNIX and adapted multiple point stimulation MUNE methods

“…MUNIX can be performed in less than 5 min (Nandedkar et al, 2010) and a couple of minutes more for the CMAP and SIP signal importation and analysis in an excel table. The mean duration for AMPS evaluation is 13 ± 4 min (n = 100) (Wang et al, 2009).…”

“…MUNIX was applied according to the description of Nandedkar et al (2010). The recordings were done on a 300 ms window.…”

“…Ten isometric contractions during abduction of digit 1 were registered as surface interference patterns (SIP) ranging from approximately 10-100% of contraction. To avoid interference with volumeconducted activity, conditions that should be fulfilled in order that the test be accepted were the following ones: SIP area >20 mV.ms; ideal case motor unit count (ICMUC) <100; SIP area/CMAP area >1; CMAP amplitude >0.5 mV (Nandedkar et al, 2010).…”

“…To eliminate the inherent problem of alternation that affects the incremental MUNE technique, the multiple point stimulation and then the adapted multiple point stimulation (AMPS) technique were introduced (Kadrie et al, 1976;Wang and Delwaide, 1995;Shefner et al, 2011). MUNIX is a more recently developed method providing a motor unit number index (Nandedkar et al, 2010). …”