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.

The surface-recorded compound muscle action potential (CMAP) and electromyographic (EMG) interference pattern is used to compute the motor unit number index (MUNIX). The MUNIX demonstrated all known changes in the number of motor units in normal subjects, and in patients with amyotrophic lateral sclerosis (ALS). In normal subjects MUNIX decreased slightly with age and showed excellent reproducibility. In many ALS patients MUNIX was reduced even when the CMAP was normal. Lower MUNIX values were seen in weaker muscles. This is a noninvasive method that requires minimal electrical stimulation. It is performed in less than 5 min. This makes it suitable for serial EMG investigations.

Pathogenic trinucleotide repeat expansions were found among 61% of the dominant kindreds. Among patients with apparently recessive or negative family histories of ataxia, 6.8% and 4.4% tested positive for a CAG expansion at one of the dominant loci, and 11.4 and 5.2% of patients with apparently recessive or sporadic forms of ataxia had FA expansions. Because of the significant implications that a dominant versus recessive inheritance pattern has for future generations, it is important to screen patients who do not have a clearly dominant inheritance pattern for expansions at both the FA and the dominant ataxia loci.

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