An overview of motor unit number index reproducibility in amyotrophic lateral sclerosis

Motor unit number index (MUNIX) is an electrophysiological technique to give an estimate of functioning motor neurons in a muscle. For any given neurophysiological technique for the use in clinical or research studies, reproducibility between different operators and in a single operator in different times is one of the most important qualities, which must be evaluated and approved by different examiners and centers. After its introduction, testing the reproducibility of MUNIX was the aim of many studies to show this quality of the technique. In this review, we aimed to summarize all the studies, which have been performed up to now to approve MUNIX reproducibility in amyotrophic lateral sclerosis comparing healthy individuals. amyotrophic lateral sclerosis (ALS) in addition to upper motor neuron involvement. Progression of ALS could be monitored by clinical measures such as Medical Research Council (MRC) Scale for Muscle Strength, the revised ALS Functional Rating Scale (ALSFRS-R), and the older electrophysiological techniques including nerve conduction studies (NCSs) and needle electromyography (EMG). None of these measures has the potential of quantification of lower motor neuron degeneration in ALS.1-4 As in the last decades, there has been increasing number of clinical trials attempting to find an effective treatment for ALS, the introduction of a clinical or electrophysiological marker with the ability of quantitative evaluation of motor neuron loss was an essential need. The first try to make a numerical estimate of motor neurons started by McComas et al. with motor unit number estimation (MUNE).5