“Motor Unit Number Index” (MUNIX) is a novel electrophysiological method to track motor neuron loss over time in neuromuscular diseases like amyotrophic lateral sclerosis (ALS). It provides an index of the number of functioning motor neurons in a given muscle. This session provides an overview about the evolution of this method and its advantages and limitations and includes a live-demonstration.
Compared to other motor unit number estimation (MUNE) techniques, MUNIX is easy and quickly performed, non-invasive and applicable to several muscles in an adequate amount of time. It can be applied in every muscle, where a compound muscle action potential (CMAP) can be elicited after supramaximal electrical nerve stimulation. However, patient’s cooperation and accurately and carefully performed standard nerve conduction study procedures are required and technical features have to be kept in mind to obtain reliable results. The most crucial steps in recording MUNIX are to carefully track the muscle belly to achieve the CMAP with the highest amplitude by replacing the recording electrode several times. Then, several surface EMG signals (surface interference pattern, SIP) are recorded at different voluntary force levels, reaching from minimal to maximal isometric contraction. The area and power of the CMAP and SIP are determined to calculate the MUNIX value. While this was done separately in the older versions of the software, newer systems can calculate MUNIX already during data collection, making the measurements more feasible.
The available literature demonstrates a good inter- and intra-rater reproducibility in healthy volunteers and ALS patients. Several longitudinal studies in ALS patients revealed that decline of MUNIX exceed other established markers of disease progression and functional rating scales. Recently it has been shown, that MUNIX is also capable to detect early motor neuron loss before clinical symptoms like weakness and atrophy and therefore functional deterioration occurs. This makes this method a favourable candidate as a sensitive biomarker in longitudinal ALS trials and other progressive neuromuscular disorders.
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