Motor unit number estimation (MUNE) is a unique electrophysiologic test used to estimate the number of surviving motor units in a muscle or group of muscles. It is used most frequently to monitor lower motor neuron loss in amyotrophic lateral sclerosis and spinal muscle atrophy. Of particular interest is its use as an endpoint measure in clinical trials for these diseases. This article describes the principles of MUNE and the factors that need to be considered, and reviews several techniques that have been used in clinical trials and in monitoring progression. It then reviews experience with MUNE in clinical trials for amyotrophic lateral sclerosis and spinal muscle atrophy and discusses how MUNE correlates with measures of function.
Motor unit number estimation (MUNE) is a unique electrophysiologic test used to estimate the number of surviving motor units in a muscle or group of muscles. It is used most frequently to monitor lower motor neuron loss in amyotrophic lateral sclerosis (ALS) and spinal muscle atrophy (SMA). Of particular interest is its use as an endpoint measure in clinical trials for these diseases.
The unique feature of MUNE is that it is not affected by collateral reinnervation. Other tests, including muscle strength (qualitative or quantitative) and compound muscle action potential (CMAP), are kept deceptively high by collateral reinnervation and do not fall in value until reinnervation cannot keep up with continuing denervation, which does not occur until 50% or more of motor units are lost. Needle electromyogram (EMG) is sensitive to denervation, as indicated by the presence of positive waves and fibrillation potentials, but the magnitude of abnormal spontaneous activity does not correlate with the degree of motor unit loss. Motor unit action potential morphology changes with collateral reinnervation, but the magnitude of motor unit action potential metric values (amplitude, duration, and complexity) does not correlate with the degree of denervation. Estimates of the degree of reduced motor unit recruitment during the needle examination are subjective and qualitative. Thus, MUNE is better suited than any other test to study the time course and degree of lower motor unit loss in motor neuron disease (MND).
This article first describes the principles of MUNE and the factors that need to be considered. MUNE can be performed using several operational techniques that differ in approach, and this article reviews techniques that have been used in clinical trials and in monitoring progression. It then reviews experience with MUNE in clinical trials for ALS and SMA and discusses how MUNE correlates with measures of function.
Motor unit number estimation principles
MUNE is based on determining the size of an average surface-recorded motor unit potential (SMUP) and dividing that value into the maximal CMAP :
MUNE = Area ( CMAP max ) Area ( SMUP average ) or MUNE = Amplitude ( CMAP max ) Amplitude ( SMUP average )
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