[Motor unit number estimation in motor neuron disease]

Motor unit number estimation (MUNE) is a special electrophysiologic technique that can provide a numeric estimate of the number of axons innervating a muscle or group of muscles. Since 1971, many investigators have suggested methods for making such a measurement; the lack of general acceptance of a single method points to both theoretical and practical problems inherent in all techniques so far proposed. MUNE consists of a number of different methods, with each having specific advantages and limitations. This review will briefly describe the available MUNE methods, the incremental, multiple point stimulation, spike-triggered averaging, F-wave, and statistical methods, including models of muscles that allow evaluation of MUNE techniques and comparisons between techniques. In addition, MUNE data relating to motor neuron disease especially ALS will be discussed. Despite the lack of a perfect single method for performing MUNE, it has great clinical value in the assessment of progressive motor axon loss in ALS. Further refinements in the method will likely increase its utility in the future. Modifications to MUNE techniques have been offered that permit more rapid acquisition of data. There is now more experience with MUNE as endpoint measures in clinical trials.