Chapter 10 Motor unit estimates in amyotrophic lateral sclerosis

Abstract

There has been increasing interest in the application of motor unit number estimation (MUNE) methodology to the diagnosis and quantitative assessment of muscle denervation and of amyotrophic lateral sclerosis (ALS), in particular. MUNE is ideally suited to the study of ALS for a number of reasons. The methodology measures directly the primary effect of the disease process, the loss of motor units, and functioning motoneurons. Further, it is quantitative, reproducible, and usually noninvasive, and, therefore, lends itself to longitudinal studies, in which the natural time course of the disease and the effects of therapeutic interventions can be investigated. In the first part of this chapter, the principle of MUNE is described, together with the advantages and limitations of the original method and of the various modifications that have appeared. The muscles, to which this type of examination has been applied, are then surveyed and the corresponding means MUNEs given for the normal population. The chapter then discusses the ALS and, by presenting the MUNEs found at the initial electromyography (EMG) examination, indicates the diagnostic usefulness of this test. By studying the same patients on subsequent occasions and by measuring the amplitudes of the putative motor unit potentials (MUPs) recorded during each MUNE, it is possible to determine the time-course of the ALS disease process. The similarities and differences between ALS and spinal muscular atrophy (SMA) are considered, not only for their intrinsic interest but also for the further light that they shed on the pathogenetic mechanisms of the two types of disorder. The potential usefulness of MUNE in therapeutic trials on ALS is also examined. Finally, the chapter discusses the nature of the etiological agent in some of the sporadic cases of ALS.