Motor Unit Number Estimation in Neuromuscular Disease

Abstract

Motoneuronal death is a major feature in many neuromuscular diseases. As a result of the remarkable ability of motor axons to sprout and to regenerate, the extent of motoneuronal loss often does not become clinically apparent until the loss is well advanced. This discrepancy is particularly striking in chronic, slowly progressive disorders in which surviving motoneurons have ample time to expand their innervation field. In those conditions, motor unit number estimation (MUNE) can be particularly valuable in gauging disease severity, disease progression and treatment response.In this review, we will briefly describe the commonly used MUNE methods. Their applications in neuromuscular diseases and the findings from these studies will be described in detail. BASIC CONCEPTS IN MUNESOnly in terminal stages of motor neuronal or axonal diseases when a few motor units remain is it possible to count all the ABSTRACT: Motor unit number estimation (MUNE) is an electrophysiological method designed to quantify motor unit loss in target muscles of interest. Most of the techniques are noninvasive and are therefore well suited for longitudinal monitoring. In this brief review, we describe the more commonly used techniques and their applications in amyotrophic lateral sclerosis, poliomyelitis, spinal muscular atrophy and hereditary sensorimotor neuropathies. Findings in some of these studies offer important pathophysiological insights. Since conventional electrophysiologic methods are not sensible measures of motor neuronal loss, MUNE could play a potentially important role in the diagnosis, monitoring of disease progression and response to treatment in neuromuscular diseases in which motor unit loss is a major feature.RÉSUMÉ: Estimation du nombre d'unités motrices dans les maladies neuromusculaires. L'estimation du nombre d'unités motrices (ENUM) est une méthode électrophysiologique pour quantifier la perte d'unités motrices dans les muscles étudiés. La plupart des techniques non effractives sont donc appropriées pour la surveillance longitudinale. Dans cette courte revue, nous décrivons les techniques les plus fréquemment utilisées et leur application dans la sclérose latérale amyotrophique, la poliomyélite, l'atrophie musculaire spinale et les neuropathies sensitivomotrices héréditaires. Les observations provenant de ces études fournissent des indices physiopathologiques importants. Étant donné que les méthodes électrophysiologiques conventionnelles ne constituent pas une mesure sensible de la perte de neurones moteurs, l'ENUM pourrait jouer un rôle important dans le diagnostic, la surveillance de la progression de la maladie et la réponse au traitement dans les maladies neuromusculaires où la perte d'unités motrices est un élément important.Can. J. Neurol. Sci. 2008; 35: 153-159 THE CANADIAN JOURNAL OF NEUROLOGICAL SCIENCES 153

Role of diffusion tensor imaging or magnetic resonance spectroscopy in the diagnosis and disability assessment of amyotrophic lateral sclerosis

“…Electromyography and motor unit number estimation play an important role in detecting LMNs and are regarded to be the most accurate confirmatory tests for ALS, but these techniques cannot evaluate disease development or the effects of treatment [6,7]. For the UMNs, neuroimaging techniques are a promising way to diagnose ALS or evaluate disease progression, and these techniques include conventional MRI, DTI, MRS and functional MRI [8].…”

Persistent West Nile Virus Associated with a Neurological Sequela in Hamsters Identified by Motor Unit Number Estimation

“…The temporal development of neuronal cell damage in the spinal cord in the acute infection followed by the chronic infection is reminiscent in some ways of ALS and poliomyelitis. The motor units consisting of cell bodies, axons, and the muscle fibers are diminished, probably due to cell body damage followed by axonal degeneration (30). Suppression of MUNE was coincident with the presence of WNV-induced neuropathology, whereas MUNE was not suppressed in hamsters infected with yellow fever virus or Punta Toro virus, which cause nonneurological diseases.…”

“…The loss of motor neurons is detected in other neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS) (30,36) and poliomyelitis (25), by using electrophysiological motor unit number estimation (MUNE) (5), in which a motor unit consists of an ␣-motor neuron and all the muscle fibers it innervates. A presumptive use of MUNE in the human WNV infection was identified in an uncontrolled clinical study (2) in which MUNE appeared to be a marker for degrees of weakness and recovery in patients with neurological sequelae.…”

“…During this later stage of infection, the motor neurons and their associated axons possessing either WNV or its proteins are dysfunctional, which causes suppressed MUNE and paralysis or muscle weakness. Since these neuronal cells survive, compensatory axonal sprouting from adjacent healthy axons (30) will not replace the dysfunctional axons, so both suppressed MUNE and muscle weakness may persist in some patients. The correlate to this hypothesis is that neurological sequelae might be reduced if the viral load is reduced by innate or acquired immunity or by antiviral therapy in the longterm phase of the disease.…”

Place de la rééducation dans la prise en charge des patients porteurs de séquelles de poliomyélite

“…Le vieillissement spécifique des unités motrices [12] est, quant à lui, de mieux en mieux précisé, et s'intègre dans la discussion étiopathogénique du « syndrome post-polio » (réduction des motoneurones alpha, surtout de grande taille).…”