Lower motor neuron dysfunction in ALS.
In the motor system there is a complex interplay between cortical structures and spinal cord lower motor neurons (LMN). In this system both inhibitory and excitatory neurons have relevant roles. LMN loss is a marker of motor neuron disease/amyotrophic lateral sclerosis (MND/ALS). Conventional needle electromyography (EMG) does not allow LMN loss to be quantified. Measurement of compound muscle action potential (CMAP) amplitude or area, and the neurophysiological index, provide a surrogate estimate of the number of functional motor units. Increased motor neuronal excitability is a neurophysiological marker of ALS in the context of a suspected clinical and electrophysiological diagnosis. In the LMN system, fasciculation potentials (FPs) are the earliest changes observed in affected muscles, a feature of LMN hyperexcitability. Reinnervation is best investigated by needle EMG although other methods can be explored. Moreover needle EMG give information about the temporal profile of the reinnervation process, important ancillary data. Quantitative motor unit potential analysis is a valuable method of evaluating reinnervation. The importance of FPs has been recognized in the Awaji criteria for the electrodiagnosis of ALS, criteria that are a sensitive adjunct to the revised El Escorial criteria. Finally, functionality of LMN's, and perhaps excitability studies in motor nerves, aids understanding of the disease process, allowing measurement of potential treatment effects in clinical trials. Other investigational techniques, such as electrical impedance myography, muscle and nerve ultrasound, and spinal cord imaging methods may prove useful in future.
Department of Neurosciences, Hospital de Santa Maria-Centro Hospitalar de Lisboa Norte, Lisbon, Portugal; Institute of Physiology, Faculty of Medicine, Instituto de Medicina Molecular, University of Lisbon, Portugal. Electronic address: mamedemg@gmail.com.
Mamede
M
de Carvalho
Institute of Physiology, Faculty of Medicine, Instituto de Medicina Molecular, University of Lisbon, Portugal; Department of Neurology, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, London, UK.
Michael
M
Swash
D000690:Amyotrophic Lateral Sclerosis / Q000175:diagnosis* / Q000503:physiopathology; D004576:Electromyography / Q000379:methods; D006801:Humans; D009046:Motor Neurons / Q000473:pathology / Q000502:physiology*; D018482:Muscle, Skeletal / Q000473:pathology / Q000503:physiopathology; D009431:Neural Conduction
Amyotrophic lateral sclerosis; Electromyography; Fasciculation potentials; Lower motor neuron; Motor neuron disease