Electromyography

Neurophysiological techniques for studying the peripheral nervous system include methods based on electrical stimulation of nerves (electroneurography) and recording of muscle traces induced by voluntary contraction (electromyography proper), which is usually performed with concentric needle electrodes (Adrian-Bronk type). Unless there is a particular need, electroneurography is performed with surface electrodes both for the stimulation of nerve trunks and for the recording of electrical potentials obtained from the sensory (Sensory Action Potential - SAP) and motor fibers (Compound Motor Action Potential - cMAP). The electromyographic recording allows to record in the muscles at rest the presence of the so-called spontaneous activity, usually related to a hypersensitivity by denervation of neuromuscular junctions that is an indication of activity of the underlying pathological process. The EMG tracing of voluntary submaximal and maximal contraction presents different characteristics depending on whether the basic pathological process is secondary to degeneration of the spino-bulbar motor neurons and/or of their axon branches, or to a primitively muscular noxa. Since the acquisition of the EMG signal depends on the voluntary contraction of the patient, the quality of the examination is in direct dependence on the cooperation of the patient: the opportunity to prescribe the examination in uncooperative patients (infants and young children, elderly adults with cognitive impairment, etc.) must be evaluated for each individual case in relation to the information content expected from the examination.