When is this exam indicated?
Electromyography is indicated in most neuromuscular diseases: motor neuron diseases, cervical and lumbosacral radiculopathies and plexopathies, polyneuropathies of various origins (metabolic, immune-mediated inflammatory), focal compressive neuropathies (Bell's palsy, carpal and tarsal tunnel syndromes, compression of the ulnar, radial, and peroneal nerves), neuromuscular junction dysfunction (myasthenia and myastheniform syndromes, Lambert-Eaton syndrome), myopathies (muscular dystrophies in pediatric and adult age, inflammatory autoimmune myopathies)
How is it performed?
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.
There are no absolute contraindications to the performance of electromyographic examination, which is usually well tolerated except in special cases (high threshold of electrical stimulation that forces high intensity stimulation; extensive electromyographic examinations with repeated insertions in different muscle groups). Particular caution should be reserved for frail individuals (children and the elderly), pacemaker wearers (electrical stimulation should be performed with a specific protocol), and patients on anticoagulant therapy (risk of intramuscular hematomas).