Glossary of common electrodiagnostic terms

TermDescriptionClinical pearls
Chronic denervationRemote axon loss identified by long-duration, high-amplitude motor units firing with a reduced recruitment patternGenerally denotes a process that started at least several months before the examination
Active or ongoing denervationA muscle exhibiting positive sharp waves or fibrillation potentials, reflecting a subacute (or more long-standing but uncompensated) neurogenic or axon-loss processDoes not always imply a truly active process. Fibrillation potentials and positive sharp waves are observed whenever a muscle fiber is awaiting reinnervation. These findings generally appear by about 3 weeks after the onset of injury and resolve within a few months, but may persist for longer in distal muscles and when reinnervation mechanisms are not fully successful or complete
Intraspinal canal lesion or processElectrodiagnostic testing characterized by neurogenic or axon-loss changes in muscles of 1 or more specific myotomes (eg, a spinal root or segment derivative) without sensory findingsThe lesion is proximal to the dorsal root ganglion. Most of these lesions are compressive radiculopathies; but infrequently; other lesions such as motor neuron disease produce similar findings
NeurogenicElectrodiagnostic features resulting from lesions of the anterior horn cell, nerve root, plexus, or nerveNeurogenic findings are further refined by distribution and the presence or absence of sensory findings
MyopathicElectrodiagnostic features of muscle disease, including low amplitude, short duration, and polyphasic motor unitsElectrodiagnostic testing may be less sensitive in many myopathies than in neurogenic conditions
Irritable myopathyWhen myopathic features are accompanied by diffuse fibrillation potentials, positive sharp waves, or bothSuggestive of inflammatory or necrotizing etiologies, but not pathognomonic
Motor unit instabilityThe same motor unit on needle electrode examination varies in morphology from one firing to the nextIndicates dysfunction at the neuromuscular junction, but it can be seen in neurogenic conditions during early reinnervation, when neuromuscular junctions are immature
Reduced activationSuboptimal voluntary activation of a muscle resulting from central nervous system causesCan result from pain, cognitive dysfunction, poor effort, or upper motor neuron pathology, and indicates that the data may be of lower yield
Conduction blockMotor response in a nerve conduction study has > 50% reduced response when stimulating at a more proximal locationIndicates focal demyelination When occurring at noncompression sites or in multiple nerves, can suggest acquired demyelinating polyneuropathies