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Electrophysiology of the central and peripheral nervous systems 

Electrophysiology of the central and peripheral nervous systems

Chapter:
Electrophysiology of the central and peripheral nervous systems
Author(s):

Christian Krarup

DOI:
10.1093/med/9780199204854.003.2432_update_001

Update:

Chapter reviewed; minor changes made.

Updated on 28 Nov 2012. The previous version of this content can be found here.
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date: 24 March 2017

Electrophysiological studies of the central nervous system and peripheral nervous system—the core investigations in clinical neurophysiology—include electroencephalography, evoked potentials, electromyography, and nerve conduction studies. These provide information from anatomical regions which may not be accessible to direct pathological examination, and are good for tracking changes over time. However, they do not provide direct information about pathological changes in the nervous system, hence it is often necessary to supplement electrophysiological findings by imaging or other laboratory studies, and it is mandatory to view all results in their clinical context.

Electroencephalography (EEG)

EEG is mainly used to identify and diagnose epileptic discharges in connection with paroxysmal events. The procedure is valuable for the evaluation and prognosis of disturbances of consciousness at diffuse disorders of the brain, including infectious, metabolic, and ischaemic disorders, when serial recordings are often used. EEG is of value to diagnose epilepsy caused by focal or diffuse brain diseases, but it is of limited value for detecting focal lesions, since those that affect subcortical regions cannot be detected by scalp electrodes.

Evoked potentials

Evoked potentials in the brain are obtained following sensory stimulation and used to determine the integrity of afferent and efferent pathways, principally threatened by disease in myelinated tracts or the synaptic connections by which sensory impulses are relayed. (1) Somatosensory and motor evoked potentials—these are useful for monitoring surgical procedures in the vertebral column or carried out for spinal lesions. (2) Visual evoked potentials—these are used to assess diseases of the optic nerve in for example multiple sclerosis; electroretinography recorded with contact lenses is used to assess retinal disease, and dark adapted studies may assist diagnosis of retinal degeneration. (3) Brainstem auditory evoked potentials—elicited by clicks, are complex multiphase responses determined by conduction within the cochlear nerve and the different relay steps in the lateral lemniscus; they assist in evaluating the integrity of the brainstem in diverse types of brain injury, and are also useful in localizing cochlear nerve lesions, lesions of the cochlear nucleus and brainstem tracts.

Electromyography (EMG) and nerve conduction studies

EMG is useful for determining whether weakness is caused by muscle disease or by loss of alpha-motor fibre innervation, for which nerve conduction studies—to search for loss of sensory or motor axons or disease of myelinated fibres—are usually required. Disturbances of neuromuscular transmission (e.g. myasthenia gravis and the Lambert–Eaton syndrome) require special examination of compound muscle responses evoked by repetitive motor stimulation. Single-fibre recordings of action potentials induced during voluntary activity or after repetitive stimulation allow the stability of neuromuscular transmission to be assessed.

Other electrophysiological techniques

Additional methods—e.g. cardiovascular reflexes in the study of the autonomic nervous system, respiratory movements and oxygen saturation in polysomnographic studies of sleep disturbances, and recording of force in the study of voluntary muscle—are becoming increasingly important.

Acknowledgement: I am indebted to Dr H. Høgenhaven MD for comments on the manuscript.

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