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CNI Neuromuscular and Peripheral Nerve Disorders
Electromyography and nerve conduction studies
by Marc M. Treihaft, MD, FAAN
Excerpted from the CNI REVIEW
Considered an extension of the neurological examination, electromyography and nerve conduction studies provide information for the evaluation of neuromuscular disorders. Advances in signal acquisition and computer-assisted analysis have increased the diagnostic sensitivity of conventional studies. This article reviews the clinical use of standard techniques and the application of newer developments such as single-fiber electromyography and center electromyography.
Electromyography (EMG) and nerve conduction studies, considered by neurologists to be an extension of the neurological examination, provide valuable information regarding localization, pathophysiology and prognosis of neuromuscular disorders. In initial experiments performed by Galvani and others at the end of the 18th century, a muscle twitch was observed following frog nerve stimulation.1 Fueled by experimentation in Galvanic electricity throughout the 19th century, an understanding of bio-electrical activity was expanded.2 Following Lindell's and Sherrington's proposal of the concept of the motor unit in 1925 3 and the invention of the concentric needle electrode by Adrian and Bronk in 1929,4 clinical applications for electrophysiological observations were developed.
Conduction velocities are calculated by dividing the difference in conduction time between the two stimulus sites by the distance the action potential has traveled (meters/second). Slowed conduction times occur with inherited sensory motor neuropathies such as Chacot-Marie-Tooth disease as well as with acquired demyelinating neuropathies such as the Gullain-Barrœ syndrome. A localized conduction demyelinating delay is present with compression of the median nerve at the wrist (carpal tunnel syndrome) or in other mononeuropathies associated with metabolic disorders such as diabetes mellitus.
Electromyography is performed by inserting a needle electrode into a muscle belly and reading volitional activity on an oscilloscope. The wave form configurations of motor unit action potentials differ in neuropathic, myopathic and normal states. In neurogenic disorders - such as polio, cervical and lumbar radiculopathies, and motor neuron disease - the motor units are often large and broad, whereas in myopathic disorders - such as polymyositis - the characteristic wave form is of brief duration and low amplitude.
Footnotes:
1. Galvani L. De Viribus Electrocitatis in Motu Musculari Commentarius. Proc Bologna Academy and Institute of Sciences and Arts. 1971: 7:363-418. Translated into English by Green RM. Elizabeth Licht, Cambridge, 1953.
2. Kimura J. Electrodiagnosis in diseases of nerve and muscle: Principles and practice. Edition 2. FA Davis Company, Philadelphia. 1989.
3. Lidell EGT, Sherrington CS. Recruitment and some other features of reflex inhibition. Proc R Soc B 1925; 97:488-518.
4. Adrian ED, Bronk DE. The discharge impulses in motor nerve fibers. Part II. The frequency of discharge reflex and voluntary contractions. J. Physiol (London) 1929; 67:119-151.
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