Electromyography
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More Specific Terms
- electrooculography
- EMG blink-reflex
- EMG F-response
- EMG H-reflex
- needle electromyography (N-EMG)
- single-fiber electromyography (EMG)
- surface electromyography (S-EMG)
Indications
-
-
- useful in determining the site of injury in a peripheral nerve
- useful in detecting spinal nerve root lesions
- nerve root compression
- primary muscle diseases
- anterior horn cell disease
- can differentiate active (inflammatory) myopathy from chronic myopathy & acute denervation from chronic denervation
-
Contraindications
Principle
- the study of electrical potentials generated by the depolarization of muscle
- EMG techniques include measurement of latencies of F-responses, H-reflexes, blink reflexes & single-fiber EMG
- the usual macro EMG techniques measure the summated electrical activity of all fibers of a motor unit
- amplitude & area are increased in re- innervation & decreased in primary muscle disease
Procedure
- Recording technique (4 phases of examination): [4]
- insertional activity:
- a needle electrode is placed in the muscle & electrode activity associated with its insertion is evaluated
- normal: brief activity with insertion
- lower motor neuron lesion: increased activity
- upper motor neuron lesion: normal
- myopathy: normal
- myotonia: myotonic discharges
- polymyositis: increased activity
- spontaneous activity
- the muscle is evaluated at rest, with the needle stationary in a relaxed muscle
- normal: none
- lower motor neuron lesion:
- fibrillation potential
- positive sharp waves
- upper motor neuron lesion: none
- myopathy: none
- myotonia: none
- polymyositis
- fibrillation potential
- positive sharp waves
- motor unit potentials
- muscle potentials evoked by isolated discharges of motor neurons are recorded with mild voluntary contraction of the muscle
- normal: 0.5-1.0 mV, 5-10 ms
- lower motor neuron lesion: large unit, limited recruitment
- upper motor neuron lesion: normal
- myopathy: small unit, early recruitment
- myotonia: myotonic discharge
- polymyositis: small unit, early recruitment
- recruitment & interference pattern
- the change in electrical potential is assessed as the level of muscle contraction gradually increases & eventually reaches a maximum
- normal: full
- lower motor neuron lesion: reduced, fast firing pattern
- upper motor neuron lesion: reduced, slow firing pattern
- myopathy: full, but low amplitude
- myotonia: full, but low amplitude
- polymyositis: full, but low amplitude
- Except for end- plate noise & brief insertional activity, there are normally no electrical discharges in a relaxed muscle.
- Under normal circumstances, the smallest unit of volitional contraction is a motor unit.
- Fibrillation potentials & positive sharp waves are derived from single, spontaneous discharges of individual motor units.
- Interference pattern:
- with greater contraction, many motor units begin to fire rapidly
- simultaneous activation of different units no longer allows recognition of individual motor unit potentials
- this summated response is referred to as the interference pattern
- it is a measure of the density or number of spikes & the amplitude of the available motor unit potentials
- Myotonic discharges consist of rhythmic discharges lasting for long periods after the external source of excitation has come to rest.
- This may be seen in association with:
- myotonia
- polymyositis (infrequent)
- glycogen storage disease, type II
More General Terms
Additional Terms
References
- nlmpubs.nlm.nih.gov/hstat/ahcpr/
- Harrison's Principles of Internal Medicine, 13th ed. Companion Handbook, Isselbacher et al (eds), McGraw-Hill Inc. NY, 1995, pg 694-95
- Medical Knowledge Self Assessment Program (MKSAP) 11, American College of Physicians, Philadelphia 1998
- Electrodiagnosis in Diseases of Nerve and Muscle, Principles and Practice, Kimura, J. FA Davis, Philadelphia, 1983
