Electromyography

enUK

Electromyography

The detection and recording of electrical activity generated by muscle fibers. The basic elements of motor control in the body are the motor units which comprise motor neurons in the brainstem or spinal cord, their axons, and from ten to several hundred muscle fibers supplied by each motor neuron. Motor units vary in the size and properties of their motoneurons, the sizes and conduction velocities of their axons, the morphology of their nerve muscle junctions, and the structure and physiological properties of the muscle fibers supplied by each motor neuron.

Impulses originating in single motoneurons in response to various command signals from the central nervous system conduct to the periphery of the unit, normally causing all the muscle fibers in the unit to discharge. The electrical activity generated by the more or less synchronous discharges of all the muscle fibers in the unit may be detected by recording electrodes on the skin surface or by needles inserted into the muscle. Such potentials reflect the electrical activity generated by the whole motor unit.

Diseases affecting motor neurons are sometimes accompanied by spontaneous discharges of the axons. Additionally, degeneration of motor axons may leave some muscle fibers deprived of their normal innervation, some of which spontaneously fire. Such single muscle-fiber discharges are called fibrillations and are readily detected for diagnostic purposes by needle electrodes inserted into the muscle.

Electromyography may also be used to study primary muscle diseases such as the muscular dystrophies, and a wide variety of other metabolic inflammatory and congenital myopathies affecting the muscle fibers rather than motor neurons or their axons. See Biopotentials and ionic currents

Electromyography

a method of investigating the bioelectric potentials that arise in the skeletal muscles of animals and humans after the excitation of muscle fibers. The method was first applied to humans in 1907 by the German scientist H. Pieper.

Because the amplitude of oscillations of muscle potential usually does not exceed several millivolts and their duration does not exceed 20–25 microseconds, electromyography is performed with an amplifier and quick-response recorder. The curve recorded on photographic paper or film is called an electromyogram (EMG). There are three principal methods. The first involves the insertion into a muscle of needle electrodes that, owing to the small deriving surface, detect the oscillations of potential arising in individual muscle fibers or group of fibers innervated by a single motoneuron. As a result, it is possible to investigate the structure and function of motor units. The second method uses cutaneous electrodes that lead off from the summary EMG, which results from interference with the oscillations of potential of many motor units in the region of derivation; such an EMG reflects excitation of a muscle as a whole. The third method of electromyography is the recording of oscillations of potential that arise in a muscle after artificial stimulation of a nerve or sense organ; this technique is used to investigate myoneural transmission, motor reflexes, and speed of passage of excitation along a nerve.

Electromyography is used to determine the condition and activity of the muscles and nerve centers participating in movement. It is used in physiology to study the motor functions of animals and, especially, humans. It is also used in applied sciences, for example, work and sports physiology and engineering psychology (for example, to study fatigue and the development of motor skills).

R. S. PERSON

Electromyography is widely used in neuropathology and some other branches of medicine to diagnose several neurological and muscular diseases. It is also used in orthopedics and prosthetics to evaluate the condition of the motor apparatus after restoration of impaired motor function.

REFERENCES

Person, R. S. Elektromiografiia v issledovaniiakh cheloveka. Moscow, 1969.
Iusevich, Iu. S. Ocherki po klinicheskoi elektromiografii. Moscow, 1972.
Baikushev, St., Z. Kh. Manovich, and V. P. Novikova. Slimuliatsionnaia elektromiografiia i elektroneirografiia v klinike nervnykh boleznei. Moscow, 1974.
Cohen, H., and J. Brumlik. Rukovodstvo po elektromiografii i elektrodiagnostike. Moscow, 1975. (Translated from English.)

electromyography

[i¦lek·trō·mī′äg·rə·fē] (medicine) A medical specialty concerned with the production and study of electromyograms.

electromyography

enUK

Electromyography

Definition

Electromyography (EMG) is an electrical recording of muscle activity that aids in the diagnosis of neuromuscular disease.

Purpose

Muscles are stimulated by signals from nerve cells called motor neurons. This stimulation causes electrical activity in the muscle, which in turn causes contraction. This electrical activity is detected by a needle electrode inserted into the muscle and connected to a recording device. Together, the electrode and recorder are called an electromyography machine. EMG can determine whether a particular muscle is responding appropriately to stimulation, and whether a muscle remains inactive when not stimulated.EMG is performed most often to help diagnose different diseases causing weakness. Although EMG is a test of the motor system, it may help identify abnormalities of nerves or spinal nerve roots that may be associated with pain or numbness. Other symptoms for which EMG may be useful include numbness, atrophy, stiffness, fasciculation, cramp, deformity, and spasticity. EMG results can help determine whether symptoms are due to a muscle disease or a neurological disorder, and, when combined with clinical findings, usually allow a confident diagnosis.EMG can help diagnose many muscle and nerve disorders, including:

muscular dystrophy
congenital myopathies
mitochondrial myopathies
metabolic myopathies
myotonias
peripheral neuropathies
radiculopathies
nerve lesions
amyotrophic lateral sclerosis
polio
spinal muscular atrophy
Guillain-Barré syndrome
ataxias
myasthenias

Precautions

No special precautions are needed for this test. Patients with a history of bleeding disorder should consult with their treating physician before the test. If a muscle biopsy is planned as part of the diagnostic work-up, EMG should not be performed at the same site, as it may effect the microscopic appearance of the muscle.

Description

During an EMG test, a fine needle is inserted into the muscle to be tested. This may cause some discomfort, similar to that of an injection. Recordings are made while the muscle is at rest, and then during the contraction. The person performing the test may move the limb being tested, and direct the patient to move it with various levels of force. The needle may be repositioned in the same muscle for further recording. Other muscles may be tested as well. A typical session lasts from 30-60 minutes.A slightly different test, the nerve conduction velocity test, is often performed at the same time with the same equipment. In this test, stimulating and recording electrodes are used, and small electrical shocks are applied to measure the ability of the nerve to conduct electrical signals. This test may cause mild tingling and discomfort similar to a mild shock from static electricity. Evoked potentials may also be performed for additional diagnostic information. Nerve conduction velocity and evoked potential testing are especially helpful when pain or sensory complaints are more prominent than weakness.

Preparation

No special preparation is needed. The doctor supervising and interpreting the test should be given information about the symptoms, medical conditions, suspected diagnosis, neuroimaging studies, and other test results.

Aftercare

Minor pain and bleeding may continue for several hours after the test. The muscle may be tender for a day or two.

Risks

There are no significant risks to this test, other than those associated with any needle insertion (pain, bleeding, bruising, or infection).

Normal results

There should be some brief EMG activity during needle insertion. This activity may be increased in diseases of the nerve and decreased in long-standing muscle disorders where muscle tissue is replaced by fibrous tissue or fat. Muscle tissue normally shows no EMG activity when at rest or when moved passively by the examiner. When the patient actively contracts the muscle, spikes (motor unit action potentials) should appear on the recording screen, reflecting the electrical activity within. As the muscle is contracted more forcefully, more groups of muscle fibers are recruited or activated, causing more EMG activity.

Abnormal results

The interpretation of EMG results is not a simple matter, requiring analysis of the onset, duration, amplitude, and other characteristics of the spike patterns.Electrical activity at rest is abnormal; the particular pattern of firing may indicate denervation (for example, a nerve lesion, radiculopathy, or lower motor neuron degeneration), myotonia, or inflammatory myopathy.Decreases in the amplitude and duration of spikes are associated with muscle diseases, which also show faster recruitment of other muscle fibers to compensate for weakness. Recruitment is reduced in nerve disorders.

Resources

Other

Falck, B., E. Stalberg, and L. Korpinen. The Expert Electromyographer. 〈http://www.tut.fi/∼korpinen/EMG.htm〉.

Key terms

Motor neurons — Nerve cells that transmit signals from the brain or spinal cord to the muscles.Motor unit action potentials — Spikes of electrical activity recorded during an EMG that reflect the number of motor units (motor neurons and the muscle fibers they transmit signals to) activated when the patient voluntarily contracts a muscle.

electromyography

[e-lek″tro-mi-og´rah-fe] the recording and study of the intrinsic electrical properties of skeletal muscle. adj., adj electromyograph´ic. When it is at rest, normal muscle is electrically silent, but when the muscle is active, an electrical current is generated. In electromyography the electrical impulses are picked up by needle electrodes inserted into the muscle and amplified on an oscilloscope screen in the form of wavelike tracings. The visual recording may be accompanied by auditory monitoring in which the sounds are amplified.
Electromyography is useful in diagnosing disorders of the nerves supplying the muscle (as in amyotrophic lateral sclerosis and poliomyelitis) and in disorders affecting the muscle tissues. Recordings usually are obtained while the muscle is relaxed, during voluntary contraction, and during muscle activity that is produced by nerve stimulation. In this way it is possible to determine the presence of a disorder, localize the site, and identify the specific disease producing muscle weakness.

e·lec·tro·my·og·ra·phy

(ē-lek'trō-mī-og'ră-fē), 1. The recording of electrical activity generated in muscle for diagnostic purposes; both surface and needle recording electrodes can be used, although characteristically the latter is employed, so that the procedure is also called needle electrode examination. 2. Umbrella term for the entire electrodiagnostic study performed in the EMG laboratory, including not only the needle electrode examination, but also the nerve conduction studies. [electro- + G. mys, muscle, + graphō, to write]

electromyography

EMG Neurology A technique that measures minute electrical discharges produced in skeletal muscle, at rest and during voluntary contraction; EMG is used to diagnose neuromuscular disease; the electrode for EMG is inserted percutaneously and the resulting electrical discharge or motor unit potential is recorded

e·lec·tro·my·og·ra·phy

(EMG) (ĕ-lek'trō-mī-og'ră-fē) 1. The recording of electrical activity generated in muscle for diagnostic purposes; both surface and needle recording electrodes can be used; the latter are more common. 2. Umbrella term for the entire electrodiagnostic study performed in the EMG laboratory, including not only the needle electrode examination, but also nerve conduction studies. [electro- + G. mys, muscle, + graphō, to write]

electromyography

A diagnostic method in which the electrical events associated with muscle contraction are amplified and recorded for analysis. The signals may be picked up by surface electrodes or a needle consisting of two insulated, coaxial conductors may be pushed into the muscle. The method allows distinction to be made between various nerve disorders, disorders affecting the junction between the nerve and the muscle and various muscle disorders.

e·lec·tro·my·og·ra·phy

(EMG) (ĕ-lek'trō-mī-og'ră-fē) 1. Recording of electrical activity generated in muscle for diagnostic purposes. 2. Umbrella term for the entire electrodiagnostic study performed in the EMG laboratory, including not only needle electrode examination, but also nerve conduction studies. [electro- + G. mys, muscle, + graphō, to write]

electromyography

e·lec·tro·my·o·graph

electromyography

electromyography

Electromyography

Electromyography

Electromyography

REFERENCES

electromyography

electromyography

Electromyography

Definition

Purpose

Precautions

Description

Preparation

Aftercare

Risks

Normal results

Abnormal results

Resources

Other

Key terms

electromyography

e·lec·tro·my·og·ra·phy

electromyography

e·lec·tro·my·og·ra·phy

electromyography

e·lec·tro·my·og·ra·phy

electromyography

Words related to electromyography

noun diagnosis of neuromuscular disorders with the use of an electromyograph

Related Words