Electromyography (EMG) is the recording of electrical activity of selected skeletal muscle groups at rest and during voluntary contraction. In this test, a needle electrode is inserted percutaneously into a muscle. The muscle’s electrical discharge (or motor unit potential) is then measured and displayed on an oscilloscope screen.
- To aid in differentiating between primary muscle disorders, such as the muscular dystrophies, and secondary disorders.
- To help assess diseases characterized by central neuronal degeneration such as ALS.
- To help diagnose neuromascular disorders such as myasthenia gravis.
- To help diagnose radiculopathies.
- Make sure the patient has signed an appropriate consent form.
- Note and report all allergies.
- Check for and note drugs that may interfere with test results such as cholinergics, anticholinergics, anticoagulants, and skeletal muscle relaxants.
- Tell the patient he need not restrict food and fluids before the test but that it may be necessary to restrict cigarettes, coffee, tea, and cola for 2 to 3 hours beforehand.
- Warn the patient that he might experience some discomfort as a needle is inserted into selected muscles.
- Explain that the test takes at least 1 hour.
- The patient is positioned in a way that relaxes the muscle to be rested.
- Needle electrodes are quickly inserted into the selected muscle.
- A metal plate lies under the patient to serve as a reference electrode.
- The resulting electrical signal is recorded during rest and contraction, amplified 1 million times, and displayed on an oscilloscope or computer screen.
- Lead wires are usually attached to an audio-amplifier so that voltage fluctuations within the muscle are audible.
- Assess the patient’s pain level. If the patient experiences residual pain, apply warm compresses and administer prescribed analgesics.
- Tell the patient that he may resume his usual medications as ordered.
- Monitor the patient for signs and symptoms of infection at the needle electrode sites.
- At rest, muscle exhibits minimal electrical activity.
- During voluntary contraction, electrical activity increased markedly.
- A sustained contraction, or one of increasing strength, produces a rapid “train” of motor unit potentials.
- Short (low-amplitute) motor unit potentials, with frequent, irregular discharges suggest possible primary muscle disease such as muscular dystrophies.
- Isolated and irregular motor unit potentials with increased amplitude and duration suggest possible disorders such as ALS and peripheral nerve disorders.
- Initially normal motor unit potentials that progressively diminish in amplitude with continuing contractions suggest possible myasthenia gravis.
- The patient’s inability to comply with instructions.
- Drugs affecting myoneural junctions, such as anticholinergics, cholinergics, and skeletal muscle relaxants.
- EMG is contraindicated in the patient with a blessing disorder.
- Infection at the insertion site.
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