Electromyography

Definition

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.

Purpose

• 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.

Procedure

Preparation

1. Make sure the patient has signed an appropriate consent form.
2. Note and report all allergies.
3. Check for and note drugs that may interfere with test results such as cholinergics, anticholinergics, anticoagulants, and skeletal muscle relaxants.
4. 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.
5. Warn the patient that he might experience some discomfort as a needle is inserted into selected muscles.
6. Explain that the test takes at least 1 hour.

Implementation

1. The patient is positioned in a way that relaxes the muscle to be rested.
2. Needle electrodes are quickly inserted into the selected muscle.
3. A metal plate lies under the patient to serve as a reference electrode.
4. The resulting electrical signal is recorded during rest and contraction, amplified 1 million times, and displayed on an oscilloscope or computer screen.
5. Lead wires are usually attached to an audio-amplifier so that voltage fluctuations within the muscle are audible.

Nursing Interventions

1. Assess the patient’s pain level. If the patient experiences residual pain, apply warm compresses and administer prescribed analgesics.
2. Tell the patient that he may resume his usual medications as ordered.
3. Monitor the patient for signs and symptoms of infection at the needle electrode sites.

Interpretation

Normal Results

1. At rest, muscle exhibits minimal electrical activity.
2. During voluntary contraction, electrical activity increased markedly.
3. A sustained contraction, or one of increasing strength, produces a rapid “train” of motor unit potentials.

Abnormal Results

1. Short (low-amplitute) motor unit potentials, with frequent, irregular discharges suggest possible primary muscle disease such as muscular dystrophies.
2. Isolated and irregular motor unit potentials with increased amplitude and duration suggest possible disorders such as ALS and peripheral nerve disorders.
3. Initially normal motor unit potentials that progressively diminish in amplitude with continuing contractions suggest possible myasthenia gravis.

Interfering Factors

• The patient’s inability to comply with instructions.
• Drugs affecting myoneural junctions, such as anticholinergics, cholinergics, and skeletal muscle relaxants.

Precaution

• EMG is contraindicated in the patient with a blessing disorder.

Complications

• Infection at the insertion site.

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