Electromyography Health Article

Definition

Electromyography (EMG) is used to detect, process, and record electrical muscle activity in order to aid in the diagnosis of neuromuscular disease.

Purpose

EMG is performed most often to help diagnose different neuromuscular diseases causing weakness. EMG can determine whether a particular muscle is responding appropriately to stimulation, and whether a muscle remains inactive when not stimulated. 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. EMG may also be useful in determining the cause of certain symptoms, including numbness, atrophy, stiffness, fasciculation, cramps, 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, mitochondrial, and metabolic myopathies
• myotonias
• compression neuropathies, such as carpal tunnel syndrome
• peripheral neuropathies
• radiculopathies
• nerve lesions
• amyotrophic lateral sclerosis (Lou Gehrig's disease)
• polio
• spinal muscular atrophy
• Guillain-Barré syndrome
• ataxias
• myasthenias

EMG is also used in gait and motion analysis. EMG is performed dynamically (while the patient executes certain movements) to evaluate gait and movement problems. Clinical applications include assessment of patients with cerebral palsy, traumatic brain injury, spinal cord injury, motor neuron lesions, evaluation of athletic injuries, examination of gait abnormalities associated with stroke, and preoperative assessment of patients having corrective orthopedic surgery.

EMG is one of the tests administered during polysomnography studies. EMG is used to measure neuromuscular activity during sleep to aid in the diagnosis of sleep disorders, such as restless legs syndrome.

Precautions

No special precautions are needed for this test. Patients with a history of bleeding disorders 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 affect the microscopic appearance of the muscle.

Description

EMG is performed using an electromyography unit consisting of electrodes and a computer-based recording unit. Electrodes are used to detect electrical activity generated by stimulating the muscles. 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 the EMG electrode and recorded by the electromyography unit computer.

During an EMG test, the electrode is applied or inserted into the muscle to be tested. Surface, needle, and fine-wire electrodes may be used, depending on the type of stimulation required. Needle electrodes 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 electrode may be

repositioned 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. Pain-relieving medications may be prescribed to relieve muscle soreness.

Complications

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

Results

The end result of an EMG test is an electromyogram, a computer display or printout of EMG waveforms.