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.
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.
There are no significant risks to this test, other than those associated with any needle insertion (pain, bleeding, bruising, or infection).
The end result of an EMG test is an electromyogram, a computer display or printout of EMG waveforms.
Fasciculation—Small involuntary muscle contractions visible under the skin.
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.
Nerve conduction velocity testing (NCV)—A type of test that uses an electromyography unit to evaluate electrical potentials from peripheral nerves by measuring how long it takes for a nerve impulse to reach a muscle after stimulation with an electrical current.
Polysomnography—A group of studies (that includes EMG) performed while a patient is sleeping to diagnosis sleep disorders.
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.
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.
Jennifer E. Sisk M.A., The Gale Group Inc., Gale, Detroit,