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Electromyography (EMG) is a neurodiagnostic test used to assess the health and function of the muscles and nerves that control them. Typically, this test is used to detect disorders such as amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease), carpal tunnel syndrome, Guillain-Barre syndrome, muscular dystrophy, myasthenia gravis, neuropathy, and sciatic nerve dysfunction. An EMG tracing is depicted below.
An EMG is usually done in conjunction with a nerve conduction study (NCS), which measures a nerve’s ability to send a signal.
During an EMG, a thin needle electrode is inserted through the skin into the muscle that is being studied. The patient is then asked to contract the muscle, and the electrical signal that is produced is displayed as waves on a monitor, and may be heard on a speaker as loud pops. The presence, size, and shape of the waves indicate the ability of the muscle to respond to nerve stimulation and can help diagnose nerve or muscle damage.
To perform a NCS, a technician tapes two sets of flat electrodes on the skin over specific muscles. The first set of electrodes is used to send small pulses of electricity in order to stimulate the nerve that directs a particular muscle. The second set of electrodes transmits the responding electrical signal to a recording machine where a physician then reviews the response to verify any nerve damage or muscle disease. The test normally takes about 30 minutes to one hour.
In general, there are no risks associated with an EMG/NCS. However, during the test, there may be some discomfort when the needle is inserted into the muscle; and following an EMG, the areas of needle insertion may feel tender for a few days.
