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Electromyography and Nerve Conduction Testing

Nerves use electrical impulses to coordinate muscle movements in our bodies. Diseases affecting the nerves and muscles can result in abnormal electrical activity.

Electromyography (EMG) and nerve conduction studies are tests used to evaluate the functioning of your nerves and muscles, and detect nerve and muscle disorders. An EMG is designed to record the electrical activity produced by muscles during rest and contraction. Nerve conduction studies measure the conductivity of the nerves.

Your doctor may arrive at a diagnosis based on the results of the EMG, along with information on your medical history, physical and neurological examinations, and results from other tests.

Indications

EMG testing can be done for the diagnosis of a variety of disorders, such as a herniated disc, amyotrophic lateral sclerosis (ALS) and myasthenia gravis (MG), that damage nerves, muscles, or the junctions between nerves and muscles. An EMG can also be used to evaluate the cause of weakness, paralysis and muscle twitching.

Nerve conduction studies (NCS) are used to detect damage or injury to the peripheral nervous system (PNS), which consists of the extracranial nerves and spinal cord. This test can be used to diagnose neurological disorders, such as carpal tunnel syndrome or Guillain-Barré syndrome.

Before the procedure

Certain medications that act on the nervous system can cause abnormal test results. Therefore, your physician may advise you to stop these medicines 3 to 6 days prior to your procedure. If you are on blood thinners, your doctor may ask you to stop taking them before the test. Also, inform your doctor if you have a pacemaker implanted. You will be instructed not to smoke, eat or drink foods that contain caffeine for 3 hours before the test.

Procedure

Electromyography

An electromyography is performed by an EMG technologist or a doctor. It may be conducted in a doctor's office, hospital or clinic. During the test, you will be asked to sit on a reclining chair, or lie down on a table or bed so that the muscles being tested are relaxed.

An EMG normally takes about 30 to 60 minutes. The skin over the test site is cleansed and a needle electrode connected by wires to a recording machine is inserted into the muscle to be tested. You may feel a quick, sharp pain during the insertion of the electrode. The doctor then records your readings with the muscle at rest and under contraction. The needle electrode may be moved several times to record the electrical activity in different areas of the muscle. The electrical activity of the muscle is depicted as wavy and spiky lines on a computer screen. Electrical impulses may also be monitored through a speaker, in which the electrical signals are denoted by a popping sound, or monitored on video. After the test, the needle electrode is removed and your skin is cleansed. Pain medications are administered if you experience soreness at the site of needle insertion.

Nerve conduction studies

A nerve conduction study usually takes about 15 to 60 minutes depending on the number of areas to be studied. Often, nerve conduction studies are done before an electromyography (EMG). This test is usually done with several flat metal disc electrodes that are pasted or taped to the skin. Your doctor places a shock-emitting electrode directly over the nerve to be studied and a recording electrode over the muscles supplied by that nerve. Small and brief electrical impulses are generated to stimulate the nerves. The time taken for the muscle to contract in response to the pulse is recorded. The speed of transmission of the impulse is called nerve conduction velocity. To compare the conduction velocity, the corresponding nerves on the other side of the body may be studied. After the test, the electrodes are removed.

Risks

Some soreness and a tingling sensation may persist for 1 or 2 hours after electromyography. Call your doctor if you experience increasing pain, swelling, tenderness or pus at any of the needle insertion sites.

With nerve conduction studies, the risk of complications is minimal as the amount of electricity that is transmitted is very small. You may experience a brief, burning pain, a tingling sensation or twitching of the muscle during the transmission of the electrical impulses.

Certain factors or conditions may interfere with the test results. These tests are usually avoided in individuals with swelling, bleeding or obesity, and in those taking medications, such as skeletal muscle relaxants and anticholinergics.