Health Encyclopedia

Electromyography

Definition

Electromyography (EMG) is a test that checks the health of the muscles and the nerves that control the muscles.

Alternative Names

EMG; Myogram

How the test is performed

The health care provider will insert a very thin needle electrode through the skin into the muscle. The electrode on the needle picks up the electrical activity given off by your muscles. This activity is displayed on a special monitor called an an oscilloscope, and may be heard through a speaker.

After placement of the electrodes, you may be asked to contract the muscle. For example, bending your arm. The presence, size, and shape of the wave form -- the action potential -- produced on the monitor provide information about your muscle's ability to respond when the nerves are stimulated.

A nerve conduction velocity test is usually performed along with an EMG.

How to prepare for the test

No special preparation is usually necessary. Avoid using any creams or lotions on the day of the test.

How the test will feel

You may feel some pain or discomfort when the electrodes are inserted, but most people are able to complete the test without significant difficulty.

Afterward, the muscle may feel tender or bruised for a few days.

Why the test is performed

EMG is most often used when people have symptoms of weakness and examination shows impaired muscle strength. It can help to differentiate primary muscle conditions from muscle weakness caused by neurologic disorders.

Normal Values

Muscle tissue is normally electrically silent at rest. Once the muscles quiet down after the needles are inserted, there should be little muscle cell electrical activity noted on the screen.

When the muscle is voluntarily contracted, activity begins to appear. As contraction is increased, more and more muscle fibers produce action potentials until a disorderly group of action potentials of varying rates and amplitudes (complete recruitment and interference pattern) appears with full contraction.

What abnormal results mean

Disorders or conditions that cause abnormal results include the following:

Alcoholic neuropathy
Amyotrophic lateral sclerosis (ALS)
Axillary nerve dysfunction
Becker's muscular dystrophy
Brachial plexopathy
Carpal tunnel syndrome
Cervical spondylosis
Common peroneal nerve dysfunction
Denervation (reduced nervous stimulation)
Dermatomyositis
Distal median nerve dysfunction
Duchenne muscular dystrophy
Facioscapulohumeral muscular dystrophy (Landouzy-Dejerine)
Familial periodic paralysis
Femoral nerve dysfunction
Friedreich's ataxia
Guillain-Barre
Lambert-Eaton Syndrome
Mononeuritis multiplex
Mononeuropathy
Myopathy (muscle degeneration caused by a number of disorders, including muscular dystrophy)
Myasthenia gravis
Peripheral neuropathy
Polymyositis
Radial nerve dysfunction
Sciatic nerve dysfunction
Sensorimotor polyneuropathy
Shy-Drager syndrome
Thyrotoxic periodic paralysis
Tibial nerve dysfunction
Ulnar nerve dysfunction

What the risks are

Bleeding (minimal)
Infection at the electrode sites (minimal risk)

Special considerations

Trauma to the muscle from EMG may cause false results on blood tests, including creatine kinase, a muscle biopsy, or other medical tests.

Review Date:9/22/2008
Reviewed By:Daniel B. Hoch, PhD, MD, Assistant Professor of Neurology, Harvard Medical School, Department of Neurology, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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