electromyography

EMG; Myogram

For an EMG, a needle electrode is inserted through the skin into the muscle. The electrical activity detected by this electrode is displayed on an oscilloscope, and may be heard through a speaker.

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

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

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

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.

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.

Muscle tissue is normally electrically silent at rest. Once the insertion activity (caused by the trauma of needle insertion) quiets down, there should be no action potential on the oscilloscope. When the muscle is voluntarily contracted, action potentials begin 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.

Disorders or conditions that cause abnormal results include the following:

• Polymyositis
• Denervation (reduced nervous stimulation)
• Carpal tunnel syndrome
• Amyotrophic lateral sclerosis (ALS)
• Myopathy (muscle degeneration, may be caused by a number of disorders, including muscular dystrophy)
• Myasthenia gravis
• Alcoholic neuropathy
• Axillary nerve dysfunction
• Becker's muscular dystrophy
• Brachial plexopathy
• Cervical spondylosis
• Common peroneal nerve dysfunction
• 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
• Peripheral neuropathy
• Radial nerve dysfunction
• Sciatic nerve dysfunction
• Sensorimotor polyneuropathy
• Shy-Drager syndrome
• Thyrotoxic periodic paralysis
• Tibial nerve dysfunction
• Ulnar nerve dysfunction

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

Trauma to the muscle from EMG may cause false results on blood tests (such as creatine kinase), a muscle biopsy, or other tests.