What is it?
Electromyography is a test that assesses the health of the muscles and the nerves controlling the muscles. How the test is performed?
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.
You may feel some pain or discomfort when the electrodes are inserted or when the nerves are stimulated, but most people are able to complete the test without significant difficulty. Afterward, the muscle may feel tender or bruised for a few days. What can the test tell my doctor?
EMG is most often used when people have symptoms of numbness or weakness and examination shows impaired muscle strength. It can help to differentiate primary muscle conditions from muscle weakness caused by disorders of the nerve or connection between the nerve and muscle (the neuromuscular junction).
EMGs help diagnose three kinds of diseases that interfere with normal muscle contraction:
-- Diseases of the muscle itself (also known as myopathy)
-- Diseases of the neuromuscular junction, which is the connection between a nerve fiber and the muscle it supplies
-- Diseases "upstream" in nerves and nerve roots (which can be due to either nerve damage or ongoing nerve injury)
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. The test can indicate a number of disorders, including:
-- Carpal tunnel syndrome
-- Amyotrophic lateral sclerosis (ALS)
-- Myopathy (muscle degeneration, may be caused by a number of disorders, including muscular dystrophy)
-- Myasthenia gravis
-- Distal median nerve dysfunction
-- Familial periodic paralysis
-- Peripheral neuropathy
-- Sciatic nerve dysfunction
For a complete list of conditions that an EMG test can help diagnose see the MedlinePlus Web site. Are there risks?
The test could lead to a minimal risk of bleeding or infection at the electrode sites. Sources: