Electromyogram (EMG)

What is an electromyogram?

An electromyogram (EMG) is a test that is used to record the electrical activity of muscles. When muscles are active, they produce an electrical current. This current is usually proportional to the level of the muscle activity. An EMG is also referred to as a myogram.

EMGs can be used to detect abnormal muscle electrical activity that can occur in many diseases and conditions, including muscular dystrophy, inflammation of muscles, pinched nerves, peripheral nerve damage (damage to nerves in the arms and legs), amyotrophic lateral sclerosis (ALS) (also known as Lou Gehrig disease), myasthenia gravis, disc herniation, and others.

Why is an EMG test done?

An EMG is most often performed when patients have unexplained muscle weakness. The EMG helps to distinguish between muscle conditions in which the problem begins in the muscle and muscle weakness due to nerve disorders. The EMG can also be used to detect true weakness, as opposed to weakness from reduced use because of pain or lack of motivation.

What kinds of EMG are there?

There are two types of EMG: intramuscular EMG and surface EMG (SEMG).

Intramuscular EMG (the most commonly used type) involves inserting a needle electrode through the skin into the muscle whose electrical activity is to be measured.

Surface EMG (SEMG) involves placing the electrodes on (not into) the skin overlying the muscle to detect the electrical activity of the muscle.

Intramuscular EMG is the "classic" form of EMG (and is the main subject here).

How is an intramuscular EMG done?

A needle is inserted through the skin into the muscle. The electrical activity is detected by this needle (which serves as an electrode). The activity is displayed visually on an oscilloscope and may also be displayed audibly through a microphone.

Since skeletal muscles are often large, several needle electrodes may need to be placed at various locations to obtain an informative EMG.

After placement of the electrode(s), the patient may be asked to contract the muscle (for example, to bend the leg).

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 to nervous stimulation. Each muscle fiber that contracts produces an action potential. The size of the muscle fiber affects the rate (how frequently an action potential occurs) and the size (the amplitude) of the action potential.

How do you prepare for an intramuscular EMG?

For adults, no special preparation is needed. For infants and children, the physical and psychological preparation depends on the child's age, behavior, and prior experience. (For instance, has the child been traumatized by another medical or dental procedure?)

Does an EMG hurt?

Yes. There is some undeniable discomfort at the time the needle electrodes are inserted. They feel like shots (intramuscular injection), although nothing is injected during an EMG. Afterwards, the muscle may feel a little sore for up to a few days.

What is the current status of a surface EMG (SEMG)?

SEMG has some attractive features. Most notably, it does not involve piercing the skin and does not hurt. However, the available medical literature indicates that the clinical value of the information gathered by SEMG has not been well established.

The American Association of Electrodiagnostic Medicine has reported: "There is in fact almost no literature to support the use of SEMG in the clinical diagnosis and management of nerve or muscle disease." Still, the SEMG may prove of value in the future in helping to monitor the progression of disorders of the nerves and muscles.

What other test is done during an intramuscular EMG?

A nerve conduction velocity (NCV) test is often done at the same time as an EMG. In this test, the nerve is electrically stimulated while a second electrode detects the electrical impulse down stream' from the first. This is usually done with surface patch electrodes (they are similar to those used for an electrocardiogram) that are placed on the skin over the nerve at various locations. One electrode stimulates the nerve with a very mild electrical impulse. The resulting electrical activity is recorded by the other electrodes. The distance between electrodes and the time it takes for electrical impulses to travel between electrodes are used to calculate the speed of impulse transmission (nerve conduction velocity). A decreased speed of transmission indicates nerve disease.

The NCV test can be used to detect true nerve disorders (such as neuropathy) or conditions whereby muscles are affected by nerve injury (such as carpal tunnel syndrome). Normal body temperature must be maintained for the NCV test, because low body temperatures slow nerve conduction.

The word "electromyography" looks dauntingly long but it is made up of three parts: "electro-" + "-myo-" + "-graphy." ("-myo-" is from the Greek "mys", meaning muscle and "graphy" comes from the Greek "grapho" meaning to write) . So electromyography literally is the writing (recording) of muscle electricity.

 

cited from

http://www.medicinenet.com/Electromyogram/article.htm

 

 

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