What is EMG?
Electromyography (EMG) measures muscle response or electrical activity in response to a nerve’s stimulation of the muscle. The test is used to help detect neuromuscular abnormalities.
During the test, one or more small needles (also called electrodes) are inserted through the skin into the muscle. The electrical activity picked up by the electrodes is then displayed on an oscilloscope (a monitor that displays electrical activity in the form of waves). An audio-amplifier is used so the activity can be heard.
EMG measures the electrical activity of muscle during rest, slight contraction, and forceful contraction. Muscle tissue does not normally produce electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen on the oscilloscope, but after that, no signal should be present.
After all of the electrodes have been inserted, you may be asked to contract the muscle, for example, by lifting or bending your leg. The action potential (size and shape of the wave) that this creates on the oscilloscope provides information about the ability of the muscle to respond when the nerves are stimulated. As the muscle is contracted more forcefully, more and more muscle fibers are activated, producing action potentials.
A related procedure that may be performed is nerve conduction velocity (NCV). NCV is a measurement of the speed of conduction of an electrical impulse through a nerve. NCV can determine nerve damage and destruction, and is often performed at the same time as EMG. Both procedures help to detect the presence, location, and extent of diseases that damage the nerves and muscles. Please see this procedure for additional information.
Interpretation of the test results:
A healthy muscle will show no electrical activity (no signs of action potential) during rest, only when it contracts. However, if the muscle is damaged or has lost input from nerves, it may have electrical activity during rest. When it contracts its electrical activity may produce abnormal patterns.
An abnormal EMG result may be a sign of a variety of muscle or nerve disorders, including polymyositis (an inflammatory muscle disease that causes decreased muscle power), muscular dystrophy (a chronic genetic disease that progressively affects muscle function), myasthenia gravis (a genetic or immune disorder that occurs at the point where the nerve connects with the muscle), and myotonic (stiff) muscles.
Reasons for the procedure
EMG is often used along with nerve conduction velocity (NCV) to differentiate a muscle disorder from a nerve disorder. NCV detects a problem with the nerve, whereas EMG can detect diseases stemming from problems with the muscle itself, as well as other problems that result from influences on the muscle from other systems, such as nerves.
EMG may be done to identify the cause of symptoms, such as muscle weakness, deformity, spasticity, atrophy, and stiffness. It may be used to detect whether someone is experiencing true muscle weakness or weakness because of pain or psychological reasons.
EMG may be used to evaluate many problems/disorders including, but not limited to, the following:
Neuromuscular diseases, such as myasthenia gravis
Motor problems, such as involuntary muscle twitching
Nerve compression or injury, such as carpal tunnel syndrome
Nerve root injury, such as sciatica
Muscle degeneration, such as muscular dystrophy
There may be other reasons for your physician to recommend EMG.
Risks of the procedure
Some discomfort, similar to the feeling of an injection or an acupuncture needle, may be felt when the needle electrodes are inserted into the muscle. Afterwards, the muscle may feel sore for a few days and a bruise may appear at the needlestick site.
The insertion of the electrodes may also cause false results on a muscle biopsy or during blood tests in which muscle enzymes are measured.
EMG is usually contraindicated in persons receiving anticoagulant therapy (blood thinning medication such as Coumadin) because the needle electrodes may cause bleeding within the muscle.
It also may be contraindicated in persons with extensive skin infections due to the risk of spreading infection from the skin to the muscle.
There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.
Certain factors or conditions may interfere with EMG test results. Swelling, bleeding, or obesity may interfere with the transmission of electrical waves to the electrodes, and thereby alter the EMG results. Medications such as skeletal muscle relaxants, cholinergics, and anticholinergics may also interfere with EMG test results.
Before the procedure
Your physician will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
You will be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something is not clear.
Generally, fasting is not required before the test. In some cases, cigarettes and caffeinated beverages, such as coffee, tea, and cola may be restricted two to three hours before testing.
Notify your physician of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
Notify your physician if you have a pacemaker.
Dress in clothes that permit access to the area to be tested or that are easily removed.
Stop using lotions or oils on your skin for a few days before your procedure - or at least stop using them on the day of the exam.
A sedative or pain reliever may be prescribed before the procedure. If a sedative is given before the procedure, you may need to have someone drive you home afterwards.
Based upon your medical condition, your physician may request other specific preparation.
During the procedure
An EMG procedure may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.
The EMG is performed by a neurologist (a physician who specializes in brain and nerve disorders), although a technologist may also perform some portions of the test.
The EMG is usually performed immediately following a nerve conduction study (a test that measures the flow of current through a nerve before it reaches the muscle rather than the response of muscle itself).
Generally, an EMG procedure follows this process:
You will be asked to remove any clothing, jewelry, hairpins, eyeglasses, hearing aids, or other metal objects that may interfere with the procedure.
If you are asked to remove clothing, you will be given a gown to wear.
You will be asked to sit or lie down for the test.
A neurologist will locate the muscle(s) to be studied.
The skin will be cleansed with an antiseptic solution. Next, a fine, sterile needle will be inserted into the muscle. A metal plate will be positioned under you.
Ten or more needle insertions may be necessary for the test. You may experience slight pain with the insertion of the electrode, but it is usually painless.
If the test is painful you must tell your examiner because this can interfere with the results.
You will be asked to relax and then perform slight or full-strength muscle contractions.
The electrical activity from your working muscle will be measured and displayed on the oscilloscope.
An audio-amplifier may also be used so that both the appearance and sound of the electrical potentials can be evaluated. If the recorder is attached to an audio-amplifier, you may hear a sound like hail on a tin roof when you contract your muscle.
After the procedure
Pain medication may be administered and warm compresses may be applied to the affected area immediately following the procedure.
Some muscle soreness may persist for a day or so following the procedure. Notify your physician if you experience increasing pain, tenderness, swelling, or pus at the needle insertion sites.
Your physician may give you additional or alternate instructions after the procedure, depending on your particular situation.