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 oscilliscope (a monitor that displays electrical activity in the form of waves). An audio-amplifier is used so the activity can be heard. 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.
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 needle stick 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 may be 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 a 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 after 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. 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. 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
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 physical medicine specialist 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.
- Five or more needle insertions may be necessary for the test. You may experience slight pain iwth 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 rom 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
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.
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