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 an electrode has 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.
What EMG Does?
The doctor may order an EMG if the patient have signs or symptoms that may indicate a nerve or muscle disorder. Such symptoms may include:
Muscle pain or cramping
Certain types of limb pain
EMG results are often necessary to help diagnose or rule out a number of conditions such as:
Muscle disorders, such as muscular dystrophy or polymyositis
Diseases affecting the connection between the nerve and the muscle, such as myasthenia gravis
Disorders of nerves outside the spinal cord (peripheral nerves), such as carpal tunnel syndrome or peripheral neuropathies
Disorders that affect the motor neurons in the brain or spinal cord, such as amyotrophic lateral sclerosis or polio
Disorders that affect the nerve root, such as a herniated disk in the spine
Before the procedure:
Your doctor will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
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 doctor of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.
Notify your doctor 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.
Based on your medical condition, your doctor 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 doctor’s practices. The EMG is performed by a neurologist (a doctor 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 ground electrode will be positioned under your arm or leg.
Five 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:
Some muscle soreness may persist for a day or so following the procedure. Notify your doctor if you experience increasing pain, tenderness, swelling, or pus at the needle insertion sites.