Electromyography
Electromyography (EMG) measures muscle response or electrical activity in response to a nerve’s stimulation of your muscle. The test is used to help detect nerve and muscle problems.
During the test, your doctor will insert one or more small needles (also called electrodes) through your skin into your muscle. The electrical activity picked up by the electrodes is then displayed on a monitor in the form of waves. An audio-amplifier is used so the activity can be heard.
EMG measures the electrical activity of your muscle during rest, slight contraction, and forceful contraction. Muscle tissue does not normally make electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen, but after that, no signal should be present.
After all of the electrodes have been inserted, you may be asked to contract your muscle, for example, by lifting or bending your leg. The action potential (size and shape of the wave) that this creates on the monitor give information about the ability of your muscle to respond to nerve stimulation. As your contract your muscle more forcefully, more and more muscle fibers are activated, producing action potentials.
A healthy muscle will show no electrical activity (no signs of action potential) during rest, only when it contracts. However, if your 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)
- Myotonic (stiff) muscles
EMG is often used along with nerve conduction velocity (NCV) to tell the difference between a muscle problem and a nerve problem. NCV detects a problem with the nerve, whereas EMG can detect a problem with the muscle.
EMG may be done to find the cause of symptoms, such as muscle weakness, deformity, stiffness, and shrinkage. It may be used to detect whether someone is having true muscle weakness or weakness because of pain or psychological reasons.
EMG may be used to evaluate many problems or disorders, including:
- 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 healthcare provider to recommend EMG.
You may feel some discomfort when the needle electrodes are inserted into your muscle. It may feel similar to that of an acupuncture needle. Afterwards, your 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 should not be done in people on anticoagulant therapy (blood thinning medicine such as warfarin) because the needle electrodes may cause bleeding within the muscle.
There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your healthcare provider before the procedure.
Ask your healthcare provider to tell you what you should do before your test. Below is a list of common steps that you may be asked to do:
- Your healthcare provider will explain the test to you and you can ask questions.
- You will be asked to sign a consent form that gives your permission to do the test. 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 2 to 3 hours before testing.
- Tell your healthcare provider of all medicines (prescribed and over-the-counter) and herbal supplements that you are taking.
- Tell your healthcare provider 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 test, or at least stop using them on the day of the exam.
- A sedative or pain reliever may be prescribed before the test. If so, you may need to have someone drive you home afterwards.
- Based on your medical condition, your healthcare provider may request other specific preparations.
An EMG may be done on an outpatient basis or as part of your stay in a hospital. Steps may vary depending on your condition and your healthcare provider's practices. Talk with your healthcare provider about what you will experience during your test.
The EMG is done by a neurologist (a doctor who specializes in brain and nerve disorders), although a technologist may also do some parts of the test.
The EMG is usually done right after a nerve conduction velocity 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 test 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 test.
- 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 needed for the test. You may have 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 do slight or full-strength muscle contractions.
- The electrical activity from your working muscle will be measured and displayed on the monitor. 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 test, the electrodes will be removed. You may be given pain medicine and warm compresses may be applied to the affected area immediately after the test.
Some muscle soreness may persist for a day or so after the test. Tell your healthcare provider if you have increasing pain, tenderness, swelling, or pus at the needle insertion sites.
Your healthcare provider may give you other instructions after the test, depending on your particular situation.
Before you agree to the test or the procedure make sure you know:
- The name of the test or procedure
- The reason you are having the test or procedure
- What results to expect and what they mean
- The risks and benefits of the test or procedure
- What the possible side effects or complications are
- When and where you are to have the test or procedure
- Who will do the test or procedure and what that person’s qualifications are
- What would happen if you did not have the test or procedure
- Any alternative tests or procedures to think about
- When and how will you get the results
- Who to call after the test or procedure if you have questions or problems
- How much will you have to pay for the test or procedure
Updated:
January 16, 2018
Sources:
Overview of eletromyography, Up To Date
Reviewed By:
Shelat, Amit, MD,Turley, Ray, BSN, MSN