Electromyogram (EMG) and Nerve Conduction Studies
An electromyogram (EMG) measures the electrical activity of muscles when they're at rest and when they're being used. Nerve conduction studies measure how well and how fast the nerves can send electrical signals.
Nerves control the muscles in the body with electrical signals called impulses. These impulses make the muscles react in certain ways. Nerve and muscle problems cause the muscles to react in ways that aren't normal.
If you have leg pain or numbness, you may have these tests to find out which nerves are being affected and how much they are affected. These tests check to see how well your spinal nerves are working. They also check the nerves in your arms and legs.
Why It Is Done
You may need an EMG to find diseases that damage your muscles or nerves or to find out why you can't move your muscles (paralysis), why they feel weak, or why they twitch. These problems may include a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG).
You may need nerve conduction studies to find damage to the nerves that connect the brain and spinal cord to the rest of the body. (This is called the peripheral nervous system.) These studies are often used to help find nerve disorders, such as carpal tunnel syndrome.
How To Prepare
- Wear loose-fitting clothing. You may be given a hospital gown to wear.
- The electrodes for the test are attached to your skin. Your skin needs to be clean and free of sprays, oils, creams, and lotions.
- You may be asked to sign a consent form that says you understand the risks of the test and agree to have it done.
- Tell your doctor ALL the medicines, vitamins, supplements, and herbal remedies you take. Some may increase the risk of problems during your test. Your doctor will tell you if you should stop taking any of them before the test and how soon to do it.
- If you take a medicine that prevents blood clots, your doctor may tell you to stop taking it before your test. Or your doctor may tell you to keep taking it. (These medicines include aspirin and other blood thinners.) Make sure that you understand exactly what your doctor wants you to do.
How It Is Done
An EMG is done in a hospital, a clinic, or a doctor's office. It may be done in a room that stops any outside electrical activity that can interfere with the test.
You will be asked to lie on a table or bed. Or you may sit in a chair that leans back so your muscles are relaxed.
The skin over the areas being tested is cleaned. A needle electrode is put into a muscle. The electrode is attached by wires to a recording machine.
When the electrodes are in place, the electrical activity in that muscle is recorded while the muscle is at rest. Then the technologist or doctor asks you to tighten (contract) the muscle slowly and steadily. This electrical activity is recorded.
The electrode may be moved a number of times. This is done to record the activity in different parts of the muscle or in different muscles.
The electrical activity in the muscle is shown as wavy and spiky lines on a video screen. It may also be heard on a speaker. You may hear popping sounds like a machine gun when you contract the muscle. The activity may also be recorded on video.
When the test is done, the electrodes are removed. The places where a needle was put in the skin are cleaned.
Nerve conduction studies
In this test, several flat metal-disc electrodes are attached to your skin with tape or a paste. An electrode that puts out electric pulses is placed right over the nerve. Then a recording electrode is placed over the muscles controlled by that nerve. Several quick electrical pulses are given to the nerve. The time it takes for the muscle to contract in response to the electrical pulse is recorded. The speed of the response is called the conduction velocity.
The same nerves on the other side of the body may be studied. The results from both sides of the body can be compared. When the test is over, the electrodes are removed.
Nerve conduction studies are done before an EMG if both tests are being done.
How long the test takes
- An EMG may take 30 to 60 minutes.
- Nerve conduction tests may take from 15 minutes to 1 hour or more. It depends on how many nerves and muscles your doctor tests.
How It Feels
During an EMG test, you may feel a quick, sharp pain when the needle electrode is put into a muscle.
With nerve conduction studies, you will be able to feel the electrical pulses. The tests make some people anxious. Keep in mind that only a very low-voltage electrical current is used. And each electrical pulse is very quick. It lasts less than a second.
An EMG is very safe. You may get some small bruises or swelling at some of the needle sites. The needles are sterile, though. There is very little chance of getting an infection.
There is no chance of problems with nerve conduction studies. Nothing is put into your skin, so there is no chance of infection. The voltage of electrical pulses is too low to cause an injury.
Your doctor may be able to tell you some of the results of your nerve studies right after the tests. A full report may take 2 to 3 days.
The EMG recording shows no electrical activity when the muscle is at rest. There is a smooth, wavy line on the recording with each muscle contraction.
The nerve conduction studies show that the nerves send electrical impulses to the muscles or along the sensory nerves at normal speeds, or conduction velocities. Sensory nerves allow the brain to feel pain, touch, temperature, and vibration. Not all nerves have the same normal conduction velocities. They generally get slower as a person gets older.
For an EMG, electrical activity in a muscle at rest shows that there may be a problem with the nerves used by the muscle. Abnormal wave lines when a muscle contracts may mean there is a muscle or nerve problem. Examples include a herniated disc, amyotrophic lateral sclerosis (ALS), and inflammation.
In nerve conduction studies, the speed of nerve impulses is slower than normal for that nerve. Slower speeds may be caused by injury to a nerve or group of nerves.
The results from these studies are looked at along with other things. These include your medical history, symptoms, physical and neurological exams, and the results of other tests. All of these things may help your doctor find the problem or see how a disease is changing.