Electromyogram (EMG) and Nerve Conduction Studies

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Test Overview

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 how well your spinal nerves are working. They also check the nerves in your arms and legs.

Why It Is Done

An EMG is done to:

  • Find problems that damage muscle tissue, nerves, or the spots where nerves and muscles join. These problems may include a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG).
  • Find the cause of weakness, paralysis, or muscle twitching. Problems in a muscle or the nerves going to a muscle can cause these symptoms. So can problems in the spinal cord or the area of the brain that controls a muscle. The EMG does not show brain or spinal cord diseases.

A nerve conduction study is done to:

  • Find damage to the peripheral nervous system. This includes all the nerves that lead away from the brain and spinal cord. It also includes the smaller nerves that branch out from those nerves. This test is often used to help find nerve problems such as carpal tunnel syndrome or Guillain-Barré syndrome.

How To Prepare

Be sure to tell your doctor about all the medicines you take, even over-the-counter ones. Many medicines can change the results of this test.

  • You may need to stop taking some medicines before you have this test.
  • If you take aspirin or some other blood thinner, be sure to talk to your doctor. He or she will tell you if you should stop taking it before your test. Make sure that you understand exactly what your doctor wants you to do.

Wear loose clothing to make it easier to do the test. You may get a hospital gown to wear.

The electrodes are attached to your skin. So make sure your skin is clean. Don't use any sprays, oils, creams, or 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.

Talk to your doctor if you have any concerns about the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

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.

Electromyogram

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.

An EMG may take 30 to 60 minutes. 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 done, the electrodes are removed.

Nerve conduction studies are done before an EMG if both tests are being done. Nerve conduction tests may take from 15 minutes to 1 hour or more. How long the tests take depends on how many nerves and muscles are studied.

How It Feels

During an EMG test, you may feel a quick, sharp pain when the needle electrode is put into a muscle. After the test, you may be sore and feel a tingling in your muscles. This may last for up to 2 days. Call your doctor if your pain gets worse. You should also call if you have swelling, tenderness, or pus at any of the needle sites.

If you have pain after the test:

  • Put ice or a cold pack on the sore area. Try this for 10 to 20 minutes at a time. Put a thin cloth between the ice and your skin.
  • Take an over-the-counter pain medicine, such as acetaminophen (Tylenol), ibuprofen (Advil, Motrin), or naproxen (Aleve). Be safe with medicines. Read and follow all instructions on the label.

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.

Risks

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.

Results

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.

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.

Electromyogram (EMG) and nerve conduction studies

Normal:

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.

Abnormal:

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 EMG and nerve conduction 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.

What Affects the Test

You may not be able to have the test, or the results may not be helpful, if:

  • You take medicines, such as muscle relaxants or anticholinergics.
  • You have bleeding, swelling, or too much fat under the skin where the nerves or muscles are being tested.
  • You aren't able to do what is asked during the test.

What To Think About

  • The levels of some enzymes in the blood may rise when muscle tissue is damaged. These enzymes include aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and creatine phosphokinase (CPK). An EMG can cause higher levels of these enzymes for up to 10 days after the EMG. So blood tests for these enzymes should not be done for 5 to 10 days after an EMG.
  • Special types of electromyograms (EMG) may also be done. For example:
    • Single-fiber EMG uses very small needles that are put into a muscle. This can show how a single muscle fiber contracts. It is a useful test for myasthenia gravis. This disease affects the nerve-muscle (neuromuscular) junctions.
    • Repetitive nerve stimulation is a useful test for myasthenia gravis and Lambert-Eaton syndrome. In this test, small, repeated shocks are given. The test shows how the muscle reacts to repeated nerve stimulation.
    • External sphincter electromyogram measures the electrical activity of the external urinary sphincter. This helps to find urination problems. The urinary sphincter is a ring-shaped band of muscles around the urethra. It helps control urination. The electrical activity can be measured by skin electrodes, by needle electrodes put through the skin, or by electrodes in an anal plug.

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health.
  • Pagana KD, Pagana TJ (2014). Mosby's Manual of Diagnostic and Laboratory Tests, 5th ed. St. Louis: Mosby.

Credits

ByHealthwise Staff

Primary Medical ReviewerAnne C. Poinier, MD - Internal Medicine

Adam Husney, MD - Family Medicine

E. Gregory Thompson, MD - Internal Medicine

Martin J. Gabica, MD - Family Medicine

Specialist Medical ReviewerKarin M. Lindholm, DO - Neurology

Current as ofOctober 14, 2016

Current as of: October 14, 2016

Author: Healthwise Staff

Medical Review: Anne C. Poinier, MD - Internal Medicine & Adam Husney, MD - Family Medicine & E. Gregory Thompson, MD - Internal Medicine & Martin J. Gabica, MD - Family Medicine & Karin M. Lindholm, DO - Neurology