Electromyogram (EMG) and Nerve Conduction Studies

 

An electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction. 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 specific ways. Nerve and muscle problems cause the muscles to react in abnormal ways.

If you have leg pain or numbness, you may have these tests to find out how much your nerves are being affected. These tests check how well your spinal cord, nerve roots, and nerves and muscles that control your legs are working.

Why It Is Done

An EMG is done to:

  • Find diseases that damage muscle tissue, nerves, or the junctions between nerve and muscle. 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, the nerves supplying a muscle, the spinal cord, or the area of the brain that controls a muscle can cause these symptoms. The EMG does not show brain or spinal cord diseases.

A nerve conduction study is done to:

  • Find damage to the peripheral nervous system, which includes all the nerves that lead away from the brain and spinal cord and 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-BarrĂ© syndrome.

How To Prepare

Tell your doctor if you:

  • Are taking any medicines. Certain medicines that affect the nervous system can change electromyogram (EMG) results. You may need to stop taking these medicines 3 to 6 days before the test.
  • Have had bleeding problems or take blood thinners, such as warfarin or heparin. If you take blood thinners, your doctor will tell you when to stop taking them before the test.
  • Have a pacemaker.

Do not smoke for 3 hours before the test.

Do not eat or drink foods that contain caffeine (such as coffee, tea, cola, and chocolate) for 2 to 3 hours before the test.

Wear loose-fitting clothing so your muscles and nerves can be tested. You may be given a hospital gown to wear.

For an EMG, you may be asked to sign a consent form. Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean.

How It Is Done

An EMG is done in a hospital, clinic, or doctor's office. It may be done in a room that stops any outside electrical interference. The test may be done by an EMG technologist or a doctor.

You will be asked to lie on a table or bed or sit in a reclining chair so your muscles are relaxed.

Electromyogram

The skin over the areas to be tested is cleaned. A needle electrode that is attached by wires to a recording machine is inserted into a muscle.

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 to record the activity in different areas of the muscle or in different muscles.

The electrical activity in the muscle is shown as wavy and spiky lines on a video monitor and may also be heard on a loudspeaker as machine gun-like popping sounds 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 and those areas of the skin where a needle was inserted are cleaned. You may be given pain medicine if any of the test areas are sore.

Nerve conduction studies

In this test, several flat metal disc electrodes are attached to your skin with tape or a paste. A shock-emitting electrode is placed directly over the nerve, and a recording electrode is placed over the muscles controlled by that nerve. Several quick electrical pulses are given to the nerve, and 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 for comparison. 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, depending 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 have a tingling feeling in your muscles for 1 to 2 hours. If your pain gets worse or you have swelling, tenderness, or pus at any of the needle sites, call your doctor.

With the nerve conduction studies, you may feel a quick, burning pain, a tingling feeling, and a twitching of the muscle each time the electrical pulse is given. It feels like the kind of tingling you feel when you rub your feet on the carpet and then touch a metal object. 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 (less than a split-second).

 

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