Electromyography (EMG) & Nerve Conduction Velocity (NCV)

General

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 by electrical signals (impulses), and these impulses make the muscles react in specific ways. Nerve and muscle disorders cause the muscles to react in abnormal ways.

Measuring the electrical activity in muscles and nerves can help find diseases that damage muscle tissue (such as muscular dystrophy) or nerves (such as amyotrophic lateral sclerosis or peripheral neuropathies). EMG and nerve conduction studies are often done together to give more complete information.

Why It Is Done

An electromyogram (EMG) is done to:

  • Find diseases that damage muscle tissue, nerves, or the junctions between nerve and muscle (neuromuscular junctions). These disorders 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.

Nerve conduction studies are 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. Nerve conduction studies are often used to help find nerve disorders, such as carpal tunnel syndrome or Guillain-Barré syndrome.

How To Prepare

Before the test:

  • Do not smoke for 3 hours.
  • Do not eat or drink foods that contain caffeine (such as coffee, tea, cola, and chocolate) for 2 to 3 hours.
  • Wear loose-fitting clothing so your muscles and nerves can be tested. You may be given a hospital gown to wear.

How It Is Done

An electromyogram (EMG) test will be performed by a neurologist.

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

The electrodes are hooked by wires to a computer that records the electrical activity. A machine can show the activity as a series of wavy lines drawn by a row of pens on a moving piece of paper or as an image on the computer screen.

Electromyogram

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

Once 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.

When the testing is done, the electrodes are removed and those areas of the skin where a needle was inserted are cleaned.

Nerve conduction studies

A nerve conduction (NCV) test will be performed by an NCV technologist or a neurologist.

In this test, several flat metal disc electrodes are attached to your skin with tape or a special paste. A shock-emitting electrode is placed directly over the nerve and a recording electrode is placed over the muscles under control of 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 completed, 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

With an electromyogram (EMG) test, you may feel a quick, sharp pain when the needle electrode is put into a muscle. After an EMG test, you may be sore and have a tingling feeling in your muscles for 1 to 2 hours.

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).