Electromyography (EMG)

Electromyographic study: Nerve Conduction and EMG

To help diagnose peripheral nerve damage, 'pinched' nerves in the neck or back, or compressed nerves in the arm or leg, an electromyographic (EMG) study is scheduled. The following information describes the test and why it is ordered.

There are two parts to an electromyographic study, nerve conduction and EMG.

Nerve Conduction

Based on measurements of thousands of patients, standard ranges of nerve conduction have been published. A nerve conduction is how the information is sent from the brain to an arm or leg that allows a person to walk or reach and hold objects. The SPEED and NUMBER of nerve fibers are evaluated in this section of the test. 

Measurements are made and marked on the skin. A recording pick-up, made of a small metal disk or sticker, is placed on the fingers or foot. A stimulator is placed at the measured mark, and when activated, the recorder displays an image on the computer screen. The waveform is then analyzed for speed and formation.

Most patients do not find this portion of the test uncomfortable. The activation of the stimulator has been described as feeling like 'static electricity,' as when a person rubs his/her feet on a carpet and then touches another person. If pain is experienced, it will not be continuous and will last a very short time, less than a second.

EMG

A small pin is placed into a muscle, and the examiner 'listens' to the sounds of that muscle at rest and during activation. The theory is to determine if the muscle is working with or without the help of the nerves. The examiner is trying to see if a single muscle fiber is activated on its own or with cooperation of the nerve.

Most patients do not find this portion of the test painful, but it will be slightly uncomfortable. In most cases, the pin placement into the muscle will not cause bleeding; however, just as immunizations or a medication shot can cause bleeding, it can easily be controlled with pressure from a cotton pad and the examiner's hand.

An example of the pin placement can be thought of as follows: If one had a microphone (the pin) in the center of a basketball court (the muscle) and wanted to hear only the females (a muscle fiber) speak, it would be very difficult. If the microphone were then brought to a section and then to a particular row, hearing the females would be more accurate. The examiner will position the pin in the muscle to find fibers in four sections and four rows.

To determine if a muscle can contract normally, if the examiner is focusing on the upper leg muscle (quadriceps), the patient will be asked to slightly straighten the leg. While the pin is in the muscle, the recording machine will 'pop' and 'snap' much like popcorn in a microwave. This sound is a muscle fiber 'firing' to move the leg. As the leg is straightened very hard, more muscle fibers can be examined at one time. Up to 10 muscles may have to be examined for proper diagnostic information to be collected.

If, for any reason, the patient cannot continue with the study, he/she should tell the examiner to stop. The information provided will help determine a diagnosis and guide the prognosis. The examiner may ask if the patient can tolerate further testing; if not, it will not continue. This information is provided in the hopes of decreasing patients' anxiety or concerns and to help explain how this test is performed, so that the entire study can be completed.