What is electromyography?

Indications and Procedures

The term “electromyography” originated in 1890, but the process of measuring and recording the electrical signals conducted by nerves as muscles move was first studied as early as the seventeenth century by such scientists as Francesco Redi, Luigi Galvani, and Emil duBois-Reymond. It was not until 1922 that an oscilloscope, which looks like a small television set, was used to show these electrical signals on a screen, making visual monitoring possible. In the 1960s, electromyography (EMG) began to be used in medical clinics and doctors’ offices to diagnose muscular and nerve injuries and disorders, such as carpal tunnel syndrome.

Today, the electrical activity measured by EMG is displayed on a computer monitor and can also be printed out or recorded as an electromyogram. The results can also be heard through an audio speaker. This electrical activity can be recorded because an electrode detects any electrical activity of the muscles. When a patient does not move the muscle, no signal is detected, but when a patient contracts a muscle by raising a leg or finger, then the nerve cells produce a smooth, wavy line. The exact size and shape of this wave form shown on the computer monitor gives information about the specific muscle’s response to the nerve cells. This wave form is called the action potential. The EMG technician may also use a small electrical stimulus to cause a specific muscle to twitch, and this twitching produces an action potential that can be measured and recorded using electromyography as well.

Uses and Complications

The EMG is very safe, and most patients find that only the insertion of the needles or pins and the mild electrical voltage are somewhat uncomfortable. Any bruises heal within a few days. Very little preparation is required, but a patient does need to follow some guidelines to prevent complications. Cream and lotion should not be used before the EMG test, and smoking as well as caffeine-containing drinks and foods should be avoided for at least three hours prior to the test to avoid inaccurate results. A patient should also avoid taking any muscle relaxant medications such as anticholinergics for at least three to six days prior to the EMG testing. A pacemaker could also cause complications.

Other tests performed along with EMG include the nerve conduction velocity test, often abbreviated as the nerve conduction test. It can also be completed by the same EMG technician in the same location but does not require any needles or small pins. Instead, small electrodes can be taped to the skin, and as the name implies, the nerve conduction velocity test can measure the speed of electrical signals, thus allowing additional disorders that affect the peripheral nervous system to be diagnosed, such as post-polio
syndrome, which can develop even years after a human has suffered from polio. An EMG cannot evaluate brain or spinal cord diseases. Thus an EMG and a nerve conduction test are often completed together to more completely diagnose a range of disorders, such as carpal tunnel syndrome,
sciatica
(nerve root injury), muscular dystrophy, myasthenia gravis, ruptured spinal disks, spinal cord injuries, and various other
nerve disorders.

Perspective and Prospects

EMG has evolved extensively since the observation by Redi in the year 1666 that the muscle of an electric ray
fish could produce electricity. It only takes thirty to sixty minutes for an EMG to record the data. Because the waves can be transmitted through a loudspeaker as well, a patient’s physician can instantly hear them broadcast from a speaker, or recorded on a video that the physician can see instantly.

There is no risk of infection from an EMG because the needles used are sterile and the electrical pulse is of very low voltage and is used for less than a second. Once the testing has been completed, the EMG technician removes the electrodes and cleans the patient’s skin. Some patients may experience bruising or tenderness of the muscles for a few days, but this minor discomfort should disappear within a week.

Research continues to expand the useful scope of conditions that can be diagnosed using EMG. Urination problems can now be diagnosed by recording the electrical signals of the external urinary sphincter, which is a group of muscles that surround the urethra. Also, myasthenia gravis can be diagnosed by using a special type of single fiber EMG that is able to monitor the contraction of a single fiber contraction.

Bibliography

Chernecky, C., and B. Berger. Laboratory Tests and Diagnostic Procedures. 4th ed. Philadelphia: Saunders, 2004.

"Electromyography." MedlinePlus, August 27, 2010.

"Electromyography (EMG)." Health Library, September 26, 2011.

"Electromyography (EMG)." Mayo Clinic, March 7, 2013.

Fischbach, F., and M. Dunning. Manual of Laboratory and Diagnostic Tests. 7th ed. Philadelphia: Lippincott Williams & Wilkins, 2004.

Pagana, K., and T. Pagana. Mosby’s Manual of Diagnostic and Laboratory Tests. 3d ed. St. Louis: Mosby/Elsevier, 2006.