Electrodiagnostic Testing (Nerveconduction testing and Needle EMG) or EMG Studies
An electrodiagnostic test is a neurodiagnostic test that measures the electrical activity of a muscle during contraction and relaxation. An electrodiagnostic test generally consists of two parts: nerve conduction studies and needle EMG. Nerve conduction studies are useful in detecting nerve-related problems, such as nerve entrapment (carpal tunnel syndrome), or more widespread nerve dysfunction. The needle EMG is useful for identifying problems arising from a “pinched nerve” in the spine, as well as, diseases of the muscles themselves. Both parts of the test are usually needed for the physician to obtain meaningful results. The results of this test help your doctor determine the cause of your symptoms and select the best treatment for you.
What happens during a nerve conduction test and needle EMG?
You will be asked to lie on an exam table, either on your back or stomach depending on the areas to be tested. Small, flat discs called electrodes will be taped to the surface of your skin. A small electrical current is used to stimulate the nerve. Since the level of electricity used during the test is very low (similar to that of static electricity), the test is very safe. Repeated small, short stimulations of varied intensity are administered. The electrical activity will be recorded on a computer connected to the EMG machine. The second part of the test involves the placement of a sterile, disposable, very thin needle electrode through the skin and into the muscle. The needle is a specialized recording device that directly records the muscle’s natural electrical activity at rest and during muscle contraction. Analysis of this activity gives important information about the health of each muscle tested as well as the controlling nerves. The stimulation of the nerve and the prick of the needle will be slightly uncomfortable, but lying still and remaining as relaxed as possible will help.
The entire test can last anywhere from 30 minutes to an hour depending on the number of areas to be tested. The results of the nerve conduction study and EMG will be discussed with you at your next appointment. Little preparation is needed before these examinations. Make sure not to place any skin lotions or moisturizers on before your examination. Wearing loose fitting clothing may also be helpful. Please inform the examiner if you take blood thinners or have a pacemaker.
What is it?
An electromyogram (EMG) looks at the function of the nerve roots leaving the spine. It does this by looking at how well the electrical currents in the nerves are being transmitted to the muscles. Pressure on the nerves or damage to the nerves changes the way they transmit electrical current. This shows up in the muscles as they react to the information being sent to them from the brain by the nerves. When ordering electrical tests to diagnose spine problems, EMG is combined with a test showing electrical signals going from the body to the brain, called somatosensory evoked potentials (SSEPs).
Why is it done?
By looking for abnormal electrical signals in the muscles the EMG can show if a nerve is being irritated or pinched as it leaves the spine on its way down the arm or leg. The EMG is similar to testing the wiring on a lamp. If a working bulb is placed into the lamp and it lights up, you can assume that the wiring is okay. If the bulb does not light up you can assume that something is probably wrong with the wiring-the lamp is unplugged or a short circuit has occurred. Using the muscles like the light bulb in the lamp, the EMG is able to determine the condition of the nerves that supply those muscles, just like the wiring on the lamp. If the EMG finds that the muscles are not working properly, your doctor can assume that the nerves must be getting pinched somewhere.
How is it done?
Tiny electrodes are inserted into the muscles of the legs. The nerve going to the muscle is stimulated to see how long it takes the electrical signal to reach the muscle and make the muscle tighten.
What are the limitations?
An EMG does not show why the problem occurred or what is causing it. The test looks primarily at how the muscles are reacting to the nerve problem. But there could be a problem in the nerve somewhere between the spine and the muscle, and it may not necessarily be in the spine itself. Still, an EMG is helpful in determining how much the nerve is being damaged and if there is a herniated disc or other source of pressure on the nerve roots. Also, the test may appear normal when the nerve is damaged. This is called a "false negative," meaning the test results look negative, even though they are actually positive.
What are the risks?
There are few risks associated with an EMG. Anytime a needle is inserted into the body there is a small chance of infection. But the risk is almost absent in this type of test.