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 an SSEP or somatosensory evoked potentials test.
Why is it done?
By looking for abnormal electrical signals in the muscles, the EMG can tell your doctor whether a nerve is being irritated or pinched as it leaves the spine on its way down your 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 each muscle is then stimulated to see how long it takes the electrical signal to reach the muscle and make the muscle tighten. The EMG procedure takes about 2-3 hours.
What are the limitations?
An EMG does not show why there is a problem with the nerves in your spine or what is causing it. The test looks primarily at how your muscles are reacting to the nerve problem. There could even be a problem in the nerve somewhere between the spine and the muscle, and it may not necessarily be in the spine itself. Another limitation of the EMG is that result of the test may normal when the nerve is actually 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 your body there is a small chance of infection. This risk is almost absent in an EMG test.
A PATIENT’S GUIDE TO ELECTROMYOGRAMS
What it is: An electromyogram (EMG) test 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.
What the test shows: 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. Think of how you test the wiring on a lamp. If you place a working bulb into the lamp, and the bulb lights up, you assume that the wiring is okay. However, what if the bulb does not light up? You can safely assume that something is probably wrong with the wiring, like the lamp is unplugged, or a short circuit has occurred. By 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, like the wiring on the lamp. If the EMG machine finds that the muscles (the light bulb) are not working properly, the doctor can assume that the nerves (the wiring) must be getting pinched somewhere.
What the test does not show: These tests do not show why the problem occurred or what is causing the problem. The EMG test looks primarily at how the muscles are reacting to the nerve problem (the wiring problem). There could be a problem in the nerve somewhere between the spine and the muscle. The problem may not necessarily be in the spine itself. Still, it is a good test to see how much the nerve is being damaged, and if there is a herniated disc or other source of pressure on the nerve roots.
How the test is done: The test is done by inserting tiny electrodes into the muscles of the lower extremity. Very small needles are inserted through the skin and down into the muscles.
What risks the test has: There are not many risks associated with the EMG or SSP tests. Anytime a needle is inserted into the body there is a small chance of infection – but the risk is almost zero in this type of test.
What the test costs: An EMG/SSP usually has two costs associated with the test. The first cost is the fee for actually doing the test. This is called the “technical fee”. The second cost is the fee of having a specialist, such as a neurologist, to read and interpret the test. This is called the “professional fee”. You may get two bills for this test: one from the hospital or clinic where you had the EMG/SSP done, and one from the specialist who read the test.
