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.
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.
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.
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.
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.