Electrodiagnostic studies consist of both nerve conduction studies (NCS) and electromyography (EMG). These studies are often performed together and the combination of the two is abbreviated an EMG/NCS or, more commonly, just EMG. They work in combination to measure the electrical activity in nerves and muscles and are used to evaluate nerve and muscle injury. These injuries may be due to nerve compression, primary nerve or muscle diseases, or secondary injuries related to other diseases. An EMG helps the physician determine if the patient’s nerves/muscles are healthy and, if not, determine the cause of the nerve or muscle injury.
Nerve conduction studies
Nerve conduction studies (NCS) are performed to determine how well nerves conduct signals. The procedure involves stimulating the nerves with a small amount of electricity and recording how each nerve conducts this signal along its course. The response to this stimulation is recorded by electrodes that are taped to the patient’s skin. The electrodes are attached to a computer that analyzes the signals and produces a visual representation of them on the screen. The physician interprets the results by making measurements of the responses and correlating them to your symptoms.
An electromyogram (EMG) uses a fine wire electrode to evaluate the electrical activity of muscles. The muscles are innervated (supplied with nerves) and it is possible to tell if there is an injury to the nerve by evaluating how its muscle fires. In this way, muscles in the arm or leg can be tested to evaluate a nerve injury in the neck or back. The fine wire electrode used in EMG is slightly larger than an acupuncture needle but not nearly as big as a hollow point needle used for blood draws or IVs. The electrode is connected by wire to a computer, where the signal is analyzed and displayed both as an image on the screen and is broadcast as a sound, typically a popping noise with muscle contraction. The muscles are tested both at rest (to make sure there are not any abnormal spontaneous signals that should not be there) and then with muscle contraction (to make sure the muscle is firing normally).
What to expect
An EMG takes about 45 minutes to an hour to complete. The nerve conduction studies are commonly performed first, followed by the EMG. The electrical stimulations used in nerve conduction studies typically feel like getting a rubber band snapped against your skin. Usually, 3-6 nerves are tested in an affected limb and, if necessary, a few in an unaffected limb are tested for comparison. The EMG feels like getting your skin pinched with electrode insertion and may cause a dull ache, but is generally well tolerated. Generally, nine to twelve muscles are tested. Arm, forearm, and hand muscles are tested to evaluate neck and upper limb injuries. Thigh, leg, and foot muscles are used to evaluate low back and lower limb injuries. Risks of an EMG are very rare. It may hurt a little at the time of the test, but this discomfort does not last long.