Electroneuromyography (EMG) is a series of tests that measure how the nerves and muscles are working and how well the nerves communicate with the muscles. This testing can tell us where a nerve or muscle problem is located and often the nature of the problem based on the abnormalities discovered. EMG is most often done in the outpatient setting and is performed by neuromuscular specialists who have specialized training in performing and interpreting this test.
There are two main components to the test, which usually takes 30 to 60 minutes. The first part consists of nerve conduction studies, which are performed by giving electrical stimulations to the nerves and then measuring how fast the nerve conducts and how big of a signal is produced a fixed distance away. The doctor or technician may use heated towels to warm your arms or legs before testing. With motor nerves, an electrode is placed on a specific muscle, and the electrical stimulation to the nerve causes a small muscle twitch which is detected by the electrode. If the signal takes longer than expected to reach the muscle, there may be a problem with the myelin cells that cover and insulate the nerves, suggesting a “demyelinating neuropathy.” If the muscle twitch is smaller than expected, it could be a problem with the axon nerve fibers that make up the nerve, suggesting an “axonal neuropathy,” or it could also be a problem with the muscle itself. With sensory nerves, the electrode picks up a tiny signal in the sensory nerve. Other tests may look at the signals that go up to the spinal cord and back down the nerve. Together, these tests show how the nerves are working. If you have weakness, the doctor may give a rapid series of electrical stimulations to the nerve which tests whether there is a problem with the chemical signals from the nerves going to the muscles. This occurs with diseases like Myasthenia gravis.
The second part of the test involves inserting a small needle electrode into specific muscles. This can be uncomfortable but most patients tolerate it well, and your doctor will monitor how you are doing at all times. This part of the test shows whether the muscles are “irritable” due to loss of nerve connections, whether the nerve has partially grown back to contact more muscle fibers, and whether there might be a problem in the muscles themselves. Your doctor may ask you to move a single muscle by increasing amounts gradually. These tests provide important information that can lead to diagnosis of a wide variety of nerve and muscle diseases, including common conditions like carpal tunnel syndrome and rare conditions such as inflammatory muscle disease. EMG can also be used by your neurologist to select specific muscles for botulinum toxin injections.