What is an EMG? In humans, nerves carry signals from the brain to the muscles. Nerves control the action of the muscles by electrical signals, called impulses. An EMG involves a series of tests, including both nerve conduction studies (NCS) and needle electromyography (needle EMG), that measure your nerve signaling and muscle response. Specifically, NCS measure how well your nerves can send signals to your muscles and the needle EMG measures the electrical activity of your muscles.
The NCS and needle EMG form the core of an EMG, however, an EMG can also include a variety of other specialized examinations. Each EMG is individualized for each person on the basis of the results of his or her neurological examination.
The EMG plays a key role in the evaluation of patients with muscle weakness. The primary aim of the EMG is to identify the location and source of the problem. In most cases, the exact cause cannot be determined by the EMG alone. Instead, the neurologist uses the information from the EMG together with other clinical evidence to determine the problem’s cause.
What happens during the EMG?
NCS: Every EMG begins with NCS and these findings are used to plan the needle examination. In the NCS, a brief electrical shock is administered to the skin over the nerves being evaluated. This shock usually feels like a static electricity charge, though sometimes it can be a bit stronger. The NCS stimulates nerves at specific points and gives an estimate of how fast the nerves are conducting the electrical impulse. The NCS also helps to determine whether there has been any damage to the nerve fibers which make up the nerve. The nerves selected for NCS depend on the individual patient’s symptoms and suspected diagnosis.
Needle EMG: In this exam, a fine needle is inserted into select muscles. First, the resting electrical activity of the muscle is recorded while the person lies still. Next, the electrical activity is recorded as the person tries to contract the muscle being tested. The needle used is very fine and there are no shocks involved in this part of the testing. Nothing is injected or withdrawn through the needle.
In the diagnosis of ALS, the EMG specialist usually checks muscles in several regions of the body even if there is no obvious weakness in those areas. These muscles often include the legs and lower back, the torso, the arms and neck, and the muscles involved in eating, speaking, and swallowing.
How does the EMG impact my diagnosis? The pattern of muscles that are involved and any abnormalities indicated by the needle EMG can help identify which neuromuscular disease an individual may have. In the diagnosis of ALS, the neurologist uses the EMG to help determine whether there is evidence of active and chronic damage to the nerves that control the muscles that were tested and whether there is evidence of damage involving different regions of the body, which suggests a progressive nerve disorder.
Does the EMG hurt? Parts of the EMG are uncomfortable, however, most people find it tolerable.
How long does an EMG take? The NCS and needle EMG are usually completed within 60 to 90 minutes.
Is there an EMG specialist at MGH? Yes. Dr. William David, one of our physicians in the MGH ALS Multidisciplinary Clinic is the director of the EMG Laboratory at Mass General. His lab is located just a few blocks from the main hospital campus, at 165 Cambridge Street in Boston, MA.
Will I need to have more than one EMG?You may. The EMG may not clearly identify a problem early on in the disease. Sometimes, it must be repeated at a later time.