Electromyography (EMG)

An EMG is one tool that a healthcare provider can use to help make the diagnosis of a muscle disorder, most often presenting with muscle weakness.

Electromyography (EMG) is a test that is used to measure and record the electrical activity of muscles. This test is often done in association with a nerve conduction study (NCS).

Every muscle movement in the body begins with an electrical signal from the brain that is routed via the spinal cord to motor nerves that connect to the muscle. This electrical signal makes the muscle contract and allows a body part to move.

If a muscle stops working properly, gets weak, or starts to lose its mass, EMG and NCS can help with making a diagnosis. Specifically they can help pinpoint whether the problem starts in the nerve or the involved muscle. Nerve conduction studies measure the electrical current in a nerve before it gets to the muscle. EMG measures whether the muscle responds to that electrical signal and whether it contracts appropriately.

An EMG is one tool that a healthcare provider can use to help make the diagnosis of a muscle disorder, most often presenting with muscle weakness.

Following the electrical signal from the brain to the muscle itself, examples of muscle disorders and weakness may be due to the following:

• Diseases that affect the central nervous system (brain and spinal cord), like amyotrophic lateral sclerosis (ALS)
• Conditions affecting the nerves as they leave the spinal canal, like sciatica or other radiculopathies
• Peripheral nerve disease or inflammation. Examples include carpal tunnel syndrome in the wrist, or peripheral neuropathy secondary to diabetes.
• Diseases that prevent normal conduction of electrical impulses from the nerves to the muscle, like myasthenia gravis.
• Diseases of the muscle tissue itself or myopathies (myo=muscle + pathy=disorder). Examples include muscular dystrophy, dermatomyositis, or polymyositis.

An EMG is usually performed by a neurologist, a physician specializing in diseases of the nervous system or a physiatrist.

During an EMG, a single muscle is tested at a time.

A thin needle, with an attached electrode, is inserted through the skin into the muscle to be tested.

The provider will ask the patient to move in certain ways so that a specific muscle will contract or relax. It might be to flex or straighten a joint, or lift or lower an arm or leg.

The electrode is attached to a machine that will record the electrical activity that the muscle produces as it goes through its contraction and relaxation phases. The electricity produced helps determine how frequently and how strong the muscle contracts.

It takes a minute or two to test a muscle. Once that muscle is tested, the electrode is removed and then inserted into the next muscle to be tested.

The EMG test needs little preparation. Usually there is nothing more than cleaning the skin and wearing loose fitting clothes.

Inserting the electrode needles into the muscle may be painful or uncomfortable. It may also cause some muscle soreness for the next couple of days. It would be similar to having a vaccination or intramuscular injection.

There are few complications of an EMG. One important complication is that of pneumothorax when muscle weakness of the chest wall is being evaluated. It is possible for the electrode needle to be inadvertently placed through the muscle and into the chest cavity causing the lung to collapse.

The EMG causes minimal disability and most people can continue with their daily activities almost immediately. There can be residual soreness in the muscles that were tested that can last for a couple of days.

What happens if EMG is not normal?

The EMG is usually not positive or negative. An EMG is just one part of the process to help make the diagnosis in a patient who has developed muscle weakness. The healthcare provider will have performed a history and physical exam, likely have done laboratory tests, and consulted with a neurologist prior to ordering the EMG and nerve conduction studies.

The goal of the test is often to give direction in making the diagnosis that a nerve-muscle condition is present. It may also be used to monitor patients who have known muscle nerve disorders and measure improvement or worsening of disease.

Based upon the results, the medical team may provide guidance as to the next step in making a diagnosis or beginning treatment, depending on the patient’s specific situation.

A nerve velocity test, also known as a nerve conduction study (NCS), is often performed just prior to the EMG. It measures how quickly a nerve can conduct electricity to a muscle and is used to identify potential nerve damage.

If the NCS that measures the nerve function until it reaches the muscle is normal, it is more likely that the muscle weakness is due to a problem with the muscle itself, or the connection between the nerve and the muscle.