Clinical Electromyography (EMG) consists of nerve conduction studies (NCS) and needle electromyography (needle EMG). In the strict sense of the word, EMG only refers to the needle electrode examination of muscles, however is has traditionally been used to refer to both NCS and needle EMG. EMG is a very powerful diagnostic modality for the evaluating the peripheral nervous system which, in competent hands, can provide valuable information that may not be obtainable with any other diagnostic test. It can also complement the information obtained from other sources, such as imaging studies, for optimal localization of a lesion.

More so than most electrodiagnostic tests, EMG is extremely dependent on the skill of the examiner. There are many technical factors which can affect the test and produce erroneous results if not recognized. It is also important to note that EMG is not a standard test like electrocardiography. There are dozens of muscles and nerves that can be tested, and it is the electromyographer's job to tailor the examination to the patient's particular clinical problem. What is tested is as important as how competently it is done.

What is an EMG?

• Electromyography (EMG) measures muscle response or electrical activity in response to a nerve's stimulation of the muscle. The test is used to help detect neuromuscular abnormalities.
• During the test, one or more small needles (also called electrodes) are inserted through the skin into the muscle. The electrical activity picked up by the electrodes is then displayed on an oscilloscope (a monitor that displays electrical activity in the form of waves). An audio-amplifier is used so the activity can be heard.
• EMG measures the electrical activity of muscle during rest, slight contraction, and forceful contraction. Muscle tissue does not normally produce electrical signals during rest. When an electrode is inserted, a brief period of activity can be seen on the oscilloscope, but after that, no signal should be present.
• After all of the electrodes have been inserted, you may be asked to contract the muscle, for example, by lifting or bending your leg. The action potential (size and shape of the wave) that this creates on the oscilloscope provides information about the ability of the muscle to respond when the nerves are stimulated. As the muscle is contracted more forcefully, more and more muscle fibers are activated, producing action potentials.

What is a NCS?

• A related procedure that will be performed is Nerve Conduction Study (NCS). NCS is a measurement of the speed of conduction of an electrical impulse through a nerve. NCS can determine nerve damage and destruction, and is often performed at the same time as EMG. Both procedures help to detect the presence, location, and extent of diseases that damage the nerves and muscles.
• A Nerve Conduction Study (NCS) is an electrical test that is used to detect nerve conditions.
• In this test, the nerve is electrically stimulated while a second electrode detects the electrical impulse down stream from the first.
• This is usually done with surface patch electrodes that are placed on the skin over the nerve at various locations.
• One electrode stimulates the nerve with a very mild electrical impulse. The resulting electrical activity is recorded by the other electrodes.
• The distance between electrodes and the time it takes for electrical impulses to travel between electrodes are used to calculate the speed of impulse transmission, which is the nerve conduction velocity.
• A decreased speed of transmission indicates nerve disease. A nerve conduction velocity test is often done at the same time as an electromyogram (EMG) in order to exclude or detect muscle conditions.

The results from EMG and NCS are used along with your history, symptoms, physical and neurological examinations, and the results of other tests to help your doctor find out what the problem is or to see how a disease is changing. The test will take between an hour and half to two hours depending on how extensive a test is required.

An Electromyogram (EMG) is done to:

1. Find diseases that damage muscle tissue, nerves, or the junctions between nerve and muscle (neuromuscular junctions). These disorders may include a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG).
2. Find the cause of weakness, paralysis, or muscle twitching. Problems in a muscle, the nerves supplying a muscle, the spinal cord, or the area of the brain that controls a muscle can cause these symptoms. The EMG does not show brain or spinal cord diseases.

Nerve conduction studies are done to:

• Find damage to the peripheral nervous system, which includes all the nerves that lead away from the brain and spinal cord and the smaller nerves that branch out from those nerves.
• Nerve conduction studies are often used to help find nerve disorders, such as Carpal Tunnel Syndrome or Guillain-Barre Syndrome.

Both EMG and nerve conduction studies can help diagnose a condition called post-polio syndrome that may develop months to years after a person has had polio.

EMG Interpretation:

1. A healthy muscle will show no electrical activity (no signs of action potential) during rest, only when it contracts. However, if the muscle is damaged or has lost input from nerves, it may have electrical activity during rest. When it contracts its electrical activity may produce abnormal patterns.
2. An abnormal EMG result may be a sign of a variety of muscle or nerve disorders, including polymyositis (an inflammatory muscle disease that causes decreased muscle power), muscular dystrophy (a chronic genetic disease that progressively affects muscle function), myasthenia gravis (a genetic or immune disorder that occurs at the point where the nerve connects with the muscle), and myotonic (stiff) muscles.

NCS Interpretation:

1. The interpretation of nerve conduction studies is complex, but in general, different pathological processes result in changes in latencies, motor and/or sensory amplitudes, or slowing of the conduction velocities to differing degrees.
2. For example, slowing of the NCS usually indicates there is damage to the myelin.
3. Another example, slowing across the wrist for the motor and sensory latencies of the median nerve indicates focal compression of the median nerve at the wrist, called carpal tunnel syndrome.
4. On the other hand, slowing of all nerve conductions in more than one limb indicates generalized sick nerves, or generalized peripheral neuropathy.
5. People with diabetes mellitus often develop generalized peripheral neuropathy.
6. Normal results from a NCS test only mean that there is no evidence of measurable damage or disease in the nerve. This test does not measure pain. In a small number of cases, nerve disease may still exist despite normal results. This is because other healthy fibers in the same nerve may show a normal reaction time.

1. EMG is often used along with NCS to differentiate a muscle disorder from a nerve disorder. NCS detects a problem with the nerve, whereas EMG can detect diseases stemming from problems with the muscle itself, as well as other problems that result from influences on the muscle from other systems, such as nerves.
2. EMG may be done to identify the cause of symptoms, such as muscle weakness, deformity, spasticity, atrophy, and stiffness. It may be used to detect whether someone is experiencing true muscle weakness or weakness because of pain or psychological reasons.
3. EMG may be used to evaluate many problems/disorders including, but not limited to, the following:
   • Motor problems, such as involuntary muscle twitching.
   • Nerve compression or injury, such as carpal tunnel syndrome.
   • Nerve damage from disk compression in the neck or back.
   • Neuromuscular diseases such as amyotrophic lateral sclerosis (ALS), poliomyelitis, myasthenia gravis, and muscular dystrophy.
   • Peripheral neuropathy caused by conditions such as diabetes, pernicious anemia, and heavy metal toxicity.
   • Nerve root injury, such as sciatica.
   • Muscle degeneration, such as muscular dystrophy.
   • There may be other reasons for your physician to recommend EMG.

• Carpal Tunnel Syndrome (CTS) - CTS is the single most common referring diagnosis in our EMG laboratory and by far the most common entrapment neuropathy.
• Ulnar Neuropathy at the Elbow - The second most frequent entrapment neuropathy is ulnar neuropathy at the elbow. Entrapment at the wrist is much less frequent.
• Peripheral Neuropathy - In this group of conditions the peripheral nerves are affected in a diffuse fashion and more or less symmetrically. This is distinguished from entrapment neuropathies where only one nerve is affected. In the USA the most common etiologies are diabetes mellitus (by far the most frequent), alcohol abuse, and uremia. Leprosy is one of the leading causes.
• Cervical and Lumbar Radiculopathy - This is the second common referral diagnosis (after CTS). Needle EMG is essential to establish the diagnosis. NCS are usually normal in radiculopathies, but are required to exclude concurrent peripheral neuropathy or mononeuropathy which may affect the needle EMG examination.
• Myopathy - In adults, polymyositis is the most common cause of acquired myopathy. Needle EMG is essential for diagnosis. NCS are performed for the same reasons as in radiculopathies.

Tell your doctor if you:

1. Are taking any medicines. Certain medicines that act on the nervous system (such as muscle relaxants and anticholinergics) can change Electromyogram (EMG) results. You may need to stop taking these medicines 2 to 3 days before the test.
2. Have had bleeding problems or take blood thinners, such as Warfarin (Coumadin) or Heparin. If you take blood thinners, your doctor will tell you when to stop taking them before the test.
3. Have a pacemaker.

• Do not smoke for 3 hours before the test.
• Do not eat or drink foods that contain caffeine (such as coffee, tea, cola, and chocolate) for 2 to 3 hours before the test.
• Wear loose-fitting clothing so your muscles and nerves can be tested. You may be given a hospital gown to wear.
• Your arms and legs should be clean and free of lotions or creams.
• Based upon your medical condition, your physician may request other specific preparation.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean.

Reasons you may not be able to have the test or why the results may not be helpful include:

1. Taking medicines, such as muscle relaxants and anticholinergics.
2. Having bleeding, swelling, or too much fat under the skin at the site of the nerves or muscles being tested.
3. Not being able to do what is asked during the test.

• An Electromyogram (EMG) is very safe. You may get some small bruises or swelling at some of the needle sites. Some discomfort, similar to the feeling of an injection or an acupuncture needle, may be felt when the needle electrodes are inserted into the muscle. Afterwards, the muscle may feel sore for a few days and a bruise may appear at the needle stick site. The needles are disposable, so there is no chance of developing an infection.
• There is no chance of problems with nerve conduction studies. Nothing is put into your skin, so there is no chance of infection. The voltage of electrical pulses is not high enough to cause an injury.

• EMG is usually contraindicated in persons; receiving anticoagulant therapy (blood thinning medication such as Coumadin) because the needle electrodes may cause bleeding within the muscle.
• It also may be contraindicated in persons with extensive skin infections due to the risk of spreading infection from the skin to the muscle.
• There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.
• Certain factors or conditions may interfere with EMG test results. Swelling, bleeding, or obesity may interfere with the transmission of electrical waves to the electrodes, and thereby alter the EMG results. Medications such as skeletal muscle relaxants, cholinergics, and anticholinergics may also interfere with EMG test results.

The results are usually ready within few days. Your results would be sent to the referring physician or other specialists in a detailed report.

Online Resources

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External links

• American Association of Neuromuscular & Electrodiagnostic Medicine
• American Board of Electrodiagnostic Medicine
• Information about measuring the NCV
• Details of NCV from National Institutes of Health
• WebMD summary of EMG and NCS
• American Academy of Neurology
• Amyotrophic Lateral Sclerosis Association
• Muscular Dystrophy Association
• Myasthenia Gravis Foundation of America
• National Institute of Neurological Disorders and Stroke (NINDS)
• National Institutes of Health (NIH)
• National Library of Medicine