What Is Electromyography (EMG)?

What to expect when undergoing this test

Electromyography (EMG) and nerve conduction study (NCS) are tests that use electrodes to detect, translate, and record the electrical signals in your muscles and nerve cells while they're active and at rest. These tests are valuable diagnostic tools that help neurologists locate and determine the causes of diseases that affect your muscles and nerves, as well as assess how much damage there is. EMG, also known as an electromyogram or myogram, and NCS, also known as a nerve conduction velocity test, are nearly always performed during the same visit.

Purpose of Test

Your doctor may opt for EMG when you have signs and symptoms such as weakness, tingling, numbness, pain in your muscles, cramping, or other abnormal sensations.

A small needle electrode is inserted through your skin into your muscle to measure the electrical activity in your muscle fibers (response to nerve stimulation). This test can help identify the cause of your symptoms, and may even help determine how long the problem has been present.

A nerve conduction study measures how fast electrical signals move through your nerves to determine the extensiveness of the issue. The electrical signal sent along the axon, or nerve fiber, of a nerve is called an action potential. In nerve conduction studies, these action potentials are artificially generated by electrical stimulation in order to assess how the axon responds.

While an EMG may be done with or without an NCS, the two tests are generally performed together. The information each yields is complementary and, when reviewed as a set, paints a more comprehensive picture than one test alone (except in specific situations).

EMG can detect if muscle weakness or numbness is due to a nervous system disorder or injury to a nerve that's attached to the muscle, and the two tests together can differentiate if you have a muscle disorder or a nerve disorder.

EMG and NCS can help rule out or diagnose a variety of conditions. Some common examples include:

They can also be used to monitor specific conditions such as:

  • Guillain-Barré syndrome
  • Carpal tunnel syndrome
  • Sciatica
  • Diabetic neuropathy
  • Lambert-Eaton syndrome
  • Myasthenia gravis
  • Charcot-Marie-Tooth disease
  • Herniated disc
  • Bell's palsy
  • Muscular dystrophy

Risks and Contraindications

Both EMG and NCS are low-risk. With EMG, there is a slight risk of getting an infection or bleeding where the electrodes were inserted. Let the neurologist who's performing the procedure know ahead of time if you're taking anticoagulants (blood thinners) or if you have hemophilia, a disorder that inhibits blood clotting.

If the muscles in your chest are being tested with EMG, there's a tiny risk that air could get into the space between your chest wall and your lungs and cause pneumothorax (lung collapse).

If you're having an NCS, be sure to let the neurologist know ahead of time if you have a pacemaker or cardiac defibrillator, since you're going to be getting small electric shocks. Precautions may need to be taken in this case.

Before the Test

When your doctor orders EMG and NCS, he or she will talk to you about what these tests are looking for and what's going to happen. This is a good time to ask questions and to inform your doctor about all the medications and supplements you're on, whether they're prescription, over-the-counter, or simply vitamins or herbs.

You should also take a shower or bathe before your test to remove any excess oil from your skin so the surface electrodes can stick well. It's best not to use any lotion, sunscreen, perfume, oil, or cream on the area you're going to have tested once you've taken your pre-test shower; avoiding these for a few days ahead of time is even better.

Timing

EMG can take 30 to 60 minutes. NCS may take from 15 minutes to more than an hour, depending on how many nerves your doctor wants to test. In the likely event that you have both tests, plan for the whole thing to take anywhere from one to three hours.

Location

These tests are performed on an outpatient basis at an office, lab, or clinic, or you may have them while you're already in the hospital. They're conducted in an examination room with a table or bed for you to lie down on, or a reclining chair to sit in so you're relaxed. The EMG machine will be close by.

What to Wear

Whether or not you can keep your clothes on will depend on what part of your body is being tested. Wearing loose clothing that can be pushed aside is helpful, and if you are asked to take your clothes off, you'll be given a gown to wear.

Food and Drink

For most people, fasting is not required before either of these tests. Your doctor will let you know if this isn't the case in your situation.

Ask your doctor if you need to stop taking any of your prescription or over-the-counter medications before the test, especially if you're on Mestinon (pyridostigmine).

Cost and Health Insurance

The cost of these tests varies from state to state and provider to provider. The price also depends on how many areas you're having tested. These tests may cost several hundred dollars up to several thousand dollars.

If you have health insurance, your policy should cover these tests, but you may need pre-authorization before you have them, and you may have to pay a co-pay and/or co-insurance. Call your insurance company for more details or have someone at your doctor's office find out for you.

If you don't have health insurance, consider calling around your area for prices as they can vary vastly from place to place. Ask about possible discounts that are based on income or for paying upfront as well.

What to Bring

Bring along your insurance card in case the neurologist's team needs your information. You may want to bring something to do as well, in case you have to wait for your test.

During the Test

A neurologist, a doctor who specializes in conditions that affect the nervous system, will likely perform most or all of your tests. There may be a technician to help attach the surface electrodes as well.

Pre-Test

You will likely need to fill out a consent form before you have this test.

Depending on which nerves and muscles are being tested, you may need to change out of your clothes into a gown. You will also need to take off jewelry, glasses, hairpins, or any other metal that may interfere with the test too.

The nerve conduction study is typically performed first, so after you're in position on the table or chair and your muscles are relaxed, the neurologist will find the nerve(s) that are going to be tested. A stimulating electrode is then pasted onto the skin overlying the nerve of interest, while a recording electrode is pasted over the muscle that the nerve controls.

Preparation for an EMG will occur after NCS is complete.

Throughout the Test

Once everything is set for the nerve conduction study, a low-level electrical shock is applied through the stimulating electrode, and the speed of the electrical impulse is recorded by the recording electrode. The shock is mild and quick, but will probably be uncomfortable for a few seconds. The procedure is then repeated for each area your doctor wants to test.

Each electrical impulse shows up on a monitor in what looks like waves that the neurologist will be able to interpret.

When the NCS is complete, the electrodes will be removed and any leftover paste will be cleaned off of your skin.

EMG is normally performed immediately following the NCS. In this case, you'll remain where you are while the neurologist finds the muscle(s) he or she wants to test. The area is cleaned with antiseptic and the fine, thin needle electrode is inserted into your muscle. This may feel like a brief, sharp pain or sting, followed by mild discomfort or pressure. Most people get through the test fine, but if you find it to be too uncomfortable or painful, let the neurologist know as this can affect your test results.

The electrode doesn't emit any electrical impulses; it simply records your muscle activity at rest and as you move. The neurologist will give you instructions on what to do to contract (tighten) and rest the muscle at the right times, such as bending or lifting your arm.

The neurologist may then move the needle electrode to another muscle or another part of the muscle and repeat the procedure, but how many times this happens will depend on the areas being tested.

Activity is measured and translated as waves on a screen, and static-like noises are played on a speaker. The neurologist who does the test listens to these sounds and watches the monitor in order to detect abnormalities.

Post-Test

When your test is complete, the electrode will be removed. You should be able to get dressed, if necessary, and go home right away. Your doctor may give you specific instructions, depending on your medical condition.

After the Test

Once your test is finished, you can usually go about your regular activities as normal, unless your doctor instructs you otherwise.

Managing Side Effects

The muscles that were tested may feel sore for a few days, and you may notice some bruising, tingling, or swelling in the area. This should go away within a few days, but you can apply ice or a cold pack for 10 to 20 minutes at a time and take Tylenol (acetaminophen) or Motrin/Advil (ibuprofen) for pain. If you notice increasing pain, swelling, tenderness, or drainage from the any of the areas in which the needles were inserted, let your doctor know.

Interpreting Results

The interpretation of EMG and NCS is not always straightforward and may not always lead to just one possible diagnosis, but the tests can reduce the number of diagnostic possibilities. The neurologist will interpret the tests and send the report to your doctor, which may take a day or two. Since these tests don't have a simple negative or positive result, your doctor will discuss the results with you at a follow-up appointment that you may have scheduled in advance.

If you do see your EMG or NCS report, here are some terms you may encounter:

  • Amplitude: The electrical signal is represented as a wave, and the amplitude is its height.
  • Conduction velocity (CV): The conduction velocity describes the speed at which the electrical impulse travels along the nerve.
  • Duration: This describes the width of an electrical wave.
  • Conduction block: This is the reduction of a signal across an anatomical region, such as the wrist. This suggests nerve entrapment as with carpal tunnel syndrome.
  • F reflex: The F wave is a kind of electrical echo in which the impulse travels up to the spine and then back down along the same fiber. It thereby gives a sense of the conduction along the entire length of a motor nerve.
  • H reflex: The H wave is the electrical equivalent of a reflex in the leg. An impulse travels to the spinal cord via a sensory nerve, then back along a motor nerve.

These measures give information about both motor and sensory components of the peripheral nervous system. They also suggest whether the axon or myelin sheath, which is the exterior layer of nerves that helps electrical impulses travel faster, is more damaged by a neuropathy.

Myelin helps action potentials travel faster, and so in problems of myelin (myelinopathy), conduction velocity is decreased. In problems with the axon (axonopathy), fibers that are intact can conduct signals at normal speeds, but there are fewer fibers, which leads to a weaker signal and decreased amplitude.

Normal EMG: When your muscles are at rest, they normally produce no or very little electrical activity. When a nerve stimulates a muscle to contract, the result is a brief burst of electrical activity called a motor unit action potential (MUP). Normally, the more you contract the muscle, the more electrical activity occurs. This activity shows a smooth action potential in a normal EMG.

Activity at rest: In diseases of peripheral nerves, like carpal tunnel syndrome and peripheral neuropathy, muscles sometimes start having spontaneous activity on their own, which shows that there's an issue with the nerve supply to that muscle. Resting electrical activity can also point to inflammation or a muscle disease. This activity while at rest can be detected by EMG as fibrillations and positive sharp waves on the monitor. Sometimes the abnormality causes visible muscle twitches called fasciculations.

Abnormal activity during contraction: Doctors interpreting EMG results may also mention the term "recruitment pattern." As your muscle is contracted, nerve fibers signal more and more bits of muscle (called motor units) to join in and help. In peripheral nerve diseases, the amplitude of different motor units is strong, but there are fewer of them because the nerve is unable to connect to as many units. In muscle diseases, the number of motor units is normal, but the amplitude is smaller.

The pattern of electrical discharges from the muscle tends to be distinctive for each potential problem, so the pattern that results from your test helps the neurologist tell if your muscles are responding appropriately.

NCS results: Abnormal results on an NCS almost always occur from nerve damage that's due to conduction block, axonopathy (the nerve fiber is damaged), or demyelination (damage to or loss of the outer insulating layer of nerves). There are a number of conditions that can lead to nerve damage and, thus, an abnormal NCS result.

Your test may be normal if the nerves that are affected are small and don't have a myelin sheath, or if you have a disorder that only affects your brain, spinal cord, spinal nerve roots, or muscle. In other words, even though you may have some nerve damage, your NCS result may still be normal.

Follow-Up

Your doctor may need to do additional testing to pinpoint a diagnosis, or one may be determined by your results and a treatment plan started. Next steps will vary greatly, depending on the cause (or potential cause) or your symptoms. Your doctor will let you know how to proceed.

If you had an NCS or EMG to monitor a condition you already have, the test will likely show your doctor how much nerve damage or change in muscle activity there has been since your last test, which can help him or her adjust your treatment plan accordingly, if needed. You may have these tests periodically to check on the progression of your disease.

Other Considerations

If you want a second opinion, talk to your doctor about getting your medical records and who he or she recommends that you see. Getting a second opinion doesn't mean you don't trust your doctor and it may help you feel reassured to hear another medical professional's take on your situation.

A Word From Verywell

Remember that while EMG and NCS are helpful diagnostic tools, they are only one piece of the puzzle. The results of your test(s) may not be able to pinpoint a specific diagnosis, but they can help narrow it down. Be open with your doctor and ask questions about what results he or she expects from these tests and what they might mean for you, and be sure to voice any concerns you have. Together, you can navigate the potentially scary road of a new diagnosis armed with information and reassurance.

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