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23 June 2007

Electromyogram (EMG) and Nerve Conduction Studies

An electromyogram (EMG) measures the electrical impulses of muscles at rest and during contraction. Nerve conduction studies, which measure nerve conduction velocity, determine how well individual nerves can transmit electrical signals. Nerves control the muscles in the body using electrical impulses, and these impulses make the muscles react in specific ways. Nerve and muscle disorders cause the muscles to react in abnormal ways.

Measuring the electrical activity in muscles and nerves can help detect the presence, location, and extent of diseases that can damage muscle tissue (such as muscular dystrophy) or nerves (such as amyotrophic lateral sclerosis). In the case of nerve injury, the actual site of nerve damage can often be located. EMG and nerve conduction studies are usually done together to provide more complete information.

Why It Is Done

An electromyogram (EMG) is done to:

Diagnose diseases that damage muscle tissue, nerves, or the junctions between nerve and muscle (neuromuscular junctions). These disorders include a herniated disc, amyotrophic lateral sclerosis (ALS), or myasthenia gravis (MG).
Evaluate the cause of weakness, paralysis, involuntary muscle twitching, or other symptoms. Problems in a muscle, the nerves supplying a muscle, the spinal cord, or the area of the brain that controls a muscle can all cause these kinds of symptoms.

Nerve conduction studies are done to:

Detect and evaluate 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 diagnose nerve disorders, such as carpal tunnel syndrome or Guillain-Barré syndrome.
Identify the location of abnormal sensations, such as numbness, tingling, or pain.

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.

How To Prepare

Tell your doctor if you:

Are taking any medications. Certain medications that act on the nervous system (such as muscle relaxants and anticholinergics) can interfere with an electromyogram (EMG) results. You may need to stop taking these medications 3 to 6 days prior to having the test.
Have had bleeding problems or are taking medications that thin the blood, such as warfarin (Coumadin) or heparin.
Have a pacemaker.

You do not need to restrict your food or fluids. Do not smoke for at least 3 hours before the test.

Wear loose-fitting clothing that permits access to the muscles and nerves to be tested. You may be given a hospital gown to wear.

For an EMG, you may be asked to sign a consent form. 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 indicate. To help you understand the importance of this test, fill out the medical test information form

How It Is Done

An electromyogram (EMG) is done in a hospital, clinic, or doctor's office. A special room that screens out electrical interference may sometimes be used. The test may be performed by an EMG technologist or a doctor specializing in diseases of the nervous system (neurologist) or in physical rehabilitation (physiatrist).

You will be asked to lie on a table or bed or sit in a reclining chair so that the muscles being tested are relaxed and easy to reach.

Electromyogram

The skin over the areas to be tested is cleaned with an antiseptic solution. An electrode that combines the reference point and a needle for recording is inserted into the specific muscle to be tested and attached by wires to a recording machine.

Once the electrodes are in place, the electrical activity in that muscle is recorded while the muscle is at rest. Then the technologist or doctor asks you to tense (contract) the muscle with gradually increasing force while the electrical activity in the muscle is being recorded.

The needle may be repositioned a number of times to record the electrical activity in different areas of the muscle or in different muscles.

The electrical activity in the muscle is displayed as wavy and spiky lines on a special video monitor (oscilloscope) and may also be heard on a loudspeaker as machine gun-like popping sounds when you contract the muscle. The activity may also be recorded on magnetic tape.

An EMG may take 30 to 60 minutes. When the testing is completed, the needle and skin electrodes are removed and those areas of the skin where a needle was inserted are cleaned. You may be given a pain reliever if any of the areas where a needle was inserted are sore.

Nerve conduction studies

In this test, several flat metal disc electrodes are attached to your skin with tape or a special paste. A shock-emitting electrode is placed directly over the nerve to be studied, and a recording electrode is placed over the muscles supplied by that nerve. Repeated, brief electrical pulses are administered to the nerve, and the time it takes for the muscle to contract in response to the electrical pulse is recorded. The speed of the response is called the conduction velocity.

The corresponding nerves on the other side of the body may be studied for comparison. When the testing is completed, the electrodes are removed.

Nerve conduction studies are usually done before an EMG if both tests are being done. Nerve conduction testing may take from 15 minutes to 1 hour or more, depending upon how many areas of the body are studied.

How It Feels

With an electromyogram (EMG) test, you will feel a brief, sharp pain each time a needle electrode is inserted into the muscle. Some people find this part of the test very uncomfortable. After EMG testing, some soreness and a tingling sensation may persist for 1 to 2 days. If you notice increasing pain, swelling, tenderness, or pus at any of the needle insertion sites, call your doctor.

With the nerve conduction studies, you will feel a brief, burning pain, a tingling sensation, and a twitching of the muscle each time the electrical pulse is applied. It feels similar to the kind of tingling you feel when you rub your feet on the carpet and then touch a metal object. The testing can be quite uncomfortable and makes some people nervous. Keep in mind that only a very low-voltage electrical current is used, and each electrical pulse is very brief (less than a millisecond).

Risks

An electromyogram (EMG) is very safe. You may develop small bruises or swelling at some of the needle insertion sites. The needles are sterilized, so there is very little chance of developing an infection.

There are no risks associated with nerve conduction studies. Nothing is inserted into the skin, so there is no risk of infection. The voltage of electrical pulses is not high enough to cause an injury or permanent damage.

Results

An electromyogram (EMG) measures the electrical impulses of muscles at rest and during contraction. Nerve conduction studies, which measure nerve conduction velocity, determine how well individual nerves can transmit electrical signals. Your doctor may be able to discuss some findings with you immediately after the tests. A full analysis of the results may take a few days.

**Electromyogram (EMG) and nerve conduction studies**
Normal:	The EMG recording should show no electrical activity when the muscle is at rest. There should be smooth, wavelike forms with each muscle contraction.
Normal:	The nerve conduction studies should show that the nerves transmit electrical impulses to the muscles or up the sensory nerves at normal speeds (conduction velocities). Sensory nerves allow the brain to respond to sensations such as pain, touch, temperature, and vibration. Different nerves have different normal conduction velocities. Nerve conduction velocities also tend to decrease as a person gets older.
Abnormal:	In an EMG, spontaneous electrical activity detected in a muscle at rest suggests that there is a problem with the nerve supply to the muscle. This kind of activity can also be caused by inflammation or disease in the muscle tissue. Abnormal levels and duration of electrical discharges when a muscle contracts also suggest the presence of a muscle or nerve disorder, such as amyotrophic lateral sclerosis (ALS), post-polio syndrome, or a herniated disc.
Abnormal:	In nerve conduction studies, the speed of nerve impulse transmission (conduction velocity) may be slower or faster than what is normal for that nerve. Slower conduction velocities may be caused by injury or may damage a nerve (such as carpal tunnel syndrome) or group of nerves (such as Guillain-Barré syndrome or post-polio syndrome).

The results from EMG and nerve conduction studies are usually not enough on their own to diagnose a condition. They can be used along with a person's history, symptoms, physical and neurological examinations, and the results of other tests to help establish a diagnosis and evaluate how a disease is progressing.

What Affects the Test

Factors that can interfere with your test and the accuracy of the results include:

Medications such as muscle relaxants and anticholinergics.
Bleeding, swelling, excess fat beneath the skin, or pain at the site of the nerves or muscles being tested.
The inability to cooperate and follow directions during the test.
Your age. Nerve conduction can vary depending on age and normally decreases as a person grows older.

What To Think About

The levels of some enzymes in the blood, such as aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and creatine phosphokinase (CPK), may rise when muscle tissue is damaged. An electromyogram can sometimes elevate these enzymes for up to 10 days after the EMG, so blood tests for these enzymes should be delayed at least 10 days after an EMG.
Special types of electromyograms (EMG) may also be done:
- Single-fiber EMG is a special type of EMG testing usually done only in universities or in specialized EMG laboratories, but it is becoming more commonly used. For this test, very small needles are inserted into a muscle to evaluate a single muscle fiber's ability to contract. It is a useful test in establishing a diagnosis of myasthenia gravis, a disease that affects the nerve-muscle (neuromuscular) junctions.
- Repetitive nerve stimulation is useful in diagnosing diseases of the nerve-muscle (neuromuscular) junctions such as myasthenia gravis and Lambert-Eaton syndrome. In this test, small, repeated shocks are given to measure the ability of the muscle to react to fast nerve stimulation.
- External sphincter electromyogram measures the electrical activity of the external urinary sphincter to help evaluate urination problems. The urinary sphincter is a ringlike band of muscles around the urethra that helps control urination. The electrical activity can be measured by skin electrodes, by needle electrodes inserted through the skin, or by electrodes in an anal plug. External sphincter electromyogram is a useful test in helping evaluate people with Parkinson-like disorders.

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