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More Diagnostic Imaging & EMG
EMG
An electromyogram (EMG) measures the electrical activity of muscles at rest and during contraction. Nerve conduction studies measure how well and how fast the nerves can send electrical signals. Nerves control the muscles in the body by electrical signals (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 find diseases that damage muscle tissue (such as Muscular Dystrophy) or nerves (such as Amyotrophic Lateral Sclerosis or Peripheral Neuropathies). EMG and nerve conduction studies are often done together to give more complete information.
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 and a recording electrode is placed over the muscles under control of that nerve. Several quick electrical pulses are given 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 same nerves on the other side of the body may be studied for comparison. When the test is completed, the electrodes are removed.
Nerve conduction studies are done before an EMG if both tests are being done. Nerve conduction tests may take from 15 minutes to one hour or more, depending on how many nerves and muscles are studied.
Why It Is Done
An EMG is done to:
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).
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-Barré 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.
How It Is Done
You will be asked to lie on a table or bed or sit in a reclining chair so your muscles are relaxed.
The skin over the areas to be tested is cleaned with a special soap. A needle electrode that is attached by wires to a recording machine is inserted into a specific muscle.
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 tighten (contract) the muscle slowly and steadily. This electrical activity is recorded.
The electrode may be moved a number of times to record the activity in different areas of the muscle or in different muscles.
The electrical activity in the muscle is shown as wavy and spiky lines on a special video monitor (oscilloscope) and may also be heard on a loudspeaker as popping sounds as you contract the muscle. The activity may also be recorded on video.
An EMG may take 30 to 60 minutes. When the testing is done, the electrodes are removed and those areas of the skin where a needle was inserted are cleaned. You may be given pain medicine if any of the test areas are sore.
How It Feels
With an EMG test, you may feel a quick, sharp pain when the needle electrode is put into a muscle. After an EMG test, you may be sore and have a tingling feeling in your muscles for one to two days. If your pain gets worse or you have swelling, tenderness or pus at any of the needle sites, call your doctor.
With the nerve conduction studies, you may feel a quick, burning pain, a tingling feeling, and a twitching of the muscle each time the electrical pulse is given. It feels like the kind of tingling you feel when you rub your feet on the carpet and then touch a metal object. The tests make some people anxious. Keep in mind that only a very low-voltage electrical current is used, and each electrical pulse is very quick (less than a split-second).
Risks
An EMG is very safe. You may get some small bruises or swelling at some of the needle sites. The needles are sterile, so there is very little 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.