Test Overview
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 with
electrical signals called impulses. These impulses make the muscles react in
specific ways. Nerve and muscle problems cause the muscles to react in
abnormal ways.
If you have leg pain or numbness, you may have these tests to find out how much your nerves are being affected. These tests check how well your spinal nerves and the nerves in your arms and legs are working.
Why It Is Done
An
EMG is done to:
- Find diseases that damage muscle tissue, nerves, or the junctions
between nerve and muscle. These problems 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.
A nerve conduction study is 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. This test is often used to
help find nerve problems such as
carpal tunnel syndrome or
Guillain-Barré syndrome.
How To Prepare
Tell your doctor if you:
- Are taking any medicines. Certain medicines that affect the
nervous system can change electromyogram (EMG)
results. You may need to stop taking these medicines 3 to 6 days before the
test.
- Have had bleeding problems or take blood thinners, such as
warfarin or heparin. If you take blood thinners, your doctor will
tell you when to stop taking them before the test.
- 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.
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 mean.
To help
you understand the importance of this test, fill out the
medical test information form(What is a PDF document?).
How It Is Done
An EMG is done in a
hospital, clinic, or doctor's office. It may be done in a room that stops any outside
electrical interference. The test may be done by an EMG
technologist or a doctor.
You will be asked to lie on
a table or bed or sit in a reclining chair so your muscles are relaxed.
Electromyogram
The skin over the areas to be
tested is cleaned. A needle electrode that is attached by
wires to a recording machine is inserted into a muscle.
When 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 video monitor 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 video.
An EMG may
take 30 to 60 minutes. When the test 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.
Nerve conduction studies
In this test, several
flat metal disc electrodes are attached to your skin with tape or a paste. A shock-emitting electrode is placed directly over the nerve, and a
recording electrode is placed over the muscles controlled by 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 done, 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 1 hour or more, depending on how many nerves
and muscles are studied.
How It Feels
During an EMG test, you
may feel a quick, sharp pain when the needle electrode is put into a muscle.
After the test, you may be sore and have a tingling feeling in your muscles
for 1 to 2 hours. If your pain gets worse or you have swelling, tenderness, or
pus at any of the needle sites, call your doctor.
With 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 getting 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.
Results
Your doctor may be able to tell you
about some of the results of your nerve studies right after the tests. A full report may take 2 to 3
days.
Electromyogram (EMG) and nerve conduction studies
|
Normal:
|
The EMG recording shows no electrical activity when the
muscle is at rest. There is a smooth, wavy line on the recording with each
muscle contraction.
|
|
The nerve conduction studies show that the nerves
send electrical impulses to the muscles or along the sensory nerves at
normal speeds, or conduction velocities. Sensory nerves allow the brain to feel
pain, touch, temperature, and vibration. Different nerves have different normal
conduction velocities. Nerve conduction velocities generally get slower as a
person gets older.
|
|
Abnormal:
|
Electrical activity in a muscle at rest shows that there
may be a problem with the nerve supply to the muscle. Abnormal wave lines when
a muscle contracts may mean a muscle or nerve problem, such as
a herniated disc, amyotrophic lateral sclerosis (ALS),
or inflammation.
|
|
In nerve conduction studies, the speed of nerve impulses is slower than what is normal for that nerve. Slower
speeds may be caused by injury to a nerve or group of nerves. Nerve
conduction velocities generally get slower as a person gets older.
|
The results from EMG and nerve conduction studies are used
along with your medical history, symptoms, physical and neurological exams, and
the results of other tests to help your doctor find out what the problem is or
see how a disease is changing.
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful include:
- Taking medicines, such as muscle relaxants and
anticholinergics.
- Having bleeding, swelling, or too much fat under the skin at the
site of the nerves or muscles being tested.
- Not being able to do what is asked during the test.
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 cause
higher levels of these enzymes for up to 10 days after the EMG, so blood tests
for these enzymes should not be done for 5 to 10 days after an EMG.
- Special types of electromyograms (EMG) may also be done. For example:
- Single-fiber EMG is a special type of EMG test. For this
test, very small needles are inserted into a muscle to see how a single muscle
fiber contracts. It is a useful test for
myasthenia gravis, a disease that affects the
nerve-muscle (neuromuscular) junctions.
- Repetitive nerve stimulation is a useful test for myasthenia
gravis and
Lambert-Eaton syndrome. In this test, small, repeated
shocks are given to measure how the muscle reacts to repeated nerve
stimulation.
- External sphincter electromyogram measures the electrical
activity of the external urinary sphincter to help find urination problems. The
urinary sphincter is a ring-shaped 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.
References
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Saunders.
- Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Credits
| By |
Healthwise Staff |
| Primary Medical Reviewer |
Anne C. Poinier, MD - Internal Medicine |
| Specialist Medical Reviewer |
Colin Chalk, MD, CM, FRCPC - Neurology |
| Last Revised |
November 30, 2011 |
