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EMG Test for Back Pain
An electromyogram (EMG) is a test that is used to record the electrical
activity of muscles. When
muscles are active, they produce an electrical current. This current is usually
proportional to the level of the muscle activity. An EMG is also referred to as
a myogram.
EMGs can be used to detect abnormal muscle electrical activity that can
occur in many diseases and conditions, including muscular dystrophy,
inflammation of muscles, pinched nerves, peripheral nerve damage (damage to
nerves in the arms and legs), amyotrophic lateral sclerosis (ALS) (also known as
Lou Gehrig disease), myasthenia gravis, disc herniation, and others.
By inserting thin needle electrodes into various muscles, an
electromyographer can map out normal and abnormal signals and determine whether
there has been subtle nerve damage or paralysis.
Results from a new University of Michigan Health System study show that the electromyogram (EMG) test can accurately diagnosis spinal stenosis, reducing
misdiagnosis of low back pain and other common neuromuscular conditions that
have similar symptoms, and even helping to avoid unnecessary back surgery.
�EMG plays an important role in the diagnosis of back pain because, unlike
MRI (magnetic resonance
imaging), EMG is more than a picture of a nerve - it can test nerve function and
show if there is actual nerve damage,� says Haig (Andrew Haig, M.D., associate
professor in the Department of Physical Medicine and Rehabilitation at the U-M
Medical School). �The EMG is really going to help doctors to avoid unnecessary
procedures because it proves that there is nerve damage in the people who
clearly have it and can accurately diagnosis spinal stenosis.�
Although EMG has been around for about 60 years, there have been no
controlled studies of EMG for spinal stenosis. Further, many past studies of
spinal stenosis had only looked at asymptomatic patients, while others failed to
include comparison groups for patients with
back pain or symptoms of spinal stenosis (narrowing of the lumbar (back) or cervical (neck) spinal canal, which
causes compression of the nerve roots).
Dr. Haig believes more doctors are not using EMG to test for back pain
because the results are harder to read and takes more time to diagnose.
The EMG is a test designed to determine how well the
muscles in the different parts of the body are
functioning.
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Types of EMG
There are two types of EMG: intramuscular EMG and surface EMG (SEMG).
Intramuscular EMG (the most commonly used type) involves inserting a needle
electrode through the skin into the muscle whose electrical activity is to be
measured.
Surface EMG (SEMG) involves placing the electrodes on (not into) the skin
overlying the muscle to detect the electrical activity of the muscle.
Intramuscular EMG is the "classic" form of EMG
How is an intramuscular EMG done?
A needle is inserted through the skin into the muscle. The electrical activity
is detected by this needle (which serves as an electrode). The activity is
displayed visually on an oscilloscope and may also be displayed audibly through
a microphone.
Since skeletal muscles are often large, several needle electrodes may need
to be placed at various locations to obtain an informative EMG.
After placement of the electrode(s), the patient may be asked to contract
the muscle (for example, to bend the leg).
The presence, size, and shape of the wave form (the action potential)
produced on the oscilloscope provide information about the ability of the muscle
to respond to nervous stimulation. Each muscle fiber that contracts produces an
action potential. The size of the muscle fiber affects the rate (how frequently
an action potential occurs) and the size (the amplitude) of the action
potential.
The EMG test consists of two parts:
-
Nerve Conduction Study / NCS which evaluates the speed and amount of
electrical activity along a nerve
-
EMG study which looks at electrical activity in muscles at rest and also
when they are voluntarily moved (if possible) to determine if the pattern of
activity is normal
Does it hurt?
Parts of the test are uncomfortable but not severely painful. The anxiety
before the test can be more of a factor than the test itself. A majority of the
time spent with the patient is setting up the studies and running the computer;
a very small amount of the time involves electrical stimulation or use of the
pin electrode.
The electrical stimulation of the nerve conduction study could be compared
to a static electricity shock from a doorknob after walking across carpet.
The pin insertion could be compared to being pinched by someone's
fingernails.
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