Intramuscular Electromyogram
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
electrical activity of muscle 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), myasthenia gravis, disc herniation, and others.
An EMG is often performed when patients
have unexplained muscle weakness. The EMG helps to distinguish between muscle
conditions in which the problem begins in the muscle and muscle weakness due to
nerve disorders. The EMG can also be used to detect true weakness, as opposed
to weakness from reduced use because of pain or lack of motivation. EMGs can
also be used to isolate the level of nerve irritation or injury.
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 detected audibly with a speaker.
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