Eelectromyogram (EMG) and Nerve Conduction Velocity (NCV)

What is an 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 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.

What is a nerve conduction velocity test?
A nerve conduction velocity test (NCV), is an electrical test that is used to detect nerve conditions. In this test, the nerve is electrically stimulated while a second electrode detects the electrical impulse 'down stream' from the first. A nerve conduction velocity test is often done at the same time as an electromyogram (EMG) in order to exclude or detect muscle disorders.

NCVs are used to detect abnormal nerve function that occurs in many different diseases, such as, Diabetes, pinched/damaged nerves, inflammatory and other conditions.

When is a nerve conduction velocity done?
The NCV test can be used to detect true nerve disorders (such as neuropathy) or conditions whereby muscles are affected by nerve injury (such as carpal tunnel syndrome). Normal body temperature must be maintained for the NCV test, because low body temperatures slow nerve conduction.

Why is an EMG test done? SAn EMG is most 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.

How is an 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, the patient may be asked to contract the muscle (for example, to bend the leg).

The presence, size, and shape of the wave form produced on the oscilloscope provide information about the ability of the muscle to respond to nervous stimulation. Each muscle fiber that contracts produces its own characteristic waveform. The size of the muscle fiber affects the rate (how frequently it occurs) and the size (the amplitude) of the waveform.

How is a NCV study done? This is usually done with surface patch electrodes (they are similar to those used for an electrocardiogram or EKG) that are placed on the skin over the nerve at various locations. One electrode stimulates the nerve with a very mild electrical impulse. The resulting electrical activity is recorded by the other electrodes. The distance between electrodes and the time it takes for electrical impulses to travel between electrodes as well as the size and shape of the impulse are used to evaluate for abnormalities.

Does an EMG/NCV hurt? Yes, unfortunately. There is some undeniable discomfort at the time the needle electrodes are inserted. They feel like very small shots (intramuscular injection), although nothing is injected during an EMG. Afterwards, the muscle may feel a little sore for up to a few days. There is no risk of excessive bleeding or infection. The NCV can be uncomfortable as well. Though, generally the electrical stimulations given are no different than that felt from static cling.

How do you prepare for an EMG? It is necessary that the skin surface is clean, dry and free from any oils, creams, lotions, etc. You can eat any meal as you normally would as well as take any medication that you normally would. Please inform your doctor is you are on blood thinners of any sort.

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