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EMG / NCV Testing

EMG

Definition:

Electromyography is a test that assesses the health of the muscles and the nerves controlling the muscles.

How is the test performed?

For an EMG, a needle electrode is inserted through the skin into the muscle. The electrical activity detected by this electrode is displayed on an oscilloscope and may be heard through a speaker. After placement of the electrodes, you may be asked to contract the muscle (for example, by bending your arm). 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 when the nerves are stimulated. A Nerve conduction velocity (NCV-a test of the speed of conduction of impulses through a nerve.) test is usually performed in conjunction with an EMG.

How to prepare for the test:

No special preparation is usually necessary. Avoid using any creams or lotions on the day of the test.

How will it feel?

You may feel some pain or discomfort when the electrodes are inserted, but most people are able to complete the test without much difficulty. Afterward, the muscle may feel tender or bruised for a few days.

Why is the test performed?

EMG is most often used when people have symptoms of weakness and examination shows impaired muscle strength. It can help to differentiate primary muscle conditions from muscle weakness caused by neurologic disorders.

Normal results or values:

Muscle tissue is normally electrically silent at rest. Once the insertion activity (caused by the trauma of needle insertion) quiets down, there should be no action potential on the oscilloscope. When the muscle is voluntarily contracted, action potentials begin to appear. As contraction is increased, more and more muscle fibers produce action potentials until a disorderly group of action potentials of varying rates and amplitudes (complete recruitment and interference pattern) appears with full contraction.

Abnormal results:

Disorders or conditions that cause abnormal results include the following:

Polymyositis
Denervation (reduced nervous stimulation)
Carpal Tunnel Syndrome
Amyotrophic Lateral Sclerosis (ALS)
Myopathy (muscle degeneration)
Myasthenia gravis
Alcoholic neuropathy
Axillary nerve dysfunction
Becker’s muscular dystrophy
Brachial plexopathy
Cervical spondylosis
Common peroneal nerve dysfunction
Dermatomyositis
Distal median nerve dysfunction
Duchenne muscular dystrophy
Facioscapulohumeral muscular dystrophy
Familial periodic paralysis
Femoral nerve dysfunction
Friedreich’s ataxia
Guillain-Barre
Lambert-Eaton Syndrome
Mononeuritis multiplex
Mononeuropathy
Peripheral neuropathy
Radial nerve dysfunction
Sensorimotor polyneuropathy
Shy-Drager syndrome
Thyrotoxic periodic paralysis
Tibial nerve dysfunction
Ulnar nerve dysfunction

What are the risks?

Bleeding (minimal)
Infection at the electrode sites (minimal risk)

NCV

NCV– This is the test that is performed most often.

Nerve conduction velocity (NCV) is a test of the speed of conduction of impulses through a nerve

How is the test performed?

The nerve is stimulated, usually with surface electrodes, which are patch-like electrodes (similar to those used for ECG) 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 are used to calculate the nerve conduction velocity.

How to prepare for the test:

Normal body temperature must be maintained (low body temperature slows nerve conduction).

How will the test feel?

The impulse may feel like an electric shock. Depending on how strong the stimulus is, you will feel it to varying degrees, and it may be uncomfortable you. You should feel no pain once the test is finished. Often the nerve conduction test is followed by electromyography (EMG) which involves needles being placed into the muscle and you contracting that muscle. This can be uncomfortable during the test, and you may feel muscle soreness at the site of the needles afterwards as well.

Why is the test performed?

This test is used to diagnose nerve damage or destruction.

Normal Results or Values:

NCV is related to the diameter of the nerve and the normal degree of myelination (the presence of a myelin sheath on the axon) of the nerve. Newborn infants have values that are approximately half that of adults, and adult values are normally reached by age 3 – 4.