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Electromyogram (EMG)

Definition:

Electromyography is a test that assesses the state 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 a display screen, 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 screen provide information about the ability of the muscle to respond when the nerves are stimulated. A Nerve conduction study (NCS) 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. You may have to remove some jewellary or metals from the site of examination.

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. Afterwards, occasionally the muscle may feel tender or bruised for a few hours or 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:

Denervation such as is seen in cervical or lumbosacral radiculopaties.This examination helps us to identify the severity and exact localization of the nerve root involvement
Polymyositis
Carpal Tunnel Syndrome
Thoracic neurogenic outlet syndrome
Lumbosacral Plexopathy
Amyotrophic Lateral Sclerosis (ALS)
Myopathy (muscle degeneration)
Myasthenia gravis
Muscular dystrophies
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 Syndrome
Lambert-Eaton Syndrome
Monoeuritis multiplex
Monoeuropathy
Peripheral neuropathy
Radial nerve dysfunction
Sensorimotor polyneuropathy
Shy-Drager syndrome
Thyrotoxic periodic paralysis
Tibial nerve dysfunction
Ulner nerve dysfunction