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© Dirk Biddle
An electromyogram (EMG) is a test that measures the electrical activity of a muscle and detects any signs of blocking or slowing down of responses to nerve stimulation. The test provides information about the muscle itself and shows how well it receives stimulation from the nerve. It is used to differentiate between muscle disorders and muscle weakness caused by neurologic disorders.
When a nerve is stimulated with a brief electrical impulse, that feels like a tiny jolt, it creates activity in the adjoining muscle. This activity can be measured. During the test, a thin needle-like electrode is pushed through the patient's skin into the muscle to be examined. It is connected to a screen that shows the electrical activity being measured by the electrode. When the patient contracts the muscle (eg; by bending it) the muscle fibres affected by the movement produce electrical activity that is measured and shown on the screen.
A muscle not in use produces no electrical impulses, and hence no signal will be seen on the screen to begin with. Inserting the electrode will cause electrical activity, that gradually dissipates, after which the screen gradually goes blank. Upon contraction of the muscle, activity will be seen on the screen, increasing as the patient contracts increasing numbers of muscle fibres.
The presence, as well as the form and size of the resultant reading provides information on the muscle's ability to respond to transmitted nerve signals. The test takes approximately half an hour. Insertion of the electrodes may cause some pain. Exactly how much pain is involved depends on the extent of the patient's sensory disturbances. The muscle may also be tender for some days.
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, may be caused by a number of disorders, including muscular dystrophy), 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, (Landouzy-Dejerine), Familial periodic paralysis, Femoral nerve dysfunction, Friedreich's ataxia, Guillain-Barre Syndrome, Lambert-Eaton Syndrome, Mononeuritis multiplex, Mononeuropathy, Peripheral neuropathy, Radial nerve dysfunction, Sciatic nerve dysfunction, Sensorimotor polyneuropathy, Shy-Drager syndrome, Thyrotoxic periodic paralysis, Tibial nerve dysfunction and Ulnar nerve dysfunction (1).
An electromyogram is often carried out in conjunction with the Nerve Conduction Velocity (NCV) test described below.
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1) http://www.nlm.nih.gov/medlineplus/ency/article/003929.htm
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