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Electromyography :
In electromyography (EMG), a small needle is inserted into a muscle to record the electrical activity of the muscle when the muscle is at rest and when it is contracting. Normally, resting muscle produces no electrical activity. A slight contraction produces some electrical activity, which increases as the contraction increases.
The record produced by EMG is called the electromyogram. It is abnormal if muscle weakness results from a problem with a spinal nerve root, peripheral nerve, muscle, or neuromuscular junction. Each type of problem produces a distinctive pattern of abnormalities, which can be identified based on the person's symptoms and results of the examination and electromyography.
Unlike CT or EEG, which can be done routinely by technicians, EMG requires the expertise of a neurologist, who chooses the appropriate nerves and muscles to test and interprets the results.
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Electromyography
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Nerve conduction studies
Nerve conduction studies measure the speed at which motor or sensory nerves conduct impulses. A small electrical current stimulates an impulse along the nerve being tested. The current may be delivered by several electrodes placed on the surface of the skin or by several needles inserted along the pathway of the nerve. The impulse moves along the nerve, eventually reaching the muscle and causing it to contract. By measuring the time the impulse takes to reach the muscle and the distance from the stimulating electrode or needle to the muscle, doctors can calculate the speed of nerve conduction. The nerve may be stimulated once or several times (to determine how well the neuromuscular junction is functioning).
Results are abnormal only if the symptom results from a problem with a nerve or neuromuscular junction. For example,
If nerve conduction is slow, the cause may be a disorder that affects one nerve, such as carpal tunnel syndrome (painful compression of a nerve in the wrist). Or the cause may be a disorder that affects nerves throughout the body (a polyneuropathy), as when diabetes damages nerves throughout the body, starting with those in the feet.
If the muscle’s response is progressively weaker after repeated stimulation, a problem with the neuromuscular junction (as occurs in myasthenia gravis) may be the cause.
However, results may be normal if the affected nerves are small and do not have a myelin sheath (the outer layer of tissues that helps nerves conduct impulses faster). Results are also normal if the disorder involves only the brain, spinal cord, spinal nerve roots, or the muscle. Such disorders do not affect the speed of nerve conduction
