Nerve Conduction Tests (EMG)

One of the first things your doctor may do when neuropathy is suspected, is an EMG (Electromyography) test to try to establish the extent of nerve damage. It has to be said here, that in many cases, the EMG is not very conclusive but that doesn't mean that you don't have neuropathy. Small fibre neuropathy for instance doesn't always show up on EMGs; skin biopsies can reveal more in that respect but aren't that frequently carried out.

The doctor will use the EMG as one form of evaluation but just as important are the patient's own symptoms and story. Neuropathy symptoms are usually pretty distinctive and are difficult to confuse with other medical problems, so just because they don't show up on an EMG doesn't mean that they don't exist. Don't be too easily persuaded that your problems are psychosomatic - if you have neuropathy your symptoms will speak for themselves.

The only exception to this is in the beginning stages when you may only be experiencing numbness, or tingling in one small area but then generally, at that stage you won't be given an EMG because the doctor will prefer to see how things develop and may investigate other potential causes.

Today's post from MD Guidelines (see link below) explains how an EMG works and what it looks for.

Nerve Conduction Studies

Definition

Nerve conduction studies measure electrical conduction along peripheral nerves, which are bundles of thread-like fibers that connect the central nervous system (brain and spinal cord) to the rest of the body. Stimulation of a nerve generates an electrical impulse (current), which travels along the nerve fibers, similar to a pulse of current traveling along a telephone cable. Sensory nerves carry incoming messages from sensory receptors to the spinal cord and brain. Motor nerves carry outgoing messages from the brain and spinal cord to the muscles.

Results from these studies provide information about abnormalities in the nerves supplying a particular muscle, or in the area where the nerve connects to the spinal cord. A nerve conduction test reflects the status of the "best" surviving nerve fibers (fastest axons), and may remain normal if even a few fibers are affected by a disease process; thus, a normal test result can occur despite extensive nerve damage. However, the test is useful in revealing abnormalities limited to a particular nerve (focal nerve injury or entrapment), to a nerve root where it connects to the spinal cord (radiculopathy), or generalized to multiple nerves (neuropathy). The age of the individual being tested may be taken into consideration because nerve conduction velocities very slowly begin to decrease after the second to fourth decades. Results may also be affected by temperature, as electrical amplitude increases by 1.7% per degree Celsius. Overall, nerve impulses travel faster in the proximal nerve segments.

The American Board of Electrodiagnostics provides specialty training and certification for physicians who perform electrodiagnostic tests such as nerve conduction studies, electromyography, and evoked potential tests.

Reason for Procedure

Nerve conduction studies are used to diagnose nerve damage or destruction, whether generalized as in alcoholic, diabetic, or other peripheral neuropathy; or localized as in compression to or stretch injury to nerve roots (radiculopathy). Nerve compression problems, such as carpal tunnel syndrome (compression of the median nerve in the wrist) or plexus syndrome (compression of the nerves coming from the neck into the arm) may also lead to abnormal test results. Nerve conduction studies evaluate the functioning of the peripheral nerves in order to determine sensory or motor dysfunction. For example, compression of the median nerve may occur near either the wrist (as in carpal tunnel syndrome) or the elbow. In nerve compression syndromes, sensory conduction is usually affected first. Motor conduction may not be affected until the disorder has progressed to a more advanced stage.

Nerve conduction studies alone cannot diagnose the cause of dysfunction, although they allow for the detection of peripheral nerve injury or disease. Nerve conduction velocity testing is often done in conjunction with electromyography (EMG), which records the electrical activity of the muscles. EMG is more helpful in diagnosing muscle disease (myopathy) as well as diseases affecting the nerves supplying the muscles (neuromuscular conditions), such as muscular dystrophy, amyotrophic lateral sclerosis (Lou Gehrig's disease), myasthenia gravis, and other disorders. EMG can help determine if the nerve injury is complete or incomplete.

How Procedure is Performed

Nerve conduction studies and electromyography (EMG) are usually performed at least 4 weeks after a suspected nerve injury has occurred, as it can take up to 4 to 6 weeks for evidence of denervation to become apparent.

There is no pre-test preparation necessary, although the individual being tested may be asked to refrain from using cigarettes, caffeine, sedatives, painkillers, or other neurologic or psychiatric medications on the day of the test.

In nerve conduction studies, electrodes (thin metal discs) are placed on the skin and a mild electrical impulse, initiated at one location, travels along the nerve and is recorded by another electrode at a different location. A device called an oscilloscope is used to measure the time it takes from initiation of nerve stimulation to the nerve response. The distance between electrodes and the time it takes for electrical impulses to travel between them is used to calculate nerve conduction velocity.

The height (amplitude) of the impulse may be diminished, reflecting a decreased number of functioning nerve fibers. In addition to measuring the amplitude, the oscilloscope can also reveal the shape of the impulse, which may have diagnostic value as well. Both sensory and motor conduction can be measured, but not all nerve disorders result in slowed conduction. In nerve compression syndromes, conduction velocity is abnormally slow where the nerve is being compressed. By measuring conduction velocities over different segments of the nerve, it is possible to determine the site of compression. The measured velocity is compared to a normal range of values for that nerve segment. It may also be helpful to compare the affected and unaffected sides. The structure of the nerve fiber is usually undamaged with peripheral neuropathy, but the nerve impulse may be blocked along the nerve pathway (conduction block) with autoimmune neuropathies such as Guillain-Barré syndrome.

In EMG, the skin is cleaned with an antiseptic solution and two electrodes are placed in the test area. The reference electrode is a flat metal disc placed on the skin, and the recording electrode is a thin sterile needle that is inserted into the muscle to be tested. Resting muscle activity is noted and then compared to muscle activity that is recorded while the individual attempts to contract the muscle with increasing force. It is abnormal for the muscle to produce significant spontaneous electrical activity at rest.

Prognosis

After the nerve conduction study is performed, a neurologist or other physician interprets the test results, giving information about diagnostic possibilities, or the need for further tests such as evoked potential studies or electromyography. Although nerve conduction velocity varies for different nerves and between individuals of different sex, age, and height, an abnormally slow conduction can help in the diagnosis of nerve injury or disease. Certain test results, such as those suggesting nerve entrapment as in carpal tunnel syndrome, may lead to consideration of surgery or other specific treatment. The outcome depends upon any underlying conditions, not on the nerve conduction test itself.

Complications

There are no side effects from nerve conduction studies, which use external electrodes (thin metal discs), except for the possible complication of a skin sensitivity to the paste used to attach the electrodes to the skin. If the nerve conduction study includes electromyography, in which a needle electrode pierces the skin in order to stimulate a muscle cell, then there can be complications of pain, skin infection, bruising or bleeding. Individuals with compromised immune system or blood clotting ability may be more likely to suffer complications related to infection or bleeding. However, disability is affected by any underlying condition, rather than by the test itself.

http://www.mdguidelines.com/nerve-conduction-studies