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Electrodiagnostic study

General information

Where It's Done Who Does It How Long It Takes Discomfort/Pain
Hospital outpatient department. Doctor or technician. Less than 1 hour for both tests. Discomfort at site of needle insertion; possible anxiety over insertion of needle electrodes.
Results Ready When Special Equipment Risks/Complications Average Cost
Within 24 hours. Needle and surface electrodes, nerve stimulator, amplifier with filters, oscilloscope, and device to store data, such as a magnetic tape recorder. Very rare possibility of short-lived bacterial infection. $$

Other names

Electrodiagnostic study (needle exam) or latency studies.

 

Purpose

  • To determine the severity and exact site of nerve entrapment (in such disorders as carpal tunnel syndrome or a herniated disk).
  • To confirm diagnosis and measure the severity of peripheral nervous system disorders, such as polyneuropathies.
  • To diagnose or evaluate disorders of the muscles and motor neurons, such as amyotrophic lateral sclerosis and myasthenia gravis.

How it works

As they contract, the muscles give off a weak electrical signal that can be detected, amplified, and tracked, giving information about how well they are working. Electrical signals traveling along nerves can be measured. By measures at two points, the velocity of conduction can be measured.
Preparation
  • Avoid taking aspirin and other nonsteroidal anti-inflammatory drugs for five to seven days before the test.
  • If you take Mestinon for myasthenia gravis, stop taking it 24 hours before the test under the discretion of your physician.
  • Wear loose-fitting clothes that will allow you to expose the necessary muscles and nerves during the test. You may have to disrobe and wear a hospital gown if your hip or shoulder muscles are being tested.
  • Do not use hand cream or skin lotion before the test (so that the electrodes adhere properly).

Test procedure

You sit or lie on the examination table and expose the muscles and nerves that need to be tested. One or both of the following studies are performed.
  • The appropriate area is cleaned with alcohol, and the needle electrode (a very thin, solid needle that is similar to a pin) is inserted into the muscle.
  • The electrical activity of the muscle during relaxation, slight contraction, and forceful contraction is picked up via the electrodes, amplified, recorded on the oscilloscope, and converted to auditory signals via a speaker.
  • Several muscles or areas of muscle may be tested one by one in this manner.
  • Recording and stimulating electrodes are placed on the skin overlying a nerve supplying a muscle or muscle group.
  • A mild and brief electrical stimulus is delivered to the stimulating electrodes.
  • The response of the muscle is picked up by the recording electrodes, amplified, and displayed. The speed with which the signal generated by the muscle travels through the nerves, called nerve conduction velocity, is measured. The amplitude (strength) of the signal is also measured.
  • The maneuver is repeated on different nerves.

After the test

You are free to return to previous activities, although you should try not to strenuously exert yourself for the rest of the day. If you feel any pain, it will be a mild muscle ache, no stronger than if you had bumped yourself.
Factors affecting results
  • The position of the electrodes.
  • Muscle-relaxing and anticholinergic medications.
  • Skin temperature.

Interpretation

  • Electromyography. Normally, there is no electrical activity in the muscle when it is relaxed, only when it contracts. If the muscle is diseased, it may have electrical activity in the relaxed state; when it contracts, its electrical activity may produce abnormal patterns. By examining these abnormalities, the doctor may determine the nature of the disease and identify the nerves and muscles affected.
  • Nerve conduction studies. Nerve conduction velocity reflects the speed with which electrical impulses travel along the nerve. Various diseases can cause the impulses to slow down, or to be slower on one side of the body than on the other. The magnitude of the response to stimulation also gives clues to diagnosis and the extent of the injury.

Advantages

  • It's only mildly invasive.
  • It can help quantitate subjective symptoms.

Disadvantages

It usually provides no definitive diagnosis although it adds clues to the physical examination.
The next step
  • Treatment.
  • There may be additional laboratory testing to look for the underlying cause of a neuropathy (e.g., diabetes or hypothyroidism).

Patient Tip

If you take an anticoagulant (blood-thinning) medication, be sure to tell your neurologist. If this is the case, it may not be necessary to have the electromyography part of the exam.
 
 
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