How An EMG Is Performed

In a typical study, the patient is laying down, usually on their back. Different circumstances may warrant other positions and the physician does his or her best to adaptable. In each session, several muscles are tested. Nerve injury will create specific patterns of muscle abnormalities, and by testing several muscles, the physician is able to tell where the site of injury is located. EMGs are only useful for testing injury to motor nerves or nerves that go to muscles. It tells nothing of sensory function. Therefore, if the injury has only been to the sensory portion of the nerve, the test will not be helpful. Also, if one thinks of a nerve as a wire with a conductor and an insulator, damage may occur to the wire, the insulator, or both. It is typically thought that more severe injuries include damage to the conductor and these are less able to heal. EMG will only show an abnormality if there is damage to the conductor. Therefore, the EMG can give information about severity of injury as well as prognosis for recovery.

In a typical study, 4 to 8 muscles are tested, and each muscle takes 5 to 10 seconds to test. While the pain of the test is minimized by using a fine gauge needle, it still is a needle, and there is some amount of pain involved. Generally, smaller muscles are more painful and larger muscles less so. The muscle tested may be sore for several hours after, but it is unusual for the soreness to last more than 24 hours.

HOW IS A NERVE CONDUCTION TEST PERFORMED?

A nerve conduction test is performed in a medical office with an EMG/NCS machine. Nerves conduct just like an electrical wire, so the test is performed in a manner similar to one used by an electrician in testing a wire. A probe is placed over a nerve, and an electrical impulse is delivered in gradually increasing intensity until the desired response is obtained on the computer screen. The maximal stimulation is enough to make the muscle jump. While many patients find it "neat" to watch their muscle jump involuntarily, most consider it a "strange" feeling. Only occasionally do patients consider it painful. The probe leaves no permanent marks, and does not cause any bleeding. Sometimes the stimulation will cause tingling that will last for more than a few minutes. Rarely does it last longer than that.

Most nerves are tested in several places, and several characteristics of the nerve are measured and recorded. In a typical study, several nerves are tested, and several aspects of each nerve are tested. For example, the physician typically tests the motor, sensory, and late wave characteristics of the nerve where there is a concern, as well as a control nerve where there is no worry, for a comparison. Sometimes, the physician will test the same nerve on the opposite side, also for a comparison.

 
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