Electrodiagnostic Testing
Technique helps practitioners differentiate between nerve
and muscle diseases

By Regina Silver, RN, BA, and Julie Silver, MD
September 6, 2004

Marcy* finally gives in. She can no longer ignore the pain in her hand and numbness in her thumb and index and long finger. On the first visit, the physician performs a careful history and physical exam. Suspecting carpal tunnel syndrome, he orders an EMG.

For nurses, this brings to mind electrodes, waveforms, needles, and sore muscles. But what, exactly, is an EMG, and how does it help with diagnosis?

Electrodiagnostic testing is a way for practitioners to differentiate many nerve and muscle diseases. Although people often refer to any electrodiagnostic testing as an EMG, in fact there are a number of different tests — including two of the more common, electromyography (EMG) and nerve conduction studies (NCS).

Figure 1. A nerve conduction study (NCS) uses electrical shocks to test how fast
and how well nerves conduct signals that control muscles.
Image adapted from Essentials of Clinical Neurophysiology, Butterworth-Heinemann, 1997.

The value of these studies is that they provide a real-time video stream of the physiological events of the nerve and muscle tissue, rather than the still-life snapshot obtained by imaging studies, such as an X-ray or MRI. Used together, they aid in the diagnosis of many neuromuscular conditions, such as amyotrophic lateral sclerosis (ALS, or Lou Gehrig’s disease), myasthenia gravis, muscular dystrophy, and other myopathies.

Electrodiagnostic testing might be considered if a patient presents with any of the following symptoms —

• Numbness
• Complaints of tingling (paresthesias)
• Pain
• Weakness
• A limp
• Muscle atrophy
• Decreased responsiveness of deep tendon reflexes
• Fatigue

Of course, the patient’s medical history and a physical examination are crucial in determining whether electrodiagnostic testing would be useful. For example, if Marcy reported that she had accidentally closed her hand in a door, then soft tissue trauma would lead the list of differential diagnoses.

If, however, she gave a history of neck pain that radiated down her arm and into her hand, the preliminary diagnosis would likely be cervical radiculopathy caused by a bulging disc putting pressure on one of the cervical nerves. A more serious scenario would be if Marcy reported new and progressive weakness in all of her limbs, causing one to consider the diagnosis of ALS.

Not all EMGs are the same

Although an EMG and NCS are usually performed together, they each offer separate information. The major difference between them is an NCS tests how fast and how well nerves conduct signals that control muscles, while an EMG tests the intrinsic electrical activity of the muscles.

An NCS involves electrical shocks. A physician or technician places electrodes on the patient’s skin and then uses a stimulator to send electrical impulses to the nerves.¹ The EMG machine records the electrical response and measures the speed and amplitude of the pulse through the nerve. The measurement is called the sensory nerve action potential (SNAP) when performed on a sensory nerve. For a motor nerve, the term is compound motor action potential (CMAP). The illustration in Figure 1 shows the setup for a nerve conduction study and the typical waveform of a SNAP.

Signals travel more slowly through a nerve that is diseased or injured. If Marcy has carpal tunnel syndrome, the SNAP measurement would be slower than normal when the median nerve is tested at her wrist.²

An EMG test does not involve electrical shocks. Instead, thin needles are inserted into the muscles being studied. The needles record what is happening to the muscle, indicate whether it is healthy or diseased, and pinpoint the cause of disease.

Figure 2. EMGs of muscles with normal nerve activity (top) vs. muscles
with nerve-supply problems, which have sharper waves.
Image courtesy of Easy EMG, Elsevier Inc., 2003.

The EMG test provides information about the health of the muscles both visually and by audio. The waveforms on EMGs have a particular look and sound, and whether they are normal or abnormal depends on the characteristics of both.

Electrodiagnostic testing helps establish the diagnosis of peripheral nerve injuries, as well as muscular disorders, but it does even more.

These tests also help determine the need for surgery and aid in predicting a patient’s prognosis. For example, Marcy has mild slowing of the sensory nerve across the carpal tunnel, but there is no evidence of motor nerve slowing. In addition, the EMG doesn’t show abnormalities in muscles supplied by the median nerve. Given this set of circumstances, she might be treated for a mild carpal tunnel syndrome with a hand splint and advised to reduce activities that exacerbate the pain (e.g., typing).

If, on the other hand, the tests showed more severe abnormalities, she would be a candidate for surgery.

For example, muscles with a nerve-supply problem often show increased insertional activity and spontaneous activity in the form of fibrillation potentials and sharp waves that are indicative of a problem. This abnormal waveform is shown in Figure 2.

Shock Value

The term EMG might bring to mind images of electrodes, needles, and sore muscles. But only some electrodiagnostic tests actually involve electric shocks. These tests measure the physicological events of nerve and muscle tissue and provide a real-time video stream, which aid in the diagnosis of many neuromuscular conditions.

Prepare the patient

Apprehension about possible pain and an unfavorable outcome describes the mind-set of most patients preparing to undergo any test. Denise Henderson, the rehab clinic nurse at Nassau University Medical Center in East Meadow, N.Y., reports that almost every patient has some anxiety about electrodiagnostic testing. Patients need to be told there will be some degree of discomfort during the test.

“A lot of the patients that are coming for EMGs have already anticipated the pain that will come from it,” Henderson says.

She suggests having patients come in early to the exam so there is plenty of time for nurses to discuss the procedure with patients and address any fears. Henderson explains to patients that the nerve conduction portion of the study is usually done first and involves electrical impulses that vary in intensity.

The impulses are quick, and the patient can request to stop at any time or to have the intensity decreased. It’s important to tell patients, however, that decreasing the intensity might inhibit obtaining critical information needed for a proper diagnosis.

For the EMG portion of the test, Henderson assures the patient that no shocks are involved. As with the NCS, the patient can request the tester to stop at any point or decline to have specific muscles or parts of the body tested if it is too painful.

This, too, can affect whether an accurate diagnosis is achieved.

Contraindications to testing may include anticoagulation therapy, a bleeding disorder, or extensive skin infections. Children and agitated adults can have a mild sedative, but overly sedating people will make it impossible to complete the EMG portion of the test, for which they need to clench their muscles.

Some patients can tolerate only one of the two tests. While this is not ideal, having some information is better than having none, Henderson says.

In the book Easy EMG, coauthor Lyn Weiss, MD, a professor at the State University of New York at Stony Brook School of Medicine, suggests telling patients the study will be stopped at their request: “The patient is always in control of what happens, and the test can be stopped at any time if the patient feels it is too uncomfortable.”³

Henderson agrees that putting patients at ease is important. To alleviate their concerns, she spends a great deal of time explaining why the test is being done, how it works, and what they can expect to feel during the exam. “Most of the time,” she says, “the patients leave feeling a lot better and [say] the test wasn’t as bad as they had anticipated.”

For more information, visit the American Association of Electrodiagnostic Medicine’s website at www.aaem.net.

Regina Silver, RN, BA, is clinical nurse manager, progressive care unit, at Beverly Hospital, Beverly, Mass. Julie Silver, MD, is an assistant professor at Harvard Medical School and a physiatrist at Spaulding Rehabilitation Hospital, both located in Boston. Silver is also one of the authors of the new book Easy EMG, published by Butterworth-Heinemann, a subsidiary of Elsevier Inc.

References

1. Misulis KE. Essentials of Clinical Neurophysiology. Newton, Mass.: Butterworth-Heinemann; 1997.

2. Brown WF, Bolton CF. Clinical Electromyography. Newton, Mass.: Butterworth-Heinemann; 1993.

3. Weiss L, Silver JK, Weiss J. Easy EMG. Oxford, United Kingdom: Butterworth-Heinemann; 2004.