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An electromyography (EMG) tests the intrinsic electrical activity of muscles. An EMG machine records the electrical response and measures the speed and amplitude of the pulse through the nerve. No electrical shocks are involved.

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.