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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.