Electrodiagnostic Study (EMG/NCS) Definition

An electrodiagnostic study measures the electrical activity in nerves and muscles. This testing may be ordered to help diagnose the cause of neck or back pain, particularly if the pain is accompanied by tingling, numbness, and/or weakness.

How Electrodiagnostic Testing Works

An electrodiagnostic study typically consists of two test types:

  1. Nerve conduction studies (NCS). This test, also called nerve conduction velocity (NCV), measures the speed in which the nerves are conducting electrical signals. Electrodes are attached to the skin’s surface, such as in the arm, then small electrical currents are applied (which may feel uncomfortable but should not cause pain). The electrodes are moved to different positions on the skin and NCS tests are repeated.
  2. Electromyography (EMG). This test, also called needle EMG, measures the electrical activity in muscles during movements. A needle electrode is inserted into the muscle being tested, and the patient is asked to move the muscle at certain times so the electrical activity can be recorded by an electromyograph machine. The needle may be moved to different areas depending on which muscle is being tested.

An abnormality found in an NCS indicates nerve dysfunction, whereas an abnormality found in an EMG may indicate muscle dysfunction, which could also be related to an underlying nerve problem. The results of EMG/NCS testing are considered within the broader examination of the patient, including the medical history, physical exam, and imaging. EMG/NCS testing alone cannot lead to a diagnosis.

When an Electrodiagnostic Study Is Considered

An electrodiagnostic study may be considered to help diagnose the cause of neck or back pain that has lasted more than a few weeks, or to assess the extent of nerve damage. For example, an electrodiagnostic study may provide more information about which nerve(s) are/or muscle(s) are behaving abnormally and to what extent. It is possible for two nerve roots to appear compressed on an MRI, but an electrodiagnostic study might show only one of them to have significant deficits in function.

An electrodiagnostic study is typically considered safe when performed by a trained professional. With any needle insertion, there is some risk for infection or other complications.