Chapter 9

Electrodiagnosis can play a crucial role in identifying the underlying problem in a patient presenting with a pain disorder. Although electrodiagnostic testing is older than some radiologic modalities such as magnetic resonance imaging (MRI), it provides unique functional information about the integrity of both the central and peripheral nervous systems. In addition to localizing the problem, electrodiagnostic testing can give insight into its chronicity and severity, while often also providing helpful prognostic information. Although minor structural abnormalities may be readily identified on MRI, their clinical significance may be uncertain. For example, disk bulges at multiple levels are routinely found on MRI studies of the lumbar spine. Electrodiagnostic testing has the unique ability to determine whether one of these bulges is actually producing nerve damage. Moreover, such testing may also demonstrate abnormalities in patients with inflammatory lesions, where neuroimaging is often normal.

In general, electrodiagnostic testing encompasses nerve conduction studies (NCSs) and electromyography (EMG), which together provide information about the peripheral nerves and muscles, and evoked potentials, which are used predominantly for evaluation of the central nervous system. Other tests, including intraoperative monitoring, autonomic function testing, and analysis of movement disorders, also fall under the rubric of electrodiagnostic testing, but generally have a limited role in the evaluation of pain disorders.

In the first part of this chapter, we review the methodology and interpretation of electrodiagnostic studies in general. As only a limited summary is provided, additional information regarding this complex topic may be obtained from several excellent texts.1–3 In the second part of the chapter, we examine the usefulness of electrophysiologic testing in specific neurologic disorders associated with pain.

### Nerve Conduction Studies

NCSs and EMG are usually performed at the same session because the procedures are complementary, each providing unique information about the peripheral nerves and muscles. Generally, NCSs are performed before EMG, with the results of the NCSs used to guide the needle electrode examination. NCSs provide quantitative information, whereas EMG, as performed in its standard fashion, is more subjective.

Only a few nerves are routinely studied. In the arms, these nerves include the median, radial, and ulnar, and in the legs, posterior tibial, deep peroneal, and sural. The facial and trigeminal nerves can also be studied. All of these nerves are easily accessible to stimulation and are commonly involved in neurogenic illness. A number of additional nerves, including musculocutaneous, superficial peroneal, and saphenous, are studied less often but are sometimes helpful in localizing a lesion.

#### Motor Nerve Conduction Studies

When performing motor studies, an active recording electrode is placed over a muscle belly and a referential recording electrode is placed over the tendon insertion of that muscle. The nerve is then stimulated at a fixed distance from the muscle (Fig. 9-1). The electrical response represents the depolarization of the muscle beneath the active electrode relative to the referential electrode ...

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