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.
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
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 ...