Electrodiagnosis (EDx) can play a crucial role in identifying the underlying problem in a patient presenting with a pain disorder. In contrast with radiologic modalities such as magnetic resonance imaging (MRI), EDx provides unique functional information about the integrity of both the central and peripheral nervous systems. In addition to localizing the problem, EDx can give insight into chronicity, severity, and prognosis. For example, disc abnormalities of uncertain clinical significance are routinely found at multiple levels on MRI studies of the lumbar spine. EDx has the unique ability to determine whether one of these disc abnormalities is actually producing nerve damage. Moreover, such testing may also demonstrate abnormalities in patients with inflammatory lesions, where neuroimaging is often normal.
In general, EDx 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. Emerging techniques are briefly mentioned, including electrical impedance myography and neuromuscular ultrasound. Other EDx tests—including intraoperative monitoring, autonomic function testing, repetitive nerve stimulation, and analysis of movement disorders—generally have a limited role in the evaluation of pain disorders and are not included in detail here.
In the first part of this chapter, we review the methodology and interpretation of EDx. In the second part, we examine the electrophysiologic testing in specific disorders associated with pain. Additional information regarding this complex topic may be obtained from several excellent texts.1-4
ELECTRODIAGNOSIS: METHODS AND INTERPRETATION
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 NCS used to guide the needle electrode examination. NCSs provide quantitative information, whereas EMG, as performed in its standard fashion, is more subjective.
NCSs assess large myelinated peripheral nerves, and can provide information about the function of myelination, number of axons, function of motor units, and function of the neuromuscular junction.5 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.
Orthodromic studies record impulses propagated in the same direction as physiologic conduction, while antidromic studies record impulses propagated in the opposite direction of physiologic conduction. Stimulation is achieved by depolarizing a nerve under a negatively charged cathode. The propagated wave is recorded with a standard differential technique via two electrodes: an “active” and a “reference.”