What is the clinical role of evoked potentials?

Updated: Oct 25, 2019
  • Author: Andrew B Evans, MD; Chief Editor: Selim R Benbadis, MD  more...
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During the last decade, the rapid advancement in magnetic resonance imaging (MRI) technology has diminished the utilization of electrophysiologic testing. To an extent, this is justified by the high yield and generally good correlation of MRI findings with the underlying pathology. In addition, MRI is often better able to visualize the pathologic anatomy underlying a disease state.

Nevertheless, there are some situations in which either the disturbance is not readily visible by an imaging modality or MRI is neither feasible nor cost-effective. For such eventualities, evoked potential (EP) studies—including visual evoked potential (VEP), brainstem auditory evoked potential (BAEP), and somatosensory evoked potential (SEP)—are uniquely suited. Their current clinical role may be briefly summarized as follows.

In a wide variety of primary and secondary visual system diseases, VEP testing provides a sensitive extension of the clinical examination. MRI is a highly accurate localizing modality, whereas VEP is useful primarily in assessing optic nerve function in the anterior (prechiasmatic) portion (it is lateralizing but not localizing to the lesion). In patients with insidious onset and appropriate MRI findings for multiple sclerosis (MS), VEPs are recommended to help confirm the diagnosis.

BAEP is useful in acoustic neuroma, but in the past few years its yield in small lesions has been surpassed by that of MRI, and in most cases MRI is clearly superior. However, MRI may be inapplicable in some patients (eg, the increasingly large percentage of elderly persons who are equipped with pacemakers), whereas BAEP studies can be done in patients with a variety of implanted devices. BAEP provides good anatomic definition.

SEP, though limited with respect to spatial localization, is a good functional tool; its primary use is to determine compromised central nervous system (CNS) conduction. It may help confirm symptoms when few physical findings are noted. It may also reveal asymptomatic lesions, thereby facilitating the workup of suspected MS. SEP testing and MRI may serve as complementary modalities. SEP is of limited value in spinal disorders. Degenerative disk disease, spinal stenosis, and compressive lesions show poor physioanatomic correlation.

SEP may confirm or reject the presence of a suspected conduction block. BAEPs and short-latency SEPs (SLSEPs) may be able to establish an anatomic region where the conduction disturbance or block occurs. They provide a sensitive tool for assessment of brainstem auditory and somatosensory tracts and nearby structures. Abnormalities demonstrated by these tests are etiologically nonspecific and must be integrated carefully into the clinical situation by a physician familiar with the clinical uses and limitations of these tests.

In patients who have experienced a cardiac arrest, MRI of the brain frequently is unremarkable for pathology and therefore is less helpful in predicting neurologic outcomes from the injury. In these situations, SEP studies may be useful for helping predict negative outcomes.

With advances in technology and increases in the frequency with which certain implants and medical devices are used in patients, the number of people who are unable to undergo MRI scanning is growing. For these patients, EP studies may be a suitable avenue for diagnosis. Finally, intraoperative EPs may provide timely information in the operating room that has the potential to improve neurosurgical or neurovascular surgical outcomes.

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