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Intraoperative Neuromonitoring and Alarm Criteria for Judging MEP Responses to Transcranial Electric Stimulation: The Threshold-Level Method

Calancie, Blair

Journal of Clinical Neurophysiology: January 2017 - Volume 34 - Issue 1 - p 12–21
doi: 10.1097/WNP.0000000000000339
Invited Review

Summary: The motor evoked potential (MEP) is used in the operating room to gauge—and ultimately protect—the functional integrity of the corticospinal tract (CST). However, there is no consensus as to how to best interpret the MEP for maximizing its sensitivity and specificity to CST compromise. The most common way is to use criteria associated with response magnitude (response amplitude; waveform complexity, etc.). With this approach, should an MEP in response to a fixed stimulus intensity diminish below some predetermined cutoff, suggesting CST dysfunction, then the surgical team is warned. An alternative approach is to examine the minimum stimulus energy—the threshold—needed to elicit a minimal response from a given target muscle. Threshold increases could then be used as an alternative basis for evaluating CST functional integrity. As the original proponent of this Threshold-Level alarm criteria for MEP monitoring during surgery, I have been asked to summarize the basis for this method. In so doing, I have included justification for what might seem to be arbitrary recommendations. Special emphasis is placed on anesthetic considerations because these issues are especially important when weak stimulus intensities are called for. Finally, it is important to emphasize that all the alarm criteria currently in use for interpreting intraoperative MEPs have been shown to be effective for protecting CST axons during surgery. Although differences between approaches are more than academic, overall it is much better for patient welfare to be using some form of MEP monitoring than to use none at all, while you wait for consensus about alarm criteria to emerge.

Department of Neurosurgery, Upstate Medical University, Syracuse, New York, U.S.A.

Address correspondence and reprint requests to Blair Calancie, PhD, Department of Neurosurgery, Upstate Medical University, 750E. Adams St, IHP #1213, Syracuse, NY 13210, U.S.A.; e-mail:

The author has no funding or conflicts of interest to disclose.

Presented as a poster at the 5th meeting of the International Society of Intraoperative Neurophysiology, Rio de Janeiro, Brazil, November 12, 2015.

© 2017 by the American Clinical Neurophysiology Society