Progressive suppression of motor evoked potentials during general anesthesia - The phenomenon of "anesthetic fade"

Transcranial motor evoked potentials (MEPs) are useful for assessing the integrity of spinal cord motor tracts during major spine surgery. Anesthetic agents depress the amplitude of MEPs in a dose-dependant fashion. Anecdotal reports suggest that MEP responses degrade or "fade" over the duration of a surgery, despite unchanged anesthetic levels or other physiologic variables. This phenomenon has not been systematically analyzed. We performed a retrospective study of 418 patients who underwent spine surgery at UCSF using intraoperative MEP monitoring. We excluded patients who experienced variations in physiologic parameters that might affect MEP signals and those who developed new neurologic deficits. We identified 46 neurologically intact patients and 16 myelopathic patients who had surgery performed using a constant desflurane/N2O /narcotic or desflurane/propofol/narcotic anesthetic regimen. The minimum voltage threshold needed to produce an MEP response of at least 50 muV in amplitude was recorded at the beginning ("baseline") and end of surgery. The voltage threshold was higher at the end of the case than at baseline for each patient, regardless of anesthetic regimen. In normal patients, the rate of rise of the threshold was similar for those receiving propofol (11.4 +/- 6.9 V/hr) or N2O (9.7 +/- 5.9 V/hr) (P = not significant). Myelopathic patients demonstrated a larger rate of rise in voltage threshold, 23.4 +/- 12.2 V/hr, versus normal subjects (P < 0.01). The rate of rise of voltage threshold is inversely proportional to anesthetic duration. Prolonged exposure to anesthetic agents necessitates higher stimulating thresholds to elicit MEP responses, separate from the dose-dependant depressant effect. This retrospective study is limited and cannot explain the mechanism for this observed fade in signals. Recognition of anesthetic fade is essential when interpreting changes to the MEP response to avoid false-positive findings.