Reproducibility of intraoperative visual evoked potentials: A review of 141 cases undergoing prone spinal procedures

Visual evoked potentials (VEPs) and surface electro-retinograms (ERGs) were recorded using flash stimulation during prolonged spine procedures performed in the prone position on 141 subjects without known visual deficits in the presence of various anesthetic protocols that included inhalation and intravenous agents. Waveforms were subjectively evaluated with respect to morphology, latency, amplitude, and reproducibility, with specific attention to the effects of minimal alveolar concentration (MAC) of the inhalant in the anesthetic regimen. Although inter-subject variability was noted in the morphology of the VEPs, waveforms were highly reproducible within individual subjects and were relatively invariant over time. Reproducibility correlated inversely with the MAC level of inhalation agent. Potentials recorded using infraorbital-to-midline electrodes were also robust, reproducible, and stable, and similarly varied inversely with the MAC of inhalation agent. These waveforms were similar in morphology to previously reported clinical and intraoperative electroretinograms (a short duration electronegative component, followed by a longer duration, higher amplitude, positive wave). Our data demonstrate that intraoperative VEPs and ERGs can be successfully performed with current anesthetic regimens, especially when a relatively low level of gaseous anesthetic is combined with opioid infusions. Reliable intraoperative recordings of VEPs and ERGs may facilitate early detection of compromise to visual pathways.