Usefulness of Intraoperative Monitoring of Visual Evoked Potentials in Transsphenoidal Surgery

Abstract

Postoperative visual outcome is a major concern in transsphenoidal surgery (TSS). Intraoperative visual evoked potential (VEP) monitoring has been reported to have little usefulness in predicting postoperative visual outcome. To re-evaluate its usefulness, we adapted a high-power light-stimulating device with electroretinography (ERG) to ascertain retinal light stimulation. Intraoperative VEP monitoring was conducted in TSSs in 33 consecutive patients with sellar and parasellar tumors under total venous anesthesia. The detectability rates of N75, P100, and N135 were 94.0%, 85.0%, and 79.0%, respectively. The mean latencies and amplitudes of N75, P100, and N135 were 76.8 ± 6.4 msec and 4.6 ± 1.8 μV, 98.0 ± 8.6 msec and 5.0 ± 3.4 μV, and 122.1 ± 16.3 msec and 5.7 ± 2.8 μV, respectively. The amplitude was defined as the voltage difference from N75 to P100 or P100 to N135. The criterion for amplitude changes was defined as a > 50% increase or 50% decrease in amplitude compared to the control level. The surgeon was immediately alerted when the VEP changed beyond these thresholds, and the surgical manipulations were stopped until the VEP recovered. Among the 28 cases with evaluable VEP recordings, the VEP amplitudes were stable in 23 cases and transiently decreased in 4 cases. In these 4 cases, no postoperative vision deterioration was observed. One patient, whose VEP amplitude decreased without subsequent recovery, developed vision deterioration. Intraoperative VEP monitoring with ERG to ascertain retinal light stimulation by the new stimulus device was reliable and feasible in preserving visual function in patients undergoing TSS.

Intraoperative monitoring of flash visual evoked potential under general anesthesia

“…Previously, VEP waveforms resulting from flash stimulation were complex, varied markedly among individuals, and posed problems regarding compatibility with optic nerve function [27]. However, recent improvements in stimulation devices and the development of improved anesthesia methods have enabled the recording of stable waveforms [4589]. When performing flash stimulation, it is possible to photostimulate the retina with up to 20,000 lux of illuminance using an LSF-101 II flash stimulation device (Unique Medical Co., Ltd., Tokyo, Japan), which uses LEDs [48].…”

“…The flash stimulation is set to an intensity that is slightly higher than that at which the VEP amplitude is attenuated by decreasing the stimulation intensity from 20,000 lux to perform maximum stimulation. When flash stimulating the retina, it is necessary to confirm that the flash stimulus has reached the retina by taking electroretinogram (ERG) recordings at the same time as the flash VEP recordings [459]. …”

“…ERG must be recorded while monitoring flash VEPs [459]. Flash stimulation-induced ERG can be easily recorded using electrodes placed anywhere around the eyes, after which the ERG is processed by the same averaging as the VEPs.…”

“…4). When the peak-to-peak distance between N75 and P100 decreases by at least 50% from the reference amplitude, this is reported to the surgeon as a significant change in the VEP [459]. However, as the VEP amplitude is low, at least two or more recordings of the same waveform must be confirmed to verify reproducibility.…”

Intraoperative Flash Visual Evoked Potential Recording and Relationship to Visual Outcome

“…Specifically, the use of restrictive bandpass filtering, optimal reject window settings, mastoid reference site, total intravenous anesthetic (TIVA), and stable retinal stimulation (ensured by concomitant electroretinogram [ERG] recording) have greatly enhanced FVEP reproducibility. [1][2][3][4][5] The FVEP was previously considered difficult to obtain during surgery and, when obtained, a poor predictor of postoperative visual function. [6][7][8][9][10] Now that it is possible to obtain more reproducible intraoperative FVEPs, there is renewed interest in determining its relationship to iatrogenic visual pathway injury.…”