The Future of Intraoperative Neuromonitoring in Skull Base Surgery: Intraoperative Flash Visual Evoked Potentials A Novel Technique Aiming to Reduce the Risk of Intraoperative Visual Pathway Injury

Intraoperative optic nerve/chiasmal injury is one the most devastating unexpected outcomes that may happen to patients undergoing endoscopic skull base surgery. Additionally, one of the main goals for most endoscopic skull base surgery is visual improvement, however currently there is limited ability of intraoperative monitoring for the visual pathway. We examine a novel technique which was developed in our center for monitor the visual pathway intraoperatively using continuous flash visual evoked potentials (FVEPs). The FVEPs provide continuous monitoring of the visual pathway including the optic nerve, optic chiasm, optic radiation, and visual cortex, as well as retina function. This technique has been developed at the Ottawa Hospital.

Intraoperatively left then right eyes were stimulated by goggle light emitting diode (LED) stimulators (3 LEDs on each side, 640 nm peak wavelength, 10 milliseconds pulse width, 3000mCd of luminous intensity was reflected back on an angle to give uniform illumination across the whole lens, which was placed over the closed eyelids, and recording was obtained from corkscrew electrodes placed at Oz-Fz (2 cm behind Fz). The stimulus rate was 1.41Hz. 150 stimulus presentations were included in a single average. The low cut filter was ≤5 Hz and the high cut filter was 100 Hz with amplifier gain 20,000 or 50,000. The total sweep time was 300 milliseconds. EEG was continuously recorded during the surgery from C4-Fz. The recorded FVEP and EEG were correlated to patients pre and postoperative visual fields and acuity. Also any droop in the FVEP will be examined in relation to any intraoperative events. Any transient FVEP amplitude changes which is caused by electrocautery were excluded from the analysis.

Thirty patients who had endoscopic skull base surgery had intraoperative VEP using FVEPs in addition to other neurophysiologic monitoring. Patients demographic data, co-morbidities, diagnosis, surgical approach, length of surgery, MAP, and blood loss during surgery were recorded. All patients’ visual acuity and field deficits were evaluated by neuro-ophthalmologist prior to their surgery and within 30 days after their surgery. Surgical anesthesia was maintained by TIVA (Propofol/remifentanil infusion). Changes in anesthetic agents and intraoperative events (e.g., sudden blood loss, use of vasopressors ...etc) were timed and recorded.

The recorded FVEP and EEG were correlated to patients pre and postoperative visual fields and acuity. We found that FVEP is reproducible throughout surgery and can predict the postsurgical outcome. Additionally, we found that FVEP is transiently affected by different stages of surgery (e.g., traction on optic chiasm, dissection, etc.). Also we found boluses of propofol and electrocautery can artificially affect FVEP.