33. Intraoperative visual evoked potential monitoring and its pitfalls

Abstract

Introduction Visual evoked potentials (VEP) have been installed as a method for intraoperative visual function monitoring. However, there have been few reports regarding pitfalls of intraoperative VEP monitoring. To clarify this, relationships between intraoperative VEP waveform changes and postoperative visual function were analysed retrospectively. Material and methods Intraoperative VEP monitoring was performed in 123 surgeries, including three intraorbital, 96 parasellar and 24 cortical lesions in Shinshu University Hospital. Red flash light was provided to each eye independently. Supramaximal stimulation was found before starting surgical procedure and control VEP amplitude was measured. Decrease by 50% of control VEP amplitude was defined as a warning sign and reported to surgeons. The surgical procedure was ceased on the surgeon’s decisions. Intraoperative VEP monitoring and postoperative visual function were analysed. Results In 238 out of 246 eyes (97%), steady VEP monitoring was recorded. Transient VEP decrease was observed in 18 sides, but visual function was preserved. A permanent VEP decrease was seen in 18 sides, which resulted in visual impairment in 10 sides, and no visual aggravation in 8 sides postoperatively. The VEP amplitude was preserved greater than 50% in 200 of 202 sides, and visual function was preserved. In a patient with tuberculum sellae meningioma, visual acuity aggravated though VEP was maintained at 77% of control amplitude. In one side, visual acuity improved but minor visual field defect was encountered postoperatively, though VEP was unchanged throughout the surgery. Conclusions Intraoperative VEP monitoring predicts postoperative visual function: a 50% decrease of VEP amplitude can detect postoperative visual aggravation with a sensitivity of 99%. Pitfalls for intraoperative VEP monitoring are: (1) preoperative severe visual dysfunction, low amplitude of control VEP may interfere with intraoperative VEP monitoring in this method. (2) Visual field defect without decrease in the visual acuity may not be predicted by VEP monitoring. Attention should be paid to these pitfalls for reliable intraoperative VEP monitoring.