Standard and limitation of intraoperative monitoring of the visual evoked potential.

BACKGROUND: Visual evoked potential (VEP) has been installed as one of the intraoperative visual function monitoring. It remains unclear, however, whether intraoperative VEP monitoring facilitates as a real time visual function monitoring with satisfactory effectiveness and sensitivity. To evaluate this, relationships between VEP waveform changes and postoperative visual function were analysed retrospectively.

METHODS: Intraoperative VEP monitoring was carried out for 106 sides (eyes) in 53 surgeries, including two intraorbital, 36 parasellar and 15 cortical lesions in Shinshu University Hospital under total intravenous anaesthesia. Red light flash stimulation was provided to each eye independently. VEP recording and postoperative visual function were analysed.

RESULTS: In 103 out of 106 sides (97%), steady VEP monitoring was recorded. Stable VEP was acquired from eyes having corrected visual acuity greater than 0.4. VEP was not recorded in one side with corrected visual acuity of 0.3 and two sides in whom sevoflurane was used incidentally for anaesthesia. Transient VEP decrease was observed in three sides, but visual function was preserved. Permanent VEP decrease was seen in seven sides, which presented visual impairment postoperatively. In one side, visual acuity improved but minor visual field defect was encountered postoperatively, though VEP unchanged throughout the surgery.

CONCLUSIONS: Intraoperative monitoring of VEP predicts postoperative visual function: reversible change in VEP means visual function to be preserved. Visual field defect without decrease in the visual acuity may not be predicted by VEP monitoring. Intraoperative VEP monitoring will be mandatory for surgeries harbouring a risk of visual impairment.