Postoperative Improvement of Visual Function Following Amplitude Increase in Intraoperative Off-Response Visual Evoked Potential (VEP) Monitoring During a Skull Base Meningioma Surgery.

Intraoperative visual evoked potential (VEP) monitoring does not generally predict improvement of postoperative visual function when there is an increase in the amplitude compared to the baseline recording. However, with a novel VEP monitoring method called "off-response" VEP, postoperative improvement of visual function was documented following an increase in the VEP amplitude during a skull base meningioma surgery. The authors present a case of a patient who was diagnosed with a skull base meningioma and underwent a left frontotemporal craniotomy. The patient initially presented with a decreased visual acuity in the right eye. The best-corrected visual acuity in the right eye was 0.1 on the Landolt C chart, approximately equivalent to 20/200 on the Snellen visual acuity chart. Both off-response and conventional VEP monitoring were performed on the right eye during the surgery because the left eye was already blind. Following tumor resection, the off-response VEP recording in the right eye showed a 40% increase in amplitude, while the conventional VEP remained unchanged. The patient's visual acuity in the right eye significantly improved after surgery. We report a case of postoperative improvement of visual function preceded by an amplitude increase in intraoperative off-response VEP, despite unchanged conventional VEP recording during a skull base meningioma surgery. Off-response VEP is effective in monitoring visual function intraoperatively and may be highly sensitive compared to the conventional flash VEP.