How should visual function monitoring be performed and interpreted in surgery for suprasellar tumors?

OBJECTIVE: Preservation of visual function is important in surgery for suprasellar tumors. Visual evoked potentials (VEPs) are expected to play an important role in monitoring visual function during surgery. Given the lack of information in this field, the authors aimed to investigate the effects of optic nerve compression caused by suprasellar tumors to understand the possible usefulness of VEP monitoring using off-response (OFR) VEP.

METHODS: Eleven healthy volunteers who underwent surgery for standard record confirmation and 32 patients with optic chiasm lesions who underwent surgery were examined. Preoperative, postoperative, and intraoperative VEPs were recorded. Propofol anesthesia was administered during intraoperative VEP monitoring. Patients who underwent surgery were monitored using the same stimulation method during surgery. Light stimulation was given from a luminant pad on the eyelids, and low-intensity stimulation with continuous 500-msec emission and 500 msec off was performed. The luminescence intensity of the stimulation was at a maximum of 8000 lx with three attenuation steps, each of which was recorded repeatedly.

RESULTS: The OFR potentials and delay latencies decreased as stimulus intensity decreased. In the patient with temporal hemianopia, monocular stimulation produced the highest OFR in the contralateral occipital lobe of the stimulated eye. The authors recorded preoperative, intraoperative, and postoperative VEP in 32 patients and observed intraoperative changes in 23 patients. In the cases where VEP declined during intraoperative recording, it recovered when surgery was discontinued. Furthermore, 3 patients eventually achieved a higher VEP than that achieved at the beginning of the surgery, and rapid recovery was confirmed with visual field examination immediately after surgery. Of the 5 patients in whom VEP did not recover during surgery, 3 showed decreased visual field and acuity after surgery. In 15 cases, potential dropped temporarily but returned to the original potential, and their visual field recovered after surgery.

CONCLUSIONS: OFR has a diagnostic element in the visual field, in which the maximal potential was recorded on the opposite side of the stimulus with monocular stimulation. Unambiguous determination required stimulation of different intensities in both eyes or 1 eye and multiple recording electrodes placed in the occiput. Monitoring the OFR provides real-time alerts, making it a valuable tool for visual function evaluation in suprasellar surgery.