P324 Improvement in the reproducibility of visual evoked potentials in intraoperative neurophysiological monitoring

Objective The importance of visual evoked potentials (VEPs) in the field of Neurosurgery has been documented previously, but their use is not standardized due to their limitations (high sensibility to the anaesthetics and low reproducibility). In those surgical interventions that imply a possible damage to the optical nerve, VEPs may have an impact in the surgical decisions. Methods There have reviewed 6 cases from the Neurosurgery Department (Ramon y Cajal Hospital) which underwent surgery of tumours next to the visual pathway (in orbital cavity, occipito-parietal glioma, craniopharyngioma and at sphenoid bone’s sellar region). The visual function was monitored by VEPs with goggles stimulation and recording with the following electrode placing: Oz, O1, O2, lateral O1 and lateral O2. In two cases, responses were recorded with subdural electrodes. Results With the use of goggles with a stimulation frequency of 2.1 Hz, the placement of additional electrodes (lateral O1 and lateral O2), and the filters settings (low-pass filter: 1 Hz; high-pass filter: 1KHz), we have obtained reproducible VEPs in all the surgeries, with a higher amplitude with subdural recording. Discussion With these modifications in the neurophysiological technique, we have avoided one of the major difficulties using VEPs in the workaday monitoring, improving their reproducibility.