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Introduction Intraoperative neuromonitoring of visual evoked potentials (IONM VEP) has been considered as unreliable because of its instability and poor association with postoperative functional outcome. The aim of this study is to analyze different IONM VEP parameters to optimize its monitoring usefulness. Methods IONM VEP was carried out for 22 sides (eyes) in 11 surgeries, including one frontal lobe lesion, 9 standard transsphenoidal (TSS), and one extended TSS surgeries at the Lozano Blesa University Hospital. A preoperative neurophysiologic examination was performed to evaluate visual function in all patients. LED goggles stimulation was given with stimulus duration of 15 ms, intensity of 1.5–3 cd/s.m-2 and repetition rate from 0.5 Hz to 3.0 Hz until waveforms were feasible and reproducible. Additional recording scalp electrodes (LO1: 5 cm left of O1 and LO2: 5 cm right of O2) were added to the well-known montage (Oz-Cz/Fz, O1-Cz/Fz, O2-Cz/Fz), as we wanted to assess recording reliability. Once VEP responses were obtained, pupil dilatation was induced with mydriatic agents, (prior ophthalmologic assessment) followed 20 min. later by at least two consecutive VEP recordings, to evaluate VEP N2-P2 component amplitude and latency. Although anesthetic regimen consisted in total intravenous anesthesia (TIVA) in all our cases, after surgical completion and VEP closing traces gathering, halogenated agents were added at ascending doses (from 0.1 CAM to 1.5 CAM), to value VEP waveforms stability. Written informed consent for the surgery and IONM were obtained from all patients. Results All patients underwent pre-operative examination. In these patients, six had visual disturbances (one was severe). Ten patients were maintained under TIVA, and sevoflurane was used incidentally in one patient. Stable VEP waveforms were obtained in all patients, except in one patient with preexisting damage to the visual pathway, and another one in whom sevoflurane was used for anesthesia. The stimulus frequency showing more stable and higher waveform was below 1 Hz. There was a significant increase in VEP amplitude, but no effect on the latency after the mydriatic drug instilling. VEP traces were recordable in all channels, but in one case in where they were only obtained throughout LO1 and LO2 recording channels. The administration of halogenates agents produced a doses-dependent decrease in VEP amplitude and latency prolongation in five patients, and a complete loss of potential in three patients with doses above 1.1 CAM, all 15–20 min after application. Conclusion Recording reliable VEPs hinge on both physiological and technical aspects. VEPs may be mitigated in patients with preoperative impaired visual function. Halogenated agents showed a major influence on IONM VEP stability. The use of cycloplegics helped in obtaining larger responses. Choosing the right stimulus frequency and including additional recording electrodes (LO1-LO2), seemed to enhance the reliability of stimulus-recordings parameters. |
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