WS5.6. Intraoperative Monitoring and Mapping of Visual Pathways

Abstract

Background and Objective . Neurosurgical procedures along the visual pathways carry a risk of visual dysfunction. A reliable method for real time visual function monitoring assists in intraoperative decision making regarding radicality of excision and intermittent maneuvers near the optic apparatus. We present our experience of intraoperative flash visual evoked potential (VEP) monitoring. Methods 1) Anesthesia: Under total intravenous anesthesia (TIVA), the VEP shows larger amplitude and stable latency with less variability. Inhaled anesthetics, such as sevoflurane and isoflurane, markedly decrease the amplitude, and these should be avoided. 2) Stimulation: Flash VEP and ERG are recorded by flush stimulation on the closed eye lids. At least 1-minute preconditioning by flash stimulation should be done to obtain a steady VEP waveform before starting averaging. The stimulation intensity is decided by the supramaximal stimulation to the retina. As the evoked ERG is easy to record and the waveform is stable, it is utilized for checking the supramaximal value and inadequate stimulation to retina due to stimulator dislocation. 3) VEP Recording: Reference electrodes are set at A1 and A2, and these are electrically connected. Recording electrodes are set near Oz, O1, and O2. Simultaneous ERG recording is recommended as described above. Warning sign was defined as 50% decrease of baseline amplitude. 4) Optic nerve action potential: In cases of parasellar lesions such as craniopharyngiomas and giant aneyrysms, optic nerves, chiasm and tracts are sometimes streched around the lesion, and it is difficult to identify the location of the optic apparatus. In these situations, flash stimulation is deliverd as same manner as VEP, and action potentials recoeded from the operative fields facilitates to identifiy the optic apparatus. Results . VEP can be monitored in a patient with visual acuity greater than (0.03). Fifty percent decrease of VEP amplitude as warning sign detected postoperative visual aggravation in sensitivity of 99%. Reversible change in VEP means visual function to be preserved. Sudden VEP change correlates with visual acuity impairment and/or hemianopsia. The flat VEP indicates postoperative severe visual disturbance (nearly blindness). Conclusions . Intraoperative flash VEP monitoring is essential and reliable method for preserving visual function.