Intraoperative monitoring of visual evoked potentials (VEPs) has been regarded as having limited significance for the preservation of visual function during neurosurgical procedures, mainly due to its poor spatial resolution and signal-to-noise ratio. The authors evaluated the usefulness of cortically recorded VEPs, instead of the usual scalp VEPs, as intraoperative monitoring focusing on the posterior visual pathway.
In 17 consecutive patients who underwent microsurgical procedures for lesions near the posterior visual pathway, cortical responses were recorded using 1-Hz flashing light-emitting diodes and subdural strip electrodes after induction of general anesthesia with sevoflurane or propofol. The detectability and waveform of the initial response, stability, and changes during microsurgical manipulations were analyzed in association with the position of electrodes and postoperative changes in visual function.
Initial VEPs were detected in 82% of all patients. The VEPs were detected in 94% of patients without total hemianopia in whom electrodes were placed sufficiently near the occipital pole; in these cases the recordings were not significantly affected by anesthesia. The detectability rates of the negative peak before 100 msec (N1), positive peak ~ 100 msec (P100), and negative peak after 100 msec (N2) were 36, 50, and 100%, respectively. The mean latencies and amplitudes of N1, P100, and N2 were 90.0 ± 15.9 msec and 61.0 ± 64.0 μV, 103.9 ± 13.5 msec and 34.3 ± 38.6 μV, and 125.7 ± 12.2 msec and 44.9 ± 48.9 μV, respectively, showing great variability. In 11 patients, the initial waveforms of VEP remained stable during microsurgical procedures, and the visual status did not change postoperatively, while it disappeared in 2 patients who presented with postoperative hemianopia.
Direct recording from the visual cortices under general anesthesia achieved satisfactory detectability of the visual response to a light-emitting diode flashing light. Although the initial waveforms varied greatly among patients, they were stable during microsurgical procedures, and the changes were consistent with postoperative visual function. Intraoperative cortical VEP monitoring is a potentially useful procedure to monitor the functional integrity of the posterior visual pathway.