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Observations on intraoperative monitoring of visual pathways using steady-state visual evoked potentials
H Wiedemayer1, B Fauser, I E Sandalcioglu
1University of Essen, Department of Neurosurgery, Essen, Germany. helmut.wiedemayer@uni-essen.de
Background And Objective:
Former studies revealed conflicting information on the usefulness of intraoperative monitoring of visual evoked potentials. This study was designed to evaluate the characteristics of visual evoked potential recording in surgically anaesthetized patients using the modality of steady-state visual evoked potentials.
Methods:
In 30 cases with non-cranial surgery steady-state visual evoked potentials were recorded in the awake and surgically anaesthetized patient using total intravenous anaesthesia. For stimulation, goggles with red light-emitting diodes at a frequency of 8.5 Hz were used. A two-channel recording with silver cup electrodes at Oz to Fz and Oz to earlobe was used. All traces were analysed for the presence of the characteristically sinusoidal waveform and amplitudes and latencies of the main peaks were measured.
Results:
Recordings during surgery demonstrated a minor latency prolongation of 16% and a more pronounced amplitude attenuation of 67% compared to the recordings in the awake patients. These differences were statistically significant (paired t-test, P < 0.001). In surgically anaesthetized patients steady-state visual evoked potentials showed a relatively high intra- and interindividual variability. In four of 30 patients completely stable recordings were obtained, whereas in 14 patients identifiable waves were recordable in only less than 50% of the intraoperative traces. Of the total 1360 traces recorded intraoperatively clearly identifiable steady-state visual evoked potentials patterns were present in 56% of the traces. There was no correlation between the magnitude of the evoked potential amplitude and its stability in intraoperative recordings.
Conclusions:
We conclude from this study, that steady-state visual evoked potential recordings in the surgically anaesthetized patient appeared to be more stable compared to our earlier findings using transient visual evoked potentials. However, further efforts are necessary to improve the stability of the recordings during surgery and thus allow for a more reliable intraoperative monitoring of visual pathways in routine clinical practice.
