gms | German Medical Science

Objective: Surgery for tumors of the sellar and parasellar regions is a common occurrence in the neurosurgical practice. Despite the proximity or involvement of the visual pathway and despite of well-established methods to monitor the function of the aforementioned pathway, a clear understanding of the electrical potential changes during surgery and their objective consequences on the visual field and visual acuity are still largely lacking.

Methods: 15 patients (30 eyes, median age=63, range=19-88) with sellar and parasellar tumors (pituitary adenomas, craniopharyngiomas and anterior skull base meningiomas) where monitored during transcranial or transnasal surgery. After induction of total intravenous anesthesia, intraoperative flash visual evoked potentials where triggered using LED pads (inomed, Emerdingen, Germany). The visual stimuli where delivered with a frequency of 1.1 Hz, the illuminance was set at 20000 Lux. Occipital needle electrodes where placed subcutaneously according to the international EEG 10-20 system, the electrical signal was band-pass filtered at 5-100 Hz, averaged 200x and stored for offline analysis (Medtronic, Minneapolis, USA). All patients underwent pre- and postoperative measurements of the visual acuity and visual field integrity.

Results: We obtained reproducible VEP waveforms in 86% (26/30) of the monitored patients before surgery and in 76% (23/30) during surgery. Of the 7 eyes where VEP could not be recorded, 1 had a severe preoperative visual loss; the other 6 could be ascribed to technical difficulties. The amplitude of the VEP decreased permanently (>40% of the baseline) in 2 eyes and the patients showed an increased neurological deficit (visual acuity) postoperatively. In one case we observed an increase in the latency of the N75 peak, without registering any new postoperative deficits. We observed no new or worsened visual field deficits.

Conclusion: Measuring visual evoked potentials is a reliable method of monitoring the global function of the anterior visual pathway. Depending on their sign, we find that amplitude changes are correlated with improved or worsened visual acuity. At the same time changes in the visual filed are more subtle and not predictable from changes of the latency or amplitude of the evoked potentials.