Intraoperative monitoring of cortically recorded visual response for posterior visual pathway

Clinical article

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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.

Abbreviations used in this paper: AVM = arteriovenous malformation; GBM = glioblastoma multiforme; LED = light-emitting diode; VEP = visual evoked potential.

Article Information

Address correspondence to: Kensuke Kawai, M.D., Ph.D., Department of Neurosurgery, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8655 Japan. email:

Please include this information when citing this paper: published online July 24, 2009; DOI: 10.3171/2009.6.JNS081272.

© AANS, except where prohibited by US copyright law.



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    Case 1. Representative waveforms of visual response recorded from the pericalcarine area in a patient without preoperative visual symptoms. The strip electrode was placed on the superior (A) and inferior (B) banks of the calcarine sulcus and lateral cortices (C).

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    Case 8. Representative waveforms of visual response recorded from the pericalcarine area in a patient with inferior quadrantanopia. The P100 peak was identifiable in the recording from the inferior bank (A) but not the superior bank (B) of the calcarine fissure.

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    Case 17. Representative waveforms of visual response recorded from the occipital cortices in a patient with larger visual field defect. The waveform was identifiable only from the most medial contact (A). She had left hemianopia sparing the lower one-eighth of her vision. There was no significant difference between the preoperative (B) and postoperative (C) visual fields.

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    Case 13. Series of visual responses before and during microsurgical manipulation recorded in a patient with temporal lobe epilepsy. The strip electrode was subdurally inserted toward the occipital pole. The manipulation was a multiple subpial transection that did not directly involve the visual pathway. Waveforms were stable throughout the procedure.

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    Case 6. Series of visual responses before and during microsurgical manipulation recorded in a patient with an AVM in the inferior cuneus who presented with inferior quadrantanopia. The strip electrode was placed on the inferior bank of the calcarine fissure. Waveforms changed because of electrode shift but recovered after correction of the electrode position and remained stable thereafter.

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    Case 5. Series of visual responses before and during microsurgical manipulation recorded in a patient with a GBM in the right cuneus who had postoperative total hemianopia. The amplitude of N2 decreased as the resection went deeper and disappeared when the resection reached the lateral ventricle.

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    Case 4. Consecutive changes in the VEP during tumor resection. The amplitude of P100 decreased as the resection of the inferolateral portion of ganglioglioma in the cuneus proceeded deeply at 15:08 (thick arrow). The surgeon stopped the dissection in this direction, and the VEPs recovered during dissection in the superior aspect after some time of interruption of manipulation.



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