Elaborate mapping of the posterior visual pathway in awake craniotomy

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OBJECTIVE

Resection of intraaxial tumors adjacent to the optic radiation (OR) may be associated with postoperative visual field (VF) deficits. Intraoperative navigation using MRI-based tractography and electrophysiological monitoring of the visual pathways may allow maximal resection while preserving visual function. In this study, the authors evaluated the value of visual pathway mapping in a series of patients undergoing awake craniotomy for tumor resection.

METHODS

A retrospective analysis of prospectively collected data was conducted in 18 patients who underwent an awake craniotomy for resection of intraaxial tumors involving or adjacent to the OR. Preoperative MRI-based tractography was used for intraoperative navigation, and intraoperative acquisition of 3D ultrasonography images was performed for real-time imaging and correction of brain shift. Goggles with light-emitting diodes were used as a standard visual stimulus. Direct cortical visual evoked potential (VEP) recording, subcortical recordings from the OR, and subcortical stimulation of the OR were used intraoperatively to assess visual function and proximity of the lesion to the OR. VFs were assessed pre- and postoperatively.

RESULTS

Baseline cortical VEP recordings were available for 14 patients (77.7%). No association was found between preoperative VF status and baseline presence of cortical VEPs (p = 0.27). Five of the 14 patients (35.7%) who underwent subcortical stimulation of the OR reported seeing phosphenes in the corresponding contralateral VF. There was a positive correlation (r = 0.899, p = 0.04) between the subcortical threshold stimulation intensity (3–11.5 mA) and the distance from the OR. Subcortical recordings from the OR demonstrated a typical VEP waveform in 10 of the 13 evaluated patients (76.9%). These waveforms were present only when recordings were obtained within 10 mm of the OR (p = 0.04). Seven patients (38.9%) had postoperative VF deterioration, and it was associated with a length of < 8 mm between the tumor and the OR (p = 0.05).

CONCLUSIONS

Intraoperative electrophysiological monitoring of the visual pathways is feasible but may be of limited value in preserving the functional integrity of the posterior visual pathways. Subcortical stimulation of the OR may identify the location of the OR when done in proximity to the pathways, but such proximity may be associated with increased risk of postoperative worsening of the VF deficit.

ABBREVIATIONS DTI = diffusion tensor imaging; EOR = extent of resection; FA = fractional anisotropy; OR = optic radiation; SPGR = spoiled gradient–recalled acquisition; US = ultrasonography; VEP = visual evoked potential; VF = visual field.

Article Information

Correspondence Zvi Ram, Department of Neurosurgery, Tel Aviv Medical Center, 6 Weizmann St., Tel Aviv 6423906, Israel. email: zviram@tlvmc.gov.il.

INCLUDE WHEN CITING Published online August 25, 2017; DOI: 10.3171/2017.2.JNS162757.

Dr. Shahar and Mr. Korn contributed equally to this work.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

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Figures

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    Baseline cortical VEP recording made using subdural electrodes positioned at the occipital pole in a patient with an intact preoperative VF (A) and in a patient with a significant VF deficit (B). Note the similar VEP appearance regardless of VF status.

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    Patient 3. This 78-year-old man underwent awake craniotomy for resection of a newly diagnosed left parietal glioblastoma. A VF test at baseline (A) demonstrated a very mild upper-right quadrantanopia. Preoperative axial contrast-enhanced T1-weighted MR image of a left parietal ring-enhancing lesion (B) demonstrating proximity (< 5 mm) of the tumor to the reconstructed DT images of the OR (white arrows). Cortical VEP recording prior to tumor resection (E) using a subdural electrode positioned toward the occipital pole exhibited a typical N75 appearance (solid arrow) and a P100 (dashed arrow) component of a VEP waveform. A subcortical recording distant from the OR failed to demonstrate the typical VEP appearance (F, left panel). The location of the recording probe (F, right panel) was demonstrated intraoperatively by the navigation system using the preoperative MR image (F, right panel, left side) and intraoperative US-based navigation (F, right panel, right side) to account for brain shift. Subcortical recording 2 mm from the OR, as depicted by the navigation system using the preoperative MR image (G, right panel, left side) and intraoperative US-based navigation (G, right panel, right side), demonstrated a typical VEP waveform (G, left panel). Note the proximity of the recording probe in the superimposed reconstructed OR to the US image in an offline analysis (G, right panel, right side). Postoperative MR image (D) demonstrated complete resection of visible tumor and integrity of the reconstructed OR (white arrows). The postoperative VF test results suggest a mild worsening deficit, predominantly in the upper-right VF (C).

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