PMC6921212_01
A-C. Sagittal, axial and coronal sections of a gadolinium enhanced T1 MRI showing the tumor situated in the atrium of the right lateral ventricle. D. Probabilistic tractography streamlines of the optic radiation superimposed on anatomical T1 image. E. Resampled track-density map of the optic radiation for fusion with T1. F. Surgical navigation plan showing a 3D reconstruction of the patient's head, tumor (green volume) and optic radiations (yellow). A parietal entry point was chosen as described in the main text and the green line shows the trajectory line towards the tumor.
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MRI was repeated following the development of recent symptoms and the examination showed considerable progression of the tumor to 40 x 31 mm, now extending from the atrium of the right lateral ventricle into the posterior horn but without intra-parenchymal invasion (Fig. 1A-C). The visual pathways were delineated from the lateral geniculate body (seed region) to the occipital cortex (inclusion region) using a multi-fiber diffusion model (CSD) and probabilistic tractography (iFOD2) as implemented in the MRtrix3 software (Fig. 1). StealthStation system (Medtronic, Inc.). Subsequently, this T1-tractography dataset was merged with the anatomical reference and a transcortical trajectory towards the tumor was chosen to avoid damage to the visual pathways (Fig. 1).
Panel A shows the mounted VEP googles fixed to the patient's face with tape and covered with aluminium foil to reduce noise. Panel B shows ERG needle electrodes placed in the lateral canthus area. In addition, corneal electrodes where placed but not used due to high impedance.
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The goggles were used without rubber bands but instead fixed with tape as shown in Fig. 2A. To minimize retinal stimulation by ambient light, both eyes were covered with aluminum foil (Fig. 2A). We performed bilateral ERG using needle electrodes in the cantus area to record retinal activity following visual flash stimulation (Fig. 2B). Corneal eye electrodes were also tried but not used due to high impedance (Fig. 2B).
ERG and VEP recordings from the left (A) and right (B) eye, respectively. ERG is shown in the upper trace and VEPs recorded from O1, O2, and Oz are shown in the three lower traces in that order.
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Right-sided ERGs were robust and showed acceptable signal-to-noise ratios (Fig. 3). Separate left- and right-sided VEPs were recorded from O1 and O2, respectively, with Oz as the reference electrode (Fig. 3) and repeated runs with 500-600 averaged stimuli were stored.
This figure shows the latency shift of the P100 on the right side (B), and the amplitude reduction on the right side (D) while the waveform on the left side (A) (C) remain unchanged. The upper trace depicts the ERG recording which remained unchanged. The red average shows the recordings done at baseline and the green ones show averages of the latest recordings. The arrow shows the latency shift and the amplitude reduction between the baseline and the latest recording.
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VEPs on the right side showed a significant P100 latency shift of more than 10 ms (Fig. 4), which occurred in connection with the placement of minimally invasive selfretaining tube-retractors. The amplitude reduction recovered completely at the end of surgery (Fig. 4), while the latency shift persisted throughout the operation.
A-C. Sagittal (T1), axial (FLAIR) and coronal (FLAIR) sections of MRI two days postoperatively showing no rest tumour.
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An MRI scan conducted two days post-operatively confirmed gross total resection and showed expected post-operative radiographic sequelae but no signs of ischemia (Fig. 5).
This figure shows half field stimulation for A-right eye and B-left eye. The upper four curves in both A and B depict the VEP curves after left half field stimulation for the right (A) and for the left eye (B). Those curves are not well defined and thus the cursors were not placed. The responses were considered not present. The lower four curves in both A and B depict the VEP curves after right half field stimulation for the right (A) and for the left eye (B). The responses were well defined with normal latencies and amplitudes.
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However, the half field stimulation showed markedly reduced amplitudes of the VEP responses from the left field compared to the right field responses (Fig. 6).