Simultaneous Motor and Visual Intraoperative Neuromonitoring in Asleep Parietal Lobe Surgery: Dual Strip Technique

Abstract

Background: The role played by the non-dominant parietal lobe in motor cognition, attention and spatial awareness networks has potentiated the use of awake surgery. When this is not feasible, asleep monitoring and mapping techniques should be used to achieve an onco-functional balance. Objective: This study aims to assess the feasibility of a dual-strip method to obtain direct cortical stimulation for continuous real-time cortical monitoring and subcortical mapping of motor and visual pathways simultaneously in parietal lobe tumour surgery. Methods: Single-centre prospective study between 19 May−20 November of patients with intrinsic non-dominant parietal-lobe tumours. Two subdural strips were used to simultaneously map and monitor motor and visual pathways. Results: Fifteen patients were included. With regards to motor function, a large proportion of patients had abnormal interhemispheric resting motor threshold ratio (iRMTr) (71.4%), abnormal Cortical Excitability Score (CES) (85.7%), close distance to the corticospinal tract—Lesion-To-Tract Distance (LTD)—4.2 mm, Cavity-To-Tract Distance (CTD)—7 mm and intraoperative subcortical distance—6.4 mm. Concerning visual function, the LTD and CTD for optic radiations (OR) were 0.5 mm and 3.4 mm, respectively; the mean intensity for positive subcortical stimulation of OR was 12 mA ± 2.3 mA and 5/6 patients with deterioration of VEPs > 50% had persistent hemianopia and transgression of ORs. Twelve patients remained stable, one patient had a de-novo transitory hemiparesis, and two showed improvements in motor symptoms. A higher iRMTr for lower limbs was related with a worse motor outcome (p = 0.013) and a longer CTD to OR was directly related with a better visual outcome (p = 0.041). At 2 weeks after hospital discharge, all patients were ambulatory at home, and all proceeded to have oncological treatment. Conclusion: We propose motor and visual function boundaries for asleep surgery of intrinsic non-dominant parietal tumours. Pre-operative abnormal cortical excitability of the motor cortex, deterioration of the VEP recordings and CTD < 2 mm from the OR were related to poorer outcomes.

Keywords: corticospinal tract; intra-operative neuro-monitoring; optic radiations; parietal lobe; subdural strip electrodes; tractography; transcranial magnetic stimulation.

Figure 1

Example of a craniotomy for tumour resection with simultaneously motor and optic radiations’ mapping. The recordings from the subdural strip over primary motor cortex (left) show positive motor evoked responses from the right bicep/tricep, brachioradialis/flexor carpi ulnaris, abductor digiti minimi/ abductor pollicis brevis, and first dorsal interossei at 100 µV amplitude and a latency of 36 ms, from direct cortical stimulation using the strip electrode with 5 anodal pulses, 500 µs pulse width, at 10 mA, with software filters of 30–1500 Hz. The recordings from the subdural strip over primary visual cortex (right) show visual evoked potentials recorded using the direct cortically placed strip electrode over the calcarine fissure, from simultaneous stimulation of the bilateral eyes through LED goggles placed over the eyelids at 16,000 lx intensity and 3.1 Hz, using software filters of 10–300 Hz. Artefact can be observed on the VEP strip electrode recording channels referenced to Cz’ scalp electrode (top 4 channels with green arrows). The bottom 4 channels observed VEP waveform responses referenced to other channels on the same strip electrode, with the strongest and largest amplitudes seen on contact 1 referenced to contacts 2 and 4 (blue arrows) at 20 µV, P2 peak at ~90 ms, N3 at ~98 ms, and P3 at ~120 ms however despite filters applied, peaks N1, P2, and N2 are difficult to identify due to external artefact.

Figure 2

Example of a minimally invasive parafascicular approach (MIPS) for tumour. The recordings from the subdural strip over primary motor cortex (left) show positive motor evoked responses from the right brachioradialis/flexor carpi ulnaris, abductor digiti minimi/ abductor pollicis brevis, and first dorsal interossei at 100 µV amplitude and a latency of ~30 ms, from direct cortical stimulation using the strip electrode with 5 anodal pulses, 500 µs pulse width, at 8 mA, with software filters of 30–2000 Hz. The recordings from the subdural strip over primary visual cortex (right) show visual evoked potentials recorded using the direct cortically placed strip electrode over the calcarine fissure, from simultaneous stimulation of the bilateral eyes through LED goggles placed over the eyelids at 10,000 lx intensity and 3.1 Hz, using software filters of 10–500 Hz. All four contacts of this strip electrode were referenced to the Fz scalp electrode placed on the left mastoid. The VEPs are observed at 100 µV amplitude, latencies of each peak are seen at N1 ~50 ms, P2 ~60 ms, N3 ~68 ms, P2 ~100 ms, N3 at ~115, and P3 at ~135 ms.

Figure 3

Schematic representation of the positive subcortical positive responses for optic radiations and corticospinal tract. Each number corresponds to the number ID of the patient in Table 1. Blue: Corticospinal tract. Red: Optic Radiations. Yellow spheres: Positive subcortical responses for optic radiations with high frequency bipolar stimulation. Green spheres: Positive subcortical responses for the corticospinal tract with high frequency monopolar stimulation.

Figure 4

Illustration of Dual Strip technique with strip placed over the motor cortex (Blue) (for continuous mapping of CST) and occipital cortex (continuous mapping of VEPs) in the patient with parietal tumour (Yellow).