Elsevier

Clinical Neurophysiology

Volume 132, Issue 10, October 2021, Pages 2351-2356
Clinical Neurophysiology

Effects of polarity of bipolar sensorimotor direct cortical stimulation on intraoperative motor evoked potentials

https://doi.org/10.1016/j.clinph.2021.06.032Get rights and content

Highlights

  • The effects of stimulus polarity on MEP in bipolar cortical stimulation were examined.

  • Higher MEP was recorded with a cathode on the precentral and an anode on the postcentral gyrus.

  • The present results will contribute to a sensitive and specific MEP evaluation.

Abstract

Objective

The present study investigated the effects of the stimulus polarity and location of motor evoked potential (MEP) to establish a stimulation protocol.

Methods

Nineteen patients who intraoperatively underwent MEP in bipolar direct cortical stimulation were enrolled in the present study. Somatosensory evoked potentials (SEP) of the contralateral median nerve stimulation were recorded to determine stimulation sites. MEP was performed under two settings in all patients: 1. Anodal bipolar stimulation: an anode on the precentral gyrus and a cathode on the postcentral gyrus, 2. Cathodal bipolar stimulation: a cathode on the precentral gyrus and an anode on the postcentral gyrus. MEP amplitudes and the coefficient of variation (CV) at a stimulation intensity of 25 mA and the thresholds of induced MEP were compared between the two settings.

Results

An electrical stimulation at 25 mA induced a significantly higher amplitude in cathodal bipolar stimulation than in anodal bipolar stimulation. Cathodal bipolar stimulation also showed significantly lower thresholds than anodal stimulation. CV did not significantly differ between the two groups.

Conclusions

These results indicate that cathodal bipolar stimulation is superior to anodal bipolar stimulation for intraoperative MEP monitoring.

Significance

MEP in cathodal bipolar cortical stimulation may be used in a safe and useful evaluation method of motor fiber damage that combines sensitivity and specificity.

Introduction

The preservation of motor function during surgery is a critical challenge in patients with lesions at or close to the motor cortex. Intraoperative motor evoked potential (MEP) allows for the reliable identification of the motor region and contributes to its functional preservation (Nagle et al., 1996); (Kim et al., 2018); (Uno et al., 2020).

Two different stimulation techniques are performed to apply an electrical current to the motor cortex: bipolar cortical stimulation and monopolar cortical stimulation (Kombos and Süss, 2009). Although both techniques are suitable for cortical stimulation, their effects on the motor cortical system differ. Monopolar cortical stimulation excites the pyramidal cell zone and requires the lowest intensity to generate MEP. This method allows for the reliable monitoring of subcortical descending motor pathways (Kombos and Süss, 2009). On the other hand, bipolar cortical stimulation is more effective with stimulation electrodes aligned transversally rather than longitudinally to the axon (Kombos and Süss, 2009). This effect depends on the level of the cortex and, thus, bipolar cortical stimulation is suitable for mapping cortical function. These techniques are complementarily used for intraoperative neurophysiological monitoring.

The effects of the stimulus polarity of monopolar cortical stimulation on MEP have been investigated and monopolar anodic stimulation has been established (Hern et al., 1962); (Gorman, 1966, Kombos and Süss, 2009). However, the polarity effect of bipolar cortical stimulation on MEP remains unclear. Therefore, the present study investigated the effects of the stimulus polarity and location of bipolar cortical stimulation on the amplitudes and thresholds of MEPs.

Section snippets

Patients

Nineteen patients (11 females) who intraoperatively underwent MEP in bipolar direct cortical stimulation with subdural electrodes at Sapporo Medical University between 2016 and 2020 were enrolled in the present study. Patients with preoperative severe motor deficit and lesions within precentral/postcentral gyri were excluded from this study. Patient profiles are shown in Table 1. Their ages ranged between 28 and 84 years (median 63 years) and all patients were right-handed. The diagnoses of 19

Results

Representative waveforms of SEP and MEP are shown in Fig. 2. Epileptic seizures were not induced by the electrical stimulation, and the procedure was safely performed in all patients. MEP in anodal bipolar stimulation was recorded in 15 out of 19 patients at a stimulus intensity of 25 mA, but not in four patients in which preoperative motor deficits were absent (patient 4), absent (patient 7), absent (patient 11), and mild (patient 16)(Table 2). Similarly, MEP in cathodal bipolar stimulation at

Discussion

Regarding intraoperative MEP, the majority of previous studies have described monopolar cortical stimulation, with limited information currently being available on bipolar cortical stimulation (Mikuni et al., 2007a, Mikuni et al., 2007b, Mikuni et al., 2007c, Mikuni et al., 2007d). The present results revealed that cathodal bipolar cortical stimulation was associated with significantly higher amplitudes at the same intensity and lower thresholds than anodal bipolar stimulation.

Previous studies

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank all the staff at the Division of Clinical Engineering who provided their support for this research. We confirm that we have read the journal’s position on issues involved in ethical publication and confirm that this study is consistent with those guidelines.

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