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