# What is the best electrode setting to elicit motor evoked potentials in the muscles of lower extremities during supratentorial surgery?

## TL;DR

This study finds the best electrode setting for eliciting lower extremity motor evoked potentials during brain surgery, balancing low stimulation thresholds and high selectivity.

## Contribution

The study introduces and evaluates a novel hemispheric electrode montage for transcranial motor evoked potentials.

## Key findings

- C3/4 ↔ Cz + 6 achieved the highest side selectivity for lower extremity motor evoked potentials.
- C3 ↔ C4 required the lowest mean motor threshold to elicit lower extremity motor evoked potentials.
- C3/4 ↔ Cz + 6 showed high success rates and may avoid bypass stimulation of deeper brain structures.

## Abstract

Eliciting lower extremity transcranial motor evoked potentials (LE-tcMEP) during supratentorial surgery can be challenging, as it often requires high current intensities, bearing the risk of bypass stimulation with false positive results. The aim of this study was to evaluate a new hemispheric electrode combination (C3/4 ↔ Cz + 6), and compare it with established ones, determining the best scalp electrode setting with the minimal motor thresholds (MT) to elicit LE-tcMEP.

Patients undergoing surgery for supratentorial lesion removal requiring intraoperative neuromonitoring were prospectively included. TcMEP were elicited using montages C1 ↔ C2, C3 ↔ C4, C3/4 ↔ Cz, C3/4 ↔ Cz + 6 and Cz ↔ Cz + 6. MT was established for each muscle and montage to determine the electrode combination with the lowest MT and highest selectivity.

Based on 5880 measurements (70 patients), we found that the mean MT for eliciting LE-tcMEPs was lowest for C3 ↔ C4 (83.0 mA, p = 0.001), followed by C3/4 ↔ Cz + 6 (96.5 mA). Successfully eliciting contralateral LE-tcMEP was best achieved with C3 ↔ C4 (99.2 %, p < 0.0001), followed by C3/4-Cz + 6 (98.9 %). C3/4 ↔ Cz + 6 achieved the highest side selectivity (p < 0.001).

While electrical stimulation at C3 ↔ C4 requires the lowest MT to elicit LE-tcMEPs, stimulation at C3/4 ↔ Cz + 6 was the most selective.

C3/4 ↔ Cz + 6 may be a superior choice in supratentorial surgery, avoiding bypass-stimulation of deeper (e.g. brainstem) corticospinal fibers.