Stimulation selectivity in transcranial motor evoked potentials for monitoring during surgery for supratentorial lesions

Objective: To identify the most selective electrode configuration for monitoring of transcranial motor evoked potentials (TcMEP) during supratentorial tumor surgery and determine the stimulation threshold at which bilateral corticospinal tract (CST) activation occurs at deeper white matter levels. Methods: Fifty-six patients undergoing resection of supratentorial tumors without preoperative motor deficits were prospectively included. TcMEPs were elicited under general anesthesia using three electrode configurations: C1 ↔ C2, C3 ↔ C4, and C3/4 → Cz. Motor thresholds (MT) were recorded bilaterally from the abductor pollicis brevis (APB), and the selectivity ratio (ipsilateral/contralateral MT) was calculated. Electrode combinations were compared using paired t-tests. Linear regression assessed the influence of age, sex, height, and weight. Results: The C3/4 → Cz montage demonstrated the highest selectivity ratio (median 248%, range: 140-364%), significantly greater than C1 ↔ C2 and C3 ↔ C4 (p < 0.001). Bilateral CST activation with C3/4 → Cz began at 140% of the contralateral MT. Age correlated with lower MTs; sex, height, and weight had no significant effect. Conclusions: C3/4 → Cz is the most selective configuration for eliciting contralateral TcMEP and requires higher stimulation intensity before activating bilateral CST fibers at deeper levels. Significance: This study supports using C3/4 → Cz to optimize TcMEP selectivity and reduce the risk of false-negative monitoring due to deep white matter stimulation.