Letter to editor re: “Stimulation selectivity in transcranial motor evoked potentials for monitoring during surgery for supratentorial lesions”

To the Editor.

We read with interest the recent article by Abboud et al (2026) examining the selectivity of different transcranial electrical stimulation (TES) montages for eliciting motor evoked potentials during surgery for supratentorial lesions. The authors should be commended for addressing an important topic in intraoperative neurophysiology, as electrode configuration may influence stimulation thresholds and the interpretation of transcranial motor evoked potentials (TCMEPs). Their prospective comparison of C1-C2, C3-C4, and C3/4-CZ configurations provides useful threshold data. However, several methodological and physiological considerations merit further discussion.

First, the study implicitly assumes that electrodes placed according to the international 10–20 system reliably correspond to the intended cortical targets across all cases. In routine supratentorial surgery, electrode placement is frequently modified to accommodate the craniotomy, surgical corridor, Mayfield fixation pins, and sterile draping. Consequently, electrodes labeled as C1, C2, C3, C4, and Cz may not consistently overlie canonical scalp coordinates relative to the motor cortex. Given the heterogeneity of lesions included in the study, the effective cortical targets of stimulation may therefore have differed between cases, potentially influencing current pathways and stimulation thresholds attributed to each montage.

Second, the methods do not clearly describe the interval between stimulation trials when testing the different electrode configurations. TCMEPs are known to demonstrate facilitation with repeated stimulation trains, particularly when trials are performed sequentially with short inter-stimulation intervals. Temporal summation within corticospinal neurons and spinal motor pools can lower apparent thresholds and increase the likelihood of bilateral responses during repeated stimulation. Without controlling for refractory or facilitation effects, sequential threshold determination across multiple montages may introduce systematic bias in the measured stimulation thresholds. In addition, although the authors report an interval of approximately 65 min between neuromuscular blockade administration and TES testing, the absence of Train-of-Four monitoring makes it difficult to exclude residual neuromuscular blockade as a potential contributor to variability in stimulation thresholds.

Third, the interpretation of ipsilateral muscle responses warrants further clarification. The authors attribute ipsilateral responses primarily to stimulation of deeper corticospinal pathways, including the brainstem. While deep pathway recruitment is possible at higher stimulus intensities, alternative explanations should also be considered. High-intensity TES produces broad current fields capable of bilateral cortical activation, cathodal excitation of the ipsilateral hemisphere, and transcallosal recruitment. Ipsilateral responses during TES have been described as a cathodal phenomenon in which activation of the cortex beneath the cathode can generate motor responses without requiring deep corticospinal tract activation (Wilkinson et al, 2023). Computational modeling studies further demonstrate that TES current density is strongly influenced by skull conductivity, electrode geometry, and gyral anatomy, often producing bilateral cortical activation even at moderate stimulation intensities (Holdefer et al., 2006, Guo et al., 2023). In this context, ipsilateral responses may not uniquely reflect deep corticospinal or brainstem activation.

Finally, the clinical implications of the proposed “selectivity ratio” remain uncertain. Although differences between montages were observed, only a single electrode configuration was used during intraoperative monitoring throughout the surgical procedure. Consequently, the study does not allow comparison of monitoring performance between montages with respect to clinically relevant outcomes such as sensitivity, specificity, or prediction of postoperative motor deficits. Additionally, recordings were obtained exclusively from the abductor pollicis brevis, which predominantly reflects distal hand motor cortex activation and may not fully represent corticospinal tract recruitment relevant to proximal or lower-extremity motor pathways. Differences in corticospinal tract displacement or infiltration between tumor types, including gliomas, metastatic tumors and meningiomas, may also influence stimulation thresholds and current distribution.

In clinical practice, stimulation montages are frequently optimized intraoperatively to obtain robust contralateral responses while minimizing ipsilateral coactivation rather than relying on a single predefined electrode configuration. Ipsilateral muscle recordings are often used as control channels to identify nonselective or unintended deep stimulation. Furthermore, outcome-based studies in brain tumor surgery have demonstrated that TCMEP monitoring may exhibit high specificity but relatively low sensitivity for postoperative motor deficits, suggesting that monitoring performance is influenced by multiple physiological and technical factors beyond stimulation montage alone (Silverstein et al 2023).

Despite these considerations, the study highlights an important topic in TCMEP monitoring and contributes useful data regarding stimulation thresholds across commonly used electrode configurations. Further investigations incorporating standardized electrode positioning, controlled stimulation sequencing, and correlation with intraoperative monitoring outcomes may help clarify the physiological mechanisms underlying montage-dependent differences in TCMEP responses.

Disclosure

The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.