Clinical Report: Impact of Clinical and Anatomical Factors on Resting Motor Threshold in nTMS

Overview

This study analyzed resting motor threshold (RMT) variability in over 600 brain tumor patients undergoing navigated transcranial magnetic stimulation (nTMS). Key factors influencing RMT included age, skull-to-cortex distance, tumor characteristics, motor deficits, and antiepileptic drug intake, highlighting the complexity of cortical excitability assessment in neurosurgical planning.

Background

Navigated transcranial magnetic stimulation (nTMS) is a critical tool for preoperative motor mapping in brain tumor patients, enabling functional delineation of motor areas. The resting motor threshold (RMT) serves as a surrogate marker of cortical excitability and guides stimulation intensity to optimize safety and efficacy. However, RMT shows substantial variability influenced by individual clinical, anatomical, and pharmacological factors. Understanding these influences is essential to refine nTMS protocols and improve surgical outcomes.

Data Highlights

Key Findings

• RMT increases with age, partly due to increased skull-to-cortex distance from brain atrophy.
• Presence of motor deficits correlates with significant RMT variability.
• Sodium channel–blocking antiepileptic drugs, such as carbamazepine, consistently elevate RMT values.
• Tumor characteristics including central location, volume, and peritumoral edema significantly affect RMT.
• Pharmacological effects on RMT are complex; some antidepressants increase RMT, while benzodiazepines show no direct effect.
• Large cohort analysis allowed simultaneous evaluation of multiple interacting factors influencing RMT.

Clinical Implications

Clinicians should consider patient age, skull-to-cortex distance, tumor-related factors, and concurrent medications when interpreting RMT values during nTMS. Adjusting stimulation parameters based on these variables may enhance mapping accuracy and patient safety. Awareness of AED effects, especially sodium channel blockers, is critical to avoid overstimulation and optimize preoperative planning.

Conclusion

This comprehensive evaluation of clinical and anatomical influences on RMT in a large brain tumor cohort underscores the multifactorial nature of cortical excitability. Integrating these factors into nTMS protocols can improve functional mapping precision and support safer neurosurgical interventions.

References

1. Multiple sources (2007-2023) -- Exploring the Resting Motor Threshold in Navigated Transcranial Magnetic Stimulation