Brain tumors involving the primary motor cortex can disrupt corticospinal physiology through mass effect, tract involvement, and maladaptive plasticity. Preoperative transcranial magnetic stimulation (TMS) provides a direct measure of cortical excitability and correlates with intraoperative mapping, but the TMS parameters most relevant to postoperative motor outcomes remain uncertain. This study evaluated whether preoperative TMS excitability measures differ from healthy norms and whether they show signals potentially useful for predicting postoperative motor deficits. We conducted a retrospective, single-center observational study of consecutive adults (18-70 years) with glioblastoma adjacent to the precentral gyrus who underwent standardized preoperative TMS motor mapping (N = 67). Key excitability measures included resting motor threshold, motor-evoked potential (MEP) amplitudes, relative paired-pulse responses, and indices of intracortical inhibition and facilitation. Patient data from the tumor-affected hemisphere were compared with a healthy Brazilian reference cohort using robust, age-stratified median tests and categorical classification based on published normative ranges. Associations with postoperative motor deficits were explored using correlation screening followed by multivariable logistic modeling. Postoperative motor deficits occurred in approximately one-third of patients. Compared with healthy controls, the tumor-affected hemisphere demonstrated consistent alterations in paired-pulse responses and intracortical excitability. Using categorical normative ranges, patients were significantly less likely to show low excitability responses for short- and long-interval paired-pulse measures and for intracortical inhibition and facilitation, indicating a shift toward higher or dysregulated excitability. Age-stratified analyses showed the most pronounced deviations in patients younger than 50 years, particularly for MEP amplitudes and paired-pulse indices. Preoperative TMS mapping revealed that brain tumors adjacent to the motor cortex are associated with selective disturbances in intracortical and paired-pulse excitability, while global corticospinal output measures remain largely preserved. The consistent alterations in intracortical inhibition and facilitation indicate that tumor-related motor dysfunction primarily affects intracortical circuitry rather than overall corticospinal integrity. Age-stratified analyses further suggest that these excitability changes are more pronounced in younger patients, underscoring age-dependent differences in motor network vulnerability. Collectively, these findings support quantitative TMS-derived intracortical measures as physiologically informative biomarkers that extend beyond spatial motor mapping and hold promise for refining surgical risk stratification. Prospective studies linking these markers to postoperative motor outcomes are needed to establish their clinical utility.
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