Quantitative Navigated Transcranial Magnetic Stimulation Motor Mapping and nTMS-Guided Corticospinal Tractography in Healthy Individuals and Patients with Motor-Eloquent Brain Tumors

Background/Objectives: Navigated transcranial magnetic stimulation (nTMS) enables presurgical motor mapping, while nTMS-seeded diffusion tensor imaging (DTI) delineates the corticospinal tract (CST). We compared cortical and tractography measures between healthy individuals and patients with motor-eloquent brain tumors and between patient hemispheres. Methods: Twenty healthy volunteers and 18 patients underwent first dorsal interosseous (FDI) mapping at 110% resting motor threshold (rMT). Motor map metrics, maximum FDI motor-evoked potential (MEP) amplitude, and nTMS-seeded CST measures were analyzed; hotspot and center-of-gravity (CoG) dispersion were assessed in common space. DTI metrics were available for 13 healthy volunteers and 13 patients. Results: Consistent with previous nTMS literature, patients showed impaired motor performance and suprathreshold corticospinal output, with lower grip strength, maximum FDI MEP amplitude, and map volume than controls. rMT did not differ significantly, indicating that threshold excitability may be unrevealing despite motor system dysfunction. Notably, the contralateral patient hemisphere also differed from controls in grip strength, maximum FDI MEP amplitude, reconstructed CST fiber length, and mean axial diffusivity, while these measures did not differ significantly between the ipsilateral and contralateral patient hemispheres. CoG dispersion was greater in patients, whereas hotspot dispersion was not. Conclusions: The study confirms tumor-associated impairment of motor performance and suprathreshold corticospinal output while identifying two potentially novel observations: abnormalities extending to the apparently unaffected hemisphere and increased heterogeneity of the amplitude-weighted motor map center. Notably, contralateral abnormalities may reflect bilateral network changes or patient-related factors, rather than isolated contralateral CST pathology. Independent healthy controls may therefore provide information not captured by within-patient hemispheric comparisons.