Resection of brain tumors located near eloquent motor regions presents an enduring surgical challenge. Neurosurgeons continually balance maximal cytoreduction against the danger of inducing permanent neurological deficits. Recent evidence indicates that individual functional adaptation determines how patients recover from peri-rolandic lesions. Therefore, evaluating the neuronal reserve in glioma cases provides crucial prognostic insights into patient rehabilitation. Advanced neurophysiological tools, including navigated transcranial magnetic stimulation and diffusion tensor imaging tractography, enable clinicians to trace motor network plasticity longitudinally. Consequently, understanding these neuroplastic dynamics helps surgical teams optimize operative approaches and protect motor function.

Mechanisms of Plasticity and Rolandic Tumor Challenges

Tumors invading the primary and secondary motor areas disrupt complex sensorimotor circuits. However, the human cerebral cortex possesses an inherent capacity to adapt to structural stress. When a slow-growing neoplasm infiltrates rolandic regions, functional representations often shift toward adjacent cortical areas or contralateral networks. Investigators term this dynamic adaptive capacity the neuronal reserve. This biological phenomenon explains why two patients with similar lesions often present with dramatically different clinical symptoms. In addition, surgical manipulation introduces sudden mechanical stress and localized vascular alterations. These acute disturbances can overwhelm preexisting compensation pathways if cortical resilience is inadequate. Therefore, neurosurgeons require precise non-invasive diagnostics to gauge neuroplastic adaptability before attempting aggressive resection. Historically, surgical teams relied primarily on anatomical landmarks and intraoperative direct electrical stimulation. Advanced tractography and magnetic stimulation effectively bridge this clinical gap. Consequently, clinicians can now characterize functional integrity before skin incision and throughout long-term rehabilitation.

Assessing Neuronal Reserve in Glioma Resection

The recent prospective study investigated motor reorganization in patients harboring World Health Organization grade II to IV motor-eloquent gliomas. The investigators deployed navigated transcranial magnetic stimulation to measure the cortical representation of the first dorsal interosseous muscle across both hemispheres. In addition, they evaluated resting motor thresholds and recruitment curve slopes to measure corticospinal excitability accurately. They also examined cortical silent periods to quantify intracortical inhibition. Furthermore, the team performed diffusion tensor imaging tractography to evaluate the structural integrity of the corticospinal tract. They assessed fractional anisotropy and apparent diffusion coefficients within key motor pathways. The researchers compared twenty-seven surgical patients against twenty-eight healthy controls to establish normative baselines. Subsequently, they re-examined seventeen patients at a median follow-up of seven months after surgery. This multi-modal framework permitted direct correlation between structural tract integrity and electrophysiological adaptation. Consequently, the findings offer objective parameters to quantify reserve capacity in motor-eloquent glioma management.

Electrophysiological Signatures of Motor Network Reorganization

The neurophysiological findings revealed significant functional shifts across the follow-up period. Preoperatively, forty-seven percent of patients presented with measurable motor paresis. However, at the seven-month follow-up evaluation, the paresis rate diminished remarkably to twenty-three percent. Navigated stimulation demonstrated that the resting motor threshold difference between the affected and unaffected hemispheres normalized significantly over time. In addition, cortical silent periods shortened during follow-up, which reflects decreased intracortical inhibition and heightened cortical excitability. This finding suggests that functional recovery depends heavily on disinhibition within surviving peri-lesional cortex. Moreover, the investigators observed clear spatial reshaping of the motor cortex, marked by hotspot shifts and area resizing. These physiological adaptations indicate that the motor cortex actively recalibrates its functional map to overcome local tissue destruction. As patients recovered motor strength postoperatively, their recruitment curve slopes gradually approached normative physiological values. Therefore, electrophysiological measurements reflect the dynamic restoration of pyramidal tract efficiency.

Structural White Matter Integrity and Recovery Determinants

Diffusion tensor imaging tractography revealed essential relationships between white matter architecture and neurological performance. Specifically, disintegrity of the corticospinal tract correlated strongly with persistent motor deficits. When fractional anisotropy values dropped along subcortical motor pathways, functional recovery remained limited. Furthermore, patients presenting preoperatively with smaller motor representation areas and lower cortical excitability faced a significantly higher risk of postoperative motor decline. This crucial observation highlights that depleted baseline cortical resources predict vulnerability to surgical disruption. Conversely, patients who maintained robust motor area volume and excitability withstood surgical manipulation far better. Therefore, structural white matter tracking must accompany cortical surface mapping during preoperative risk stratification. Preserving subcortical tract integrity provides the essential physical substrate through which cortical reorganization occurs. Without intact subcortical connectivity, even highly plastic cortical areas cannot transmit motor commands effectively to peripheral effectors. Consequently, combining subcortical diffusion metrics with cortical stimulation metrics yields a comprehensive map of functional vulnerability.

Translating Neuroplasticity Insights into Surgical Strategy

These neurophysiological insights transform modern neuro-oncological management from static boundary demarcation to dynamic functional preservation. When planning resections near rolandic cortex, neurosurgeons should integrate preoperative magnetic stimulation maps into intraoperative navigation systems. Furthermore, identifying patients with reduced baseline neuronal reserve alerts the surgical team to high functional vulnerability. In these susceptible patients, surgeons can modify intraoperative trajectories, employ continuous motor evoked potential monitoring, and adjust resection margins prudently. Additionally, mapping identifies displaced motor hotspots that deviate from standard anatomical landmarks. This prevents accidental injury to functionally reorganized cortical zones located outside the classic precentral gyrus. Postoperatively, longitudinal stimulation mapping tracks functional recovery and guides targeted neuro-rehabilitation protocols. Neurologists and physiatrists can tailor rehabilitation therapies to harness reduced intracortical inhibition during the early postoperative window. Ultimately, incorporating non-invasive mapping into standard clinical workflows maximizes safe tumor resection while safeguarding patient independence and quality of life.

What defines neuronal reserve in the context of brain tumor surgery?

Neuronal reserve represents the brain's innate capacity to adapt, compensate, and reorganize functional networks following pathological stress or surgical trauma. In glioma surgery, this reserve manifests as cortical map reshaping, decreased intracortical inhibition, and preserved corticospinal tract integrity, allowing patients to maintain or regain motor function despite localized structural tissue disruption.

How does navigated transcranial magnetic stimulation aid glioma surgery planning?

Navigated transcranial magnetic stimulation non-invasively maps cortical motor representations with millimeter precision before surgery. It identifies displaced motor hotspots, measures baseline corticospinal excitability, and quantifies intracortical inhibition. Surgical teams integrate these mapping data into intraoperative neuronavigation systems to select safe surgical corridors and avoid critical functional cortex during maximal resection.

Why is diffusion tensor imaging tractography essential alongside cortical mapping?

While cortical stimulation maps surface motor representations, diffusion tensor imaging tractography visualizes the underlying subcortical corticospinal tracts. Preserving subcortical white matter integrity is vital because damaged descending fibers prevent functional recovery regardless of cortical plasticity. Combining both techniques ensures comprehensive structural and functional protection throughout the entire motor network pathway.

Disclaimer: This content is for informational and educational purposes only... Refer to the latest local and national guidelines for clinical practice.

References

1. Roth F et al. The neuronal reserve in glioma surgery: functional reorganization of the motor network examined by navigated transcranial magnetic stimulation and diffusion tensor imaging tractography. J Neurosurg. 2025 Sep 01. doi: 10.3171/2025.1.JNS241103. PMID: 40315603.
2. Jefferson D, Picht T, Moritz I, Roth F, Burzlaff M, Vajkoczy P, Zdunczyk A. The Neuronal Reserve Capacity Defines the Neurological Outcome in Motor-Eloquent Glioma Patients. Neurosurgery. 2026 Jul 13. doi: 10.1227/neu.0000000000004143. PMID: 42439542.
3. Krieg SM, Sabih FS, Bulubas L, et al. Navigated transcranial magnetic stimulation improves the treatment outcome in patients with brain tumors in motor eloquent locations. Neuro Oncol. 2014;16(10):1365-1372. doi: 10.1093/neuonc/nou110.