Intraoperative Functional Mapping and Neurophysiological Monitoring in the Resection of Supratentorial Gliomas in Eloquent Areas: A Critical Narrative Review and Algorithm for Responding to Alerts

Abstract

Introduction. Resection of supratentorial gliomas located near the cortex or functionally relevant tracts requires balancing the oncological benefit of extensive resection with the risk of persistent neurological disability. Functional mapping and intraoperative neurophysiological monitoring provide distinct and complementary information: the former locates functional tissue; the latter monitors the integrity of neural systems during surgery.

Objective. To critically analyze the rationale, indications, technical parameters, alarm criteria, limitations, and clinical utility of the main mapping and monitoring modalities used in supratentorial gliomas in eloquent locations, and to propose a structured algorithm for responding to neurophysiological changes.

Methods. A structured critical narrative review was conducted, guided by the SANRA quality domains. The search was updated through June 19, 2026, in PubMed/MEDLINE; PubMed Central was used to retrieve full-text articles, and the identification process was supplemented by reference tracking. Priority was given to guidelines and position statements, systematic reviews and meta-analyses, prospective studies, large cohort studies, and case series with direct relevance to supratentorial glioma surgery. The synthesis was organized by function at risk, modality, alarm criteria, and clinical applicability.

Results. The most robust strategy is multimodal and is defined by the vulnerable function, not by a fixed combination of tests. In motor surgery, motor evoked potentials obtained through transcranial or direct cortical stimulation should be integrated with cortical and subcortical mapping. Persistent loss of response constitutes the most serious warning sign; a reduction in amplitude of more than 50%, threshold elevation, and low subcortical thresholds are paradigm-dependent indicators and not universal rules. The approximate ratio of 1 mA per 1 mm is a valid heuristic only under comparable technical conditions. Somatosensory evoked potentials provide localization through phase inversion and sensory monitoring, but they do not replace motor assessment. For speech, direct electrical stimulation with the patient awake remains the gold standard when clinical selection allows; cortico-cortical evoked potentials are complementary and still lack standardized alarm criteria. Visual monitoring shows promise, although it exhibits technical variability and is supported by limited evidence. Any alert requires ruling out technical, anesthetic, and physiological causes before attributing it to surgical injury.

Conclusions. Intraoperative neurophysiology should be used as a predefined, individualized, and documented clinical decision-making system. Its usefulness depends on consistency across modalities, anesthetic stability, expert interpretation, and an immediate response from the team. The available evidence supports an association with greater functional safety and improved extent of resection, but it comes primarily from observational studies and does not justify attributing causality to any single modality.