Interpretation and Strategy to Resolve Neuromonitoring Changes Associated With Brain Sag.

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Intraoperative neuromonitoring (IONM) aides in safe maximal resection of brain tumors in eloquent areas. During the resection of large or deep-seated tumors, shifts in the peri-resectional parenchyma and cortical surface relative to cranial electrodes, or brain sag, can cause false-positive loss of sensorimotor signals. We describe patterns of electrophysiological changes associated with brain sag and a facile technique to rapidly resolve these changes intraoperatively by saline infusion. We reviewed cases operated on by the senior author and identified cases where brain sag was noted intraoperatively. Preoperative and postoperative imaging, operative details, and IONM electrophysiological traces were analyzed to surmise generalizable patterns. We observe stepwise decreases in somatosensory-evoked potentials and transcranial motor-evoked potentials that occur with removal of a large intraparenchymal mass or after significant loss of cerebrospinal fluid. The pattern of upper extremity or lower extremity signal change depends on the location of the lesion and the positioning of the head, depending on which regions of the motor or somatosensory cortices are most affected by brain sag. Recovery of the somatosensory-evoked potentials and transcranial motor-evoked potentials during saline infusion into the resection cavity or ventricle, which reconstitutes the cortical topography, suggests that the IONM signal change was artifactual, rather than reflecting true physiological injury. We present 6 illustrative cases that highlight distinct patterns of electrophysiological change associated with brain sag and the extent of signal restoration. Brain sag is commonly encountered during the resection of large or deep-seated tumors. Improved understanding of the spatiotemporal electrophysiological signatures of brain sag and application of the saline infusion technique to reverse false-positive drops in neuromonitoring signals improves the utility of IONM and enhances safe resection.