In Reply: Interpretation and Strategy to Resolve Neuromonitoring Changes Associated With Brain Sag

To the Editor: We appreciate the shared enthusiasm for multimodality intraoperative neuromonitoring (IONM) shared by Sampath et al.1 Our study highlighted the recognition and interpretation of neuromonitoring changes associated with brain sag and introduced saline irrigation strategies to distinguish false-positive loss of sensorimotor IONM signals caused by brain sag from true injuries.2 Alarms that were attributed to brain sag were consistently resolved with saline irrigation in our practice, whereas those persisting despite irrigation were considered true neurophysiological changes. This serves to augment our standard intraoperative neuromonitoring repertoire, which incorporates multimodality techniques including transcranial motor-evoked potentials (MEPs), direct cortical MEPs when the primary motor cortex is accessible, and dynamic subcortical motor mapping, among other strategies appropriate to the individual case.3 Brain sag was estimated by visual inspection of the distance between the cortical surface and the inner surface of the calvarium. Intraoperative ultrasound is used in nearly all cases by our team, but does not necessarily augment the quantification of brain sag, as ultrasound contacts the cortical surface at the operative site, whereas brain sag can occur distant from the operative site depending on positioning, tumor location, tumor size, and the approach chosen. The incidence of brain shift–related IONM alarms varies with tumor size, location, and experience. Resection of larger intra-axial tumors, leading to greater deformation of the overlying cortex, as well as approaches within or abutting the ventricle, leading to significant egress of cerebrospinal fluid, are more commonly associated with brain sag. Saline irrigation is delivered to any accessible cavity that allows restoration of the cortical surface, whether it is the tumor cavity for superficial pathologies or the ventricle when operating in or next to the ventricle. With experience, the recognition of neurophysiological changes suggestive of brain sag becomes readily apparent and prompts saline infusion as a rapid intraoperative assay with minimal interruption of flow. The exact incidence of brain sag–related changes is difficult to quantify retrospectively because this has become a routine observation during deep-seated or large tumor resections in our practice. Electrode placement for supratentorial surgeries is optimized through close collaboration between the neurosurgeon and intraoperative neurophysiologists. Our team has previously used hemispheric C3-Cz and C4-Cz montages with success. However, our practice has evolved to rely on the interhemispheric-linked quadripolar stimulation paradigm in recent years because of its more consistent reliability of responses with low instances of contralateral crossover stimulation. As highlighted by Sampath et al, confidence in the location of the central sulcus and primary motor cortex is a prerequisite for guiding grid placement to elicit reliable direct cortical MEPs. We incorporate dynamic subcortical motor mapping with a monopolar stimulating suction as a standard practice for deep-seated tumors involving or adjacent to corticospinal tract to facilitate safe and effective resection.3,4 Signals of subcortical mapping were rarely influenced by brain sag and, thus, were not extensively discussed in this study. We hope that such exchange of experiences will continue to motivate constant innovation in the field of intraoperative neuromonitoring, for the collective goal of optimizing patient outcomes.