Subcortical Stimulation in Brain Tumor Surgery: A Closer Look Beneath the Surface

Maximizing the patient’s onco-functional balance is the central tenet of brain tumor surgery. Consequently, numerous surgical adjuncts have been developed to facilitate identification of the tumor–brain interface and preservation of functional anatomy. Among these, intraoperative neurophysiologic monitoring (IONM) with direct cortical stimulation (DCS) and subcortical stimulation (SCS) remains the gold standard for real-time, functional mapping of motor and language activity. Stimulation techniques are not standardized and vary significantly across institutions, however. This is particularly true with SCS for mapping motor function. We reviewed the state of subcortical IONM and mapping techniques, including historical and predicate literature as well as new and emerging techniques. Here we discuss their evolution, clinical utility, and limitations to direct future research and application. Our evaluation of the background and current clinical use of DCS and SCS techniques and protocols and identification of current trends and limitations focuses specifically on methods of SCS, given the heterogeneity in the published literature. We suggest directions to optimize the clinical utility of these tools. Despite significant heterogeneity in published techniques, trends support use of the Taniguchi method for SCS. Novel dynamic stimulation techniques may improve accuracy. Prospective studies to define standardized guidelines are needed.