Surgery of language-eloquent tumors in patients not eligible for awake surgery: the impact of a protocol based on navigated transcranial magnetic stimulation on presurgical planning and language outcome, with evidence of tumor-induced intra-hemispheric plasticity

Objectives: Awake surgery and intraoperative monitoring represent the gold standard for surgery of brain tumors located in the perisylvian region of the dominant hemisphere due to their ability to map and preserve the language network during surgery. Nevertheless, in some cases awake surgery is not feasible. This could increase the risk of postoperative language deficit. Navigated transcranial magnetic stimulation (nTMS) and nTMS-based DTI fiber tracking (DTI-FT) provide a preoperative mapping and reconstruction of the cortico-subcortical language network. This can be used to plan and guide the surgical strategy to preserve the language function. The objective if this study is to describe the impact of a non-invasive preoperative protocol for mapping the language network through the nTMS and nTMS-based DTI-FT in patients not eligible for awake surgery and thereby operated under general anesthesia for suspected language-eloquent brain tumors. Patients and methods: We reviewed clinical data of patients not eligible for awake surgery and operated under general anaesthesia between 2015 and 2016. All patients underwent nTMS language cortical mapping and nTMS-based DTI-FT of subcortical language fascicles. The nTMS findings were used to plan and guide the maximal safe resection of the tumor. The impact on postoperative language outcome and the accuracy of the nTMS-based mapping in predicting language deficits were evaluated. Results: Twenty patients were enrolled in the study. The nTMS-based reconstruction of the language network was successful in all patients. Interestingly, we observed a significant association between tumor localization and the cortical distribution of the nTMS errors (p = 0.004), thereby suggesting an intra-hemispheric plasticity of language cortical areas, probably induced by the tumor itself. The nTMS mapping disclosed the true-eloquence of lesions in 12 (60%) of all suspected cases. In the remaining 8 cases (40%) the suspected eloquence of the lesion was disproved. The nTMS-based findings guided the planning and surgery through the visual feedback of navigation. This resulted in a slight reduction of the postoperative language performance at discharge that was completely recovered after one month from surgery. The accuracy of the nTMS-based protocol in predicting postoperative permanent deficits was significantly high, especially for false-eloquent lesions (p = 0.04; sensitivity 100%, specificity 57.14%, negative predictive value 100%, positive predicitive value 50%). Conclusions: The nTMS-based preoperative mapping allows for a reliable visualization of the language network, being also able to identify an intra-hemispheric tumor-induced cortical plasticity. It allows for a customized surgical strategy that could preserve post-operative language function. This approach should be considered as a support for neurosurgeons whenever approaching patients affected by suspected language-eloquent tumors but not eligible for awake surgery.