Object: Surgery for insular gliomas incurs a considerable risk of motor morbidity. In this study the authors explore the validity and utility of continuous motor tract monitoring to detect and reverse impending motor impairment during insular glioma resection.
Methods: Motor evoked potentials (MEPs) were successfully monitored during 73 operations to remove insular gliomas. Seventy-two cases were assessable, and one patient died during the early postoperative course. In this prospective observational approach, MEP monitoring results were correlated with intraoperative events and perioperative clinical data. Intraoperative recordings of MEPs remained stable in 40 cases (56%), indicating unimpaired motor outcome and allowing safe completion of the hazardous steps of the procedure. Deterioration of MEPs occurred in 32 cases (44%). This deterioration was reversible after intervention in 21 cases (29%), and there was no new motor deficit except for transient paresis in nine of these cases (13%). Surgical measures could not prevent irreversible MEP deterioration in 11 cases (15%). Transient mild or moderate paresis occurred if complete MEP loss was avoided. Irreversible MEP loss in seven cases (10%) occurred after completion of resection in four of these seven cases, and was consistently an indicator of both a stroke within the deep motor pathways and permanent paresis, which remained severely disabling in three patients (4%). In contrast, permanently severe paresis occurred in two (18%) of 11 cases without useful MEP monitoring.
Conclusions: Continuous MEP monitoring is a valid indicator of motor pathway function during insular glioma surgery. This method indicates that remote ischemia, in this study the leading cause of impending motor deterioration, helps to avert definitive stroke of the motor pathways and permanent new paresis in the majority of cases. The rate of permanently severe new deficit appears to be greater in unmonitored cases.