Combined Motor Evoked Potential Monitoring and Subcortical Dynamic Mapping in Motor Eloquent Tumors Allows Safer and Extended Resections

BACKGROUND: Subcortical motor mapping is crucial to ensure preservation of motor tracts during resections of tumors. Continuous dynamic mapping using a modified monopolar suction probe is a novel and effective way of achieving this goal. We describe our experience using this technique. METHODS: Forty patients were operated on between June 2017 and February 2018. Transcranial electric stimulation (TES) and direct cortical strip (DCS) motor evoked potentials (MEPs) were monitored. Subcortical stimulation (SCS) mapping was attempted in all cases by adapting the technique using a monopolar suction stimulator. Neurologic outcomes and extent of resection were analyzed. RESULTS: Motor fibers were between 0 and 7.6 mm away from the tumor boundaries in 26 patients. TES MEP were monitored in all patients. DCS MEPs were attempted in 31 and successfully monitored in 26 patients. SCS using the monopolar suction probe worked in all patients except one. SCS elicited no responses at 15-mA motor thresholds in 16 patients. The motor threshold ranged from 3 to 10 mA in the remainder. Overall, MEP changes were noted in 5 patients. Ten patients experienced neurologic worsening (6 transient, 3 prolonged, and 1 delayed). There was no permanent deficit at 3 months. DCS MEPs predicted neurologic worsening better than did TES MEPs. Radical resections were achieved in 68%. Neuromonitoring inputs resulted in premature termination of resection in 14 patients (35%). Radical resections (near-total resections) were still achieved in 8 of the 14 patients. CONCLUSIONS: Continuous dynamic subcortical mapping is a reliable method to map the motor tracts. This process is crucial to correctly identify truly eloquent tumors and tailor the surgical procedure as per planned goals, maximizing the resections with acceptable morbidity.