P40. Intraoperative monitoring of corticobulbar motor evoked potentials in children

Abstract

The motor function of cranial nerves (CN) can be continuously monitored by transcranial corticobulbar motor evoked potentials (CoMEPs) during neurosurgical interventions. Despite their advantages, CoMEPs are not yet widely used. While there are several publications of CoMEPs in adults (Sarnthein et al., 2013), the feasibility and safety of CoMEPs in children has not yet been documented. We included 13 consecutive procedures involving 12 patients (median age 5 y, range 1–15 y, 4 female) that were operated since 2013 and in whom CoMEPs were monitored. While most authors use a 50% reduction of CoMEP response amplitudes as a warning criterion, our approach was to keep the response amplitude constant by increasing the stimulation intensity and to establish a warning criterion based on the “threshold-level” method. For the facial nerve, a threshold increase greater than 20 mA for eliciting CoMEPs in the most reliable facial nerve target muscle was considered a prediction of reduced postoperative facial nerve function, and subsequently a warning was issued to the surgeon. Preoperative and early postoperative function was documented using the House-Brackmann (HB) grading system. Monitoring of CoMEPs was feasible in all 13 surgeries in at least one facial nerve target muscle. The mentalis muscle yielded the best result (92% of trials), followed by orbicularis oris (91%) and orbicularis oculi muscles (86%). The median stimulation threshold was initially 60 mA (range 40–100 mA) for CoMEPs and 60 mA (15–95 mA) for MEP of the thenar muscles. The initial CoMEP threshold exceeded the MEP threshold in 7/13 cases. CoMEP deterioration showed specificity for HB deterioration of 83%. Intraoperative CoMEP monitoring is feasible and safe also in young children. Stable facial nerve CoMEP predict stable HB grade. We found no evidence that procedures and thresholds should differ from CoMEP in adults. CoMEP monitoring is a valid indicator of CN function in skull base surgery. It should be used as an adjunct to direct electrical CN stimulation and continuous EMG monitoring of CN target muscles.