gms | German Medical Science

Objective: To determine whether the facial nerve (FN) transcranial motor-evoked potential (FNMEP) during neurosurgical interventions is a robust modality of intraoperative neurophysiological monitoring.

Method: We included 200 consecutive surgeries 2011 - 2015 (mean patient age 43 ± 21y, 102 female) that were operated on with FNMEP monitoring. We routinely stimulated transcranially with a train of 5 pulses (width 0.5 ms, interval 2 ms) and ruled out peripheral conduction with an single control pulse preceding the train by >20ms. During surgery, we maintained constant response amplitude by increasing the stimulation intensity and aimed to establish a warning criterion based on the “threshold-level” method. A threshold increase of greater than 20 mA for eliciting the FNMEP in the most reliable facial nerve target muscle was considered evidence for possibly reduced postoperative facial nerve function and communicated to the surgeon.

Results: Monitoring of the FNMEP was feasible in 186/200 surgeries (93%) in at least one facial nerve target muscle. The orbicularis oris muscle yielded the best result (80% of the trials). The mean stimulation threshold was 74 mA for the FNMEP and 60 mA for the motor evoked potential (MEP) of the thenar muscles. A threshold increase was communicated in 50/186 surgeries (27%).

Conclusions: FNMEP monitoring can be applied routinely during neurosurgical interventions. Stimulation intensity is in the same range as upper limb MEP. FNMEP is complementary to direct electrical FN stimulation and continuous EMG monitoring of FN target muscles.