gms | German Medical Science

Objective: During surgeries that put the facial nerve (FN) at risk of injury, FN function can be continuously monitored by transcranial FN motor evoked potentials (FNMEPs) in FN target muscles. Despite their advantages, FNMEPs are not yet widely used. While most authors use a 50% reduction of FNMEP 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.

Method: We included 31 consecutive adult patients (median age 46 years) where FNMEPs were monitored. A threshold increase >20mA for eliciting FNMEPs in the most reliable FN target muscle was considered a prediction of reduced postoperative FN function and subsequently a warning was issued to the surgeon. Pre-and early postoperative function was documented using the House-Brackmann (HB) grading system.

Results: FNMEP monitoring was feasible in all 31 surgeries in at least one FN target muscle. The mentalis muscle yielded best results. HB grade deteriorated in 14/31 patients (45%). The warning criterion was reached in 16/31 patients (52%), which predicted an 81% risk of HB deterioration. Sensitivity amounted to 93% (CI 66%-100%) and specificity to 82% (CI 57%–96%). FNMEP deterioration and deteriorated HB grade showed a high degree of association (p<0.001). The impact of FNMEP monitoring on surgical strategy is exemplified in one illustrative case.

Conclusions: In surgeries that put the facial nerve at risk, the intraoperative increase in FNMEP stimulation threshold was closely correlated to postoperative facial nerve dysfunction. FNMEP monitoring is a valid indicator of FN function in skull base surgery. It should be used as an adjunct to direct electrical FN stimulation and continuous EMG monitoring of FN target muscles.