gms | German Medical Science

Objective: We have reported the usefulness of ‘threshold-method’ transcranial facial nerve motor evoked potential (fMEP) (Goto, Skull Base, 2010). In this method, however, the threshold of fMEP sometimes changes during the procedures which did not damage the facial nerve. We hypothesized that this false threshold change resulted from the changes in measuring environment such as CSF evacuation and brain retraction. These can be reflected by the threshold change of the abductor brevis pollicis (ABP) MEP which cannot be affected by the surgical procedure. We examined whether the ratio of facial/ ABP MEP threshold change can be used for quantitative analysis for postoperative facial nerve function and exclude the false threshold change.

Methods: Among 66 consecutive cases of facial MEP monitoring, facial/ ABP ratio was added for 30 posterior extraaxial mass removal surgeries harboring facial nerve palsy between 2007 and 2011. Twenty-seven patients had no facial palsy and 3 had House-Brackmann (H-B) grade 2 palsy preoperatively. The threshold of the orbicularis oris muscle and ABP MEP were recorded, and the ratio of oris/ ABP (R) was calculated simultaneously. Chronological change of the threshold and the R was feed backed to a surgeon, and manipulation was changed or ceased on the surgeon's decision. We calculated the change ratio of R_final/ R_initial and analyzed correlation between the change and postoperative facial nerve function at one month after surgeries.

Results: Twenty-seven patients without preoperative facial palsy resulted in 22 of H-B grade 1 and 2, and 4 of grade 3 and 1 each of grade 4 and 5. Value of R_final/ R_initial and postoperative facial nerve function showed statistical significance between H-B grade 1 and 2, and grade 3 at the cut-off value of 1.15 (p < 0.05, Student t-test). Three patients with preoperative facial palsy showed no statistical significance between R and postoperative facial nerve function. R was sometimes unchanged when threshold of facial and ABP MEP increased simultaneously during manipulation which did not affect the facial nerve function.

Conclusions: The ‘Threshold-method’ of transcranial facial MEP monitoring is useful for intraoperative facial nerve function monitoring. The combination with ratio change of facial/ ABP MEP threshold can facilitate a more specific and quantitative intraoperative facial nerve function monitoring.