Recording nasal muscle F waves and electromyographic activity of the facial muscles: a comparison of two methods used for intraoperative monitoring of facial nerve function

Object. A comparison of two electrophysiological methods used to assess fac ial nerve function intraoperatively was conducted in 33 patients with tumor s of the cerebellopontine angle. Methods. All 33 patients had presented with normal facial nerve function pr eoperatively. After general anesthesia had been induced by a mixture of mid azolam and fentanyl, continual online EMG recordings from the orbicularis o culi and oris. muscles were alternated with nasal muscle F-wave recordings. Facial nerve outcomes, assessed using a modified House-Brackmann scale, va ried among good (48%), moderate (18%), and poor (33%). Analysis of electrom yographic (EMG) data resulted in a significant correlation between the find ing of only transient manipulation-evoked activity and a good outcome, wher eas in cases in which there was poor outcome, an increase in the amplitude or duration of ongoing. activity was detected. A permanent loss of nasal mu scle F waves specifically appeared to indicate a severe dysfunction of the facial nerve. that was linked to a poor outcome. All patients with latency and/or amplitude changes or even a transient loss of the F wave achieved go od or moderate facial nerve outcomes. A transient loss of the F wave, howev er, was detected significantly more frequently in patients, with moderate o utcomes. None of these patients exhibited normal facial function (House-Bra ckmann Grade I) postoperatively. Conclusions. Online EMG monitoring can provide some information on imminent or even present damage to the facial nerve intraoperatively. The diagnosti c sensitivity, specificity, and positive predictive values of a permanent F -wave loss, however, are much higher than those of EMG monitoring. Addition ally, this loss of the F wave is supposed to be transient if the surgical p rocedure is stopped until the F wave recovers. Therefore, F-wave monitoring serves to alert the surgeon that the facial nerve is about to receive a le sion.