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.

ABSTRACT

OBJECT A comparison of two electrophysiological methods used to assess facial nerve function intraoperatively was conducted in 33 patients with tumors of the cerebellopontine angle. METHODS: All 33 patients had presented with normal facial nerve function preoperatively. After general anesthesia had been induced by a mixture of midazolam and fentanyl, continual online EMG recordings from the orbicularis oculi and oris muscles were alternated with nasal muscle F-wave recordings. Facial nerve outcomes, assessed using a modified House-Brackmann scale, varied among good (48%), moderate (18%), and poor (33%). Analysis of electromyographic (EMG) data resulted in a significant correlation between the finding of only transient manipulation-evoked activity and a good outcome, whereas 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 muscle 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 good or moderate facial nerve outcomes. A transient loss of the F wave, however, was detected significantly more frequently in patients with moderate outcomes. None of these patients exhibited normal facial function (House-Brackmann Grade I) postoperatively. CONCLUSIONS: Online EMG monitoring can provide some information on imminent or even present damage to the facial nerve intraoperatively. The diagnostic sensitivity, specificity, and positive predictive values of a permanent F-wave loss, however, are much higher than those of EMG monitoring. Additionally, this loss of the F wave is supposed to be transient if the surgical procedure 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 lesion.