F-wave recordings from nasal muscle for intraoperative monitoring of facial nerve function.

The generation of the F-wave in muscle after peripheral motor nerve stimulation depends upon the functional integrity of the entire motoaxon between its axon hillock and the motor endplates. Cerebellopontine angle (CPA) surgery bears the risk of damaging the facial nerve in its root exit zone; functional degradation should therefore be reflected by changes of the nasal muscle F-wave. Constant current pulses supraliminal for direct (M) muscular responses were applied subcutaneously to the zygomatic branch of the facial nerve ipsilateral to the operated side, EEG needle electrodes were placed in the nasal muscle and referred to glabella for recording of myoelectric activity. Anesthesia was achieved by application of fentanyl, midazolame and N2O and low doses of short acting muscular relaxants; the above-mentioned narcotics did not influence F-wave recordings, whereas the administration of muscle relax ants was often deleterious to F-wave monitoring. 22 patients with different tumors in the CPA, the cerebellum or the vicinity of the brainstem were subject to this monitoring procedure up to now. F-wave monitoring was effective in 12/22 cases. In 7/12 patients the signal was stable without any resulting dysfunction postoperatively. In 3/12 cases changes in amplitude and/or latency were observed indicating a subsequent facial palsy of a moderate degree. In 2 cases the F-wave was lost-these suffered from a facial paralysis afterwards. As an illustrative case the course of one of these is pointed out. In 10/22 procedures F-wave monitoring was ineffective due to the prolonged administration of muscle relaxants. The F-wave appears to be a valid tool for intraoperative prognosis of postoperative facial nerve function in CPA surgery. The borderlines of F-wave changes indicating damage to neural function by surgical manipulation are to be defined next.