1246 The Impact of Latency Variations on the Predictive Value of Facial Nerve Proximal-to-distal Amplitude Ratio During Vestibular Schwannoma Surgery

INTRODUCTION: Intraoperative facial nerve (FN) electromyographic (EMG) monitoring is now considered an essential part of vestibular schwannoma (VS) surgery and is used for both intraoperative monitoring and postoperative outcome prediction. METHODS: Forty-eight patients underwent FN stimulation at the brainstem (proximal) and the internal acoustic meatus (distal) using a current intensity of 2 mA. The proximal latency and the P/D amplitude ratio were assessed. House-Brackmann grades I-II indicated good FN function; and grades III-VI were considered fair/poor function. A P/D amplitude ratio >0.6 was used as a cutoff to indicate good FN function, while a ratio of =0.6 indicated fair/poor FN function. RESULTS: The P/D amplitude ratio was measured for all patients, and the calculated sensitivity (SE), specificity (SP), positive predictive value (PPV), and negative predictive value (NPV) were 85.2%, 85.7%, 88.5%, and 81.8% respectively. Then the CMAPs from the mentalis muscle were classified based on their proximal latency into: Group I (<6 msec); group II (6-8 msec); and group III (>8 msec). The SE, SP, PPV, and NPV became 90.5%, 90.9%, 95%, and 83.3% respectively in group II. In group I, SE and NPV increased whereas, SP and PPV decreased. While in group III, SP and PPV increased, whereas SE and NPV decreased. CONCLUSIONS: At a latency between 6-8msec, the P/D amplitude ratio was predictive of outcome with high SE and SP. When the latency was <6 msec or >8 msec, the same predictive ability was not observed. Knowing the impact of latency variations of P/D amplitude ratio measurement is important for a better understanding of the ability and limitations of this method in predicting postoperative FN function.