Evaluation of the predictive value of intraoperative changes in motor evoked potentials of caudal cranial nerves for the postoperative functional outcome

Abstract

Objective: In this present study, we investigated the predictive value of changes in intraoperatively acquired motor evoked potentials (MEPs) of the caudal cranial nerves CN.IX (glossopharyngeal nerve) and CN.XII (hypoglossal nerve) for the operative outcome. Methods: MEPs of the glossopharyngeal (CN.IX) and hypoglossal nerve (CN.XII) were recorded intraoperatively from 63 consecutive patients undergoing brain surgery. The collected data included the patient’s age, gender, diagnosis, positioning during surgery, MEP baseline, final and final- to- baseline MEP ratio of CN.IX and CN.XII and the contralateral abductor pollicis brevis muscle of the hand as a control as well as the pre- and postoperative nerve function of CN.IX and CN.XII. We correlated the changes of the MEPs to postoperative nerve function such as dysphagia, impairment of the gag reflex, uvula deviation, and tongue deviation. Results: For the glossopharyngeal nerve, we found a significant correlation between the amplitude (µV) of the final-to-baseline MEP ratio and uvula deviation (p=0.028) and the amplitude duration (ms) of the final MEP and gag reflex function (p=0.027). The analyses of the risk estimate revealed that patients with a final-to-baseline MEP ratio of the glossopharyngeal amplitude ≤ 1.47 V have a 3.4 times increased risk to develop a uvula deviation. Patients with a final MEP of the glossopharyngeal width ≤11.6 ms have a 3.6 times increased risk for their gag reflex to become extinct. For the hypoglossal nerve, we found a significant correlation between the amplitude width (ms) of the final-to-baseline MEP ratio and swallowing function (p=0.049). The analysis of the risk estimate revealed that patients with a final-to-baseline MEP ratio of the hypoglossal amplitude width ≤ 1.03 ms have a 1.5 times increased risk to develop dysphagia. Conclusion: In conclusion, our study greatly contributed to the current knowledge of intraoperative MEPs as a predictor for postoperative nerve function. We were able to extent previous findings on MEP values of the facial nerve on postoperative nerve function to two further cranial nerves. We could show a significant relationship between the MEP values of the glossopharyngeal nerve and postoperative function of the uvula and gag reflex. For the hypoglossal nerve, we were able to show a significant relationship between the MEP values and swallowing function. Furthermore, we observed a statistical trend for the correlation between the MEP values of the hypoglossal nerve and tongue deviation; further studies including a larger sample size could confirm this result. Finding reliable predictors for postoperative nerve function is of great importance to the overall quality of life for a patient undergoing brain surgery.