Analysis of Electrocochleography Morphology Changes Recorded Intraoperatively during Cerebellopontine Angle Tumor Surgery

Introduction: During cerebellopontine angle tumor (CPAT) surgery, a serious risk of damaging the content of the internal auditory canal (IAC) occurs. The internal auditory artery, the vestibulocochlear nerve, the facial nerve, and the anterior inferior cerebellar artery are the structures that may be destroyed reversibly or irreversibly during tumor removal. Although monitoring of facial nerve function intraoperatively has already been effectively developed, efficacious monitoring of auditory function remains to be established. The aim of this study was to estimate the utility of online analysis of transtympanic electrocochleography (TT-ECochG) morphology changes to prevent irreversible hearing damage and to predict postoperative hearing results. Method: Thirty-five patients with CPAT were operated on using a middle fossa approach. Auditory function was monitored intraoperatively, simultaneously by TT-ECochG and ABR. The auditory system was stimulated acoustically with clicks of 80–85 dB nHL intensity. The authors developed software to analyze all collected data, which were displayed as ECochG morphology changes online. Results: Compound action potentials (CAPs) and summation potentials (SPs) of TT-ECochG were analyzed automatically. Also ABR waves III and V were visualized online. In all cases readable and repeatable-click CAPs were recorded. In general, 128 sweeps were a sufficient number to process the recorded data and to present all details of CAP (amplitude, latency, N1, P1). Analyzed online CAP data were displayed as CAP-N1-Amplitude and CAP-N1-Latency in time domain every 3–6 seconds. Intraoperative fluctuations of CAP-N1-Amplitudes and Latencies corresponded to clinical situations occurring during CPAT surgery. Auditory brainstem response needed more sweeps, and, in practice, did not show clinical utility to online analysis in real-time domain due to a poor quality and reproducibility. Conclusions: During intraoperative monitoring, TT-ECochG effectively revealed both minimal and serious changes in auditory function in the real-time domain. Our developed strategy of online analysis of intraoperatively recorded data has made hearing status assessment faster and easier. Removal of the IAC wall, tumor debulking and removal, bipolar cautery used for microbleeding, and stretching of the IAC content were found to be the most dangerous moments of CPAT surgery. During these intraoperative situations, the most serious changes in ECochG morphology were observed. Online-analyzed ECochG recordings provided the surgeon an early warning concerning the hearing status and prevented irreversible damage of the cochlear function and the eighth nerve.