Objective: Hearing preservation is the main focus of small acoustic neurinoma (AN) removal. Refinement of intraoperative auditory monitoring may improve postoperative hearing. We have introduced a newly designed intracranial electrode enabling continuous monitoring of cochlear nerve compound action potential (CNAP). We performed the simultaneous monitoring of ABR and CNAP during retrosigmoid small AN removal, and clarified the surgical outcome and the usefulness of CNAP monitoring.
Methods: Twenty-two consecutive patients with small AN underwent retrosigmoid tumor removal with attempting hearing preservation. ABR and CNAP were simultaneously monitored during tumor removal.
Results: AN was totally removed in all patients without facial palsy. Preservation rate of useful and serviceable hearing was 82% and 91%, respectively. During microsurgical tumor removal, various surgical equipments and procedures intensified artifacts of ABR and reliable ABR monitoring with distinct wave V was obtained in 9/22 patients. Not affected by artifacts, reliable CNAP monitoring was obtained more frequently (in 20/22 patients) than ABR (p=0.0005). CNAP at the completion of tumor removal predicted hearing preservation with no false positive or negative (100% sensitivity and 100% specificity). CNAP changed dynamically and stepwise with surgical manipulations.
Conclusion: Retrosigmoid approach using auditory monitoring for small AN can accomplish total tumor removal with excellent hearing preservation rate. CNAP provides reliable auditory monitoring more frequently than ABR, reflects the intraoperative auditory function almost in real-time, predicts postoperative hearing with excellent sensitivity and specificity, and is more useful monitoring in the removal of small AN with hearing preservation.