Detailed insight into transtympanic electrocochleography (TT-ECochG) and direct cochlear nerve action potential (CNAP) for intraoperative hearing monitoring in patients with vestibular schwannoma – methodology of measurements and interpretation of results

Background: The aim was to present the methodology and interpretation of intraoperative hearing monitoring with simultaneous Transtympanic Electrocochleography (TT-ECochG) and direct Cochlear Nerve Action Potential (CNAP) measurements during vestibular schwannoma removal. Materials and Methods: Detailed methodology of measurements and interpretation of results are presented in three exemplary patients who underwent tumor removal via middle fossa approach (MFA) with the use of intraoperative monitoring of hearing with TT-ECochG and direct CNAP performed in real time. In addition, all responses were automatically recorded and stored along with surgical information and subjected to detailed analyses and calculation after surgery. Results: The following changes in TT-ECochG and direct CNAP responses were observed: Patient #1 – TT-ECochG and CNAP responses with minor, but transient, morphology changes (hearing was preserved); Patient #2 – CNAP responses changed significantly but, temporarily, from triphasic into biphasic responses later, with marked but partially reversible desynchronization of CNAP; changes in TT-ECochG responses were also observed but, at the end, returned to baseline (surgery-related deterioration of hearing); Patient #3 – irreversible changes of TT-ECochG and direct CNAP (complete loss of hearing). Conclusions: A combination of TT-ECochG and direct CNAP allows for real-time monitoring of auditory function during vestibular schwannoma resection and surgical manipulation which contribute to the risk of hearing loss. Therefore, the surgeon can be instantly informed about changes which could increase the possibility of preserving the patient’s hearin