gms | German Medical Science

Introduction: Proper coupling of the actuator of an active middle ear implant (AMEI) to an ossicle or the cochlea is essential for optimal transfer of vibration to the inner ear. Currently available indicators of contact only allow to indirectly evaluate the efficacy of stimulation of the inner ear by an AMEI. In contrast, recording the physiological response of the inner ear directly during stimulation via the AMEI actuator, has the potential for providing a direct indicator of the efficacy of stimulation. Electrocochleography (ECochG) is a widely available standard clinical technique whose intra-operative applications are being intensely investigated in the realm of cochlear implants (Helmstaedter et al., 2018). Two components of the ECochG signal are of interest for use as an indicator of stimulation efficacy, namely the compound action potential (CAP) — whose transient nature and delay allow a clear separation between physiological response and stimulation artifact — and the cochlear microphonic (CM) — whose high amplitude may result in a signal-to-noise ratio high enough to enable real-time monitoring. We sought to investigate the feasibility of using ECochG as a method for guiding coupling of an AMEI actuator to the middle ear.

Methods: In a clinical study approved by our local ethics committee, ECochG recordings were made during implantation of a Cochlear™ Carina^® AMEI. Growth functions were obtained in two patients for both CAPs and CM potentials before, during and after coupling of the Carina actuator to the incus. The AMEI actuator was connected to the headphones output of the clinical stimulation–recording system via an electrically isolating link. An electrode placed on the cochlear promontory was used for recording responses.

Results: Initial results indicate that recording of CAPs and CMs when stimulating with this AMEI actuator is possible. When the recording electrode is properly placed in the vicinity of the cochlear promontory, physiological responses are distinguishable from the artifact produced by the electromagnetic actuator, down to a stimulation level of 80 dB HL eq. and 60–70 dB HL eq. for CAPs and CMs respectively. Placement of the recording electrode outside the middle ear (e.g., on the temporal bone exposed by the atticotomy or mastoidectomy), as well as placement of that electrode overly close to the reference electrode, may both preclude recording physiological responses with an amplitude high enough to be distinguishable from the sizeable stimulation artifact.

Conclusion: Intra-operative recording of ECochG could potentially be used as a tool for characterizing the relative output of an AMEI actuator. Despite the fact that the CM is a less direct indicator of hearing than the CAP, the CM may nevertheless prove to be a valuable intra-operative indicator of coupling due to its amplitude being one order of magnitude greater than that of the CAP. Further research is necessary to solidify these initial findings.