Long Term Monitoring of the Inner Ear Function During and After Cochlear Implant Insertion Using Cochlear Microphonics

Introduction:

To preserve residual hearing during cochlear implant (CI) surgery it is desirable to use intraoperative monitoring during electrode insertion. A promising method is the recording of cochlear microphonics (CM). The aim of the monitoring is to identify critical steps as well as to modify the ongoing insertion procedure immediately if necessary. Within this project the relation between intraoperatively recorded CMs and long term preservation of residual hearing shall be investigated.

Methods:

During insertion of hearing preservation electrodes, intraoperative CM recordings were performed extracochlearly using a cotton wick electrode as well as intracochlearly directly after insertion. Postoperative CM recordings were done during follow up appontments. The intracochlear recordings were conducted using the CI electrode (MED-EL) and a special software tool. For acoustic stimulation low frequency tone bursts were used (250 Hz to 1000 Hz). The follow up recordings take place up to one year after implantation. So far 10 patients are included who currently have passed multiple appointments.

Results:

Extracochlearly recorded CMs showed spectral peaks of maximal 1 µV at the stimulus frequency bin. Intracochlearly, peaks of up to 30 µV were detected. In the primary long-term data, amplitudes of CMs measured at consecutive appointments seem to correspond well to audiometric pure tone thresholds measured at the same appointments.

Conclusion:

Both setups were found to be feasible for recording CM. The amplitudes of intracochlearly recorded CMs were observed to be much larger than the extracochlearly recorded CMs. Long-term thresholds detected via the CM response correlated to audiometric pure tone thresholds.