In-vivo characterisation of an implanted microphone and totally implantable active middle ear implant.

OBJECTIVE

To objectively evaluate acoustic sensitivity of the implanted microphone, and maximum stable gain of a totally implantable active middle ear implant.

DESIGN

Prospective, single centre evaluation.

STUDY SAMPLE: Fourteen adult patients.

RESULTS

Microphone sensitivity is approx. 10 dB lower than an externally worn conventional hearing aid, at frequencies up to 4000 Hz, and substantially lower at higher frequencies. The masking level due to microphone noise, which determines the softest test tones that can be detected, is estimated at <20 to <30 dB HL up to 1000 Hz, and <40 dB HL at higher frequencies. Maximum stable effective gain is the maximum amplification achievable without causing feedback whistling. In sensorineural hearing loss (SNHL) cases, it is 30-40 dB at frequencies up to 2000 Hz, allowing to compensate for even the maximum recommended hearing loss (60-70 dB HL). In both SNHL and mixed hearing loss (MHL) cases, maximum stable effective gain is lower (+20 to -30 dB) around 3000-6000 Hz.

CONCLUSIONS

Microphone sensitivity is high enough to achieve aided thresholds of 20-40 dB HL. A strong correlation between actuator coupling efficiency and maximum stable effective gain implies that any effort to improve actuator efficiency should also increase the available gain.