Sound and Vibrations in Acoustic Hearing Implants

Acoustic hearing implants are accessible for several decades and are a viable treatment option for individuals with sensorineural hearing loss, conductive hearing loss, or a combination of both types, i.e., mixed hearing loss (MHL). Today a choice of acoustic implant types and technologies is available. They include bone conduction implants (BCI), active middle ear implants (AMEI), and direct acoustic cochlear implants (DACI). A range of patient-dependent factors defines which type is best suited for the individual patient. These factors include type and grade of the hearing loss, underlying pathophysiology, age, personal preference, lifestyle, and many other factors. In this PhD thesis, the focus lies on treatment options for individuals with MHL. The treatment of severe to profound MHL continues to be challenging. A couple of the acoustic hearing implants are only used clinically for a few years and there are still several knowledge gaps about these implants. This PhD project aimed at closing some of these knowledge gaps by investigating specific aspects of a DACI and a fully implantable AMEI. In the first study, the influence of a DACI on the post-operative bone conduction thresholds was investigated in cadaver heads. This was done via measurements of the relative round window membrane motion before and after DACI implantation. The outcomes of this study suggest that the implantation of the DACI does not effectively contribute to the cochlear response produced by bone conduction stimulation. The improvements in bone conduction thresholds seen in patients implanted with a DACI likely have their origin in the changed impedance at the oval window after DACI surgery leading to a more efficient contribution from the inner ear components to bone-conducted sound. The goal of the second study was to investigate the sensitivity of an implanted subcutaneous microphone over time in patients, i.e., in-situ. The in-situ sensitivity was measured at several moments after surgery and was compared to the pre-operative sensitivity measured on the bench. Discrepancies between the pre-operative and post-operative in-situ sensitivities were identified as a major finding. It showed the importance of measuring the individual microphone sensitivity as part of the fitting process. If the in-situ microphone sensitivity differs from the expected input to the fitting software, it may cause the gain and maximum power output set in the fitting software to be non-optimal. This, in turn, may adversely affect the subject's aided thresholds and speech performance. Accurate prediction of microphone sensitivity is thus an important factor in audiological outcomes. Data on the mid-term in-situ sensitivity suggested no clinically relevant changes over time. The purpose of the third study was to compare the directional response, i.e., the sensitivity to sound as a function of the angle of sound incidence, of the same subcutaneous microphone as in the second study in different positions on the head. Microphone performance related to the anatomical position was not taken into account so far. As the microphone performance cannot be measured in different microphone positions in the same patient (the microphone is fixed in one specific position on the skull), the directional sensitivity was measured in a dummy head to study the influence of the head and pinna on the microphone performance in different microphone positions. Of all microphone positions considered in this study, the one on the mastoid tip showed the best broadband sensitivity to the front and the values were significantly better than these values in the other microphone positions. The directivity index (DI), a method to estimate the impact of directional sensitivity on speech intelligibility in noise, was negative in most of the cases. It indicates that noise sources in the direction of the microphone may affect the speech intelligibility in noise, depending on the hearing capacity of the other ear. In the last study of the PhD project, the aim was to characterize the directional response of the same subcutaneous microphone in-situ. The DI was used to estimate the impact on speech intelligibility in noise. An additional aim was to compare the in-situ directional response with the directional response that was measured on the dummy head in study three. The microphone showed a directional sensitivity pattern with the highest sensitivity towards the side of the head, mostly in line with the microphone location on the head. The DI values were negative in the majority of cases owing to the higher sensitivities towards the side of the head compared to the front. This could lead to difficult speech intelligibility in noise if noise is coming from the direction of the highest microphone sensitivity. However, the subjects in this study showed good speech discrimination in noise when speech and noise were presented from the front (S0N0). The direction of highest sensitivity was comparable between the measurements in the subjects and on the dummy head, confirming that preclinical testing of implantable microphone technology on dummy heads gives a good first indication of the directional behavior. The research findings of the four studies investigating specific aspects of acoustic hearing implants unraveled crucial knowledge gaps that exist for these types of hearing implants and yield promising directions for future research in implantable hearing technology.

Jaar van publicatie:2022

Toegankelijkheid:Closed