Fitting and evaluating binaural hearing in adults using electric and acoustic stimulation
AffiliationAudiology and Speech Pathology
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2023-12-02. This item is currently available to University of Melbourne staff and students only, login required.
© 2021 Paola Incerti
Abstract Background. Advances in the design of the cochlear implant electrode arrays and improved surgical techniques have resulted in the successful preservation of residual hearing after implantation. The preservation of residual hearing following cochlear implantation provides the capacity for combining electrical and acoustic stimulation in the same ear to benefit the listener. The effectiveness of electric and acoustic stimulation in the implanted ear as compared with electrical and acoustic stimulation alone has been consistently demonstrated in the research literature. There is also accumulating research evidence that supports the provision of acoustic stimulation for residual hearing in the implanted ear together with the non-implanted ear of recipients of a unilateral cochlear implant. A variety of approaches have been used in research studies to fit devices that provide electric and acoustic stimulation in either the implanted ear or in opposite ears, however, there is no comprehensive study evaluating binaural fitting strategies in adult cochlear implant users with residual hearing in both ears. Investigation of a fitting procedure that could maximise hearing outcomes for adult recipients who use electric and acoustic stimulation in the implanted ear and acoustic stimulation in the non-implanted ear would be of benefit. Objective. The overarching goal of this thesis is to examine the fitting of devices that combine electric and acoustic stimulation for cochlear implant recipients with residual hearing in both ears. Aims 1. To systematically investigate how to adjust the hearing aid acoustic output in both ears, for adults who use electric and acoustic stimulation in the implanted ear together with acoustic stimulation in the non-implanted ear. 2. To investigate the effect of varying cross-over frequency settings for electric and acoustic stimulation in the implanted ear combined with acoustic stimulation in the non-implanted ear on speech perception in noise, localisation, functional performance in real life, and music perception. 3. To investigate the relationship between the measured cochlear dead region edge frequency in the implanted ear and the selection of the preferred cross-over frequency setting in devices that provide electric and acoustic stimulation. Methods Research design. A repeated-measures, within-subject research design, in which each participant acted as their own control was used. Participants. Eleven adults with bilateral sensorineural hearing loss, who had residual hearing in both ears, and who used a unilateral cochlear implant in one ear, and a hearing aid in the contralateral ear, participated in the study. Data collection and analysis. The thesis was structured in two distinct studies. Study 1. Participants were fitted with new hearing aids in both ears. A two-staged, systematic fine-tuning procedure was used to evaluate the hearing aid output requirements for the group of eleven adults who use a Nucleus cochlear implant system in one ear together with acoustic stimulation in both ears. Firstly, a paired comparisons technique was used to determine the participant’s preferred hearing aid gain for speech intelligibility in the implanted and the non-implanted ear. Secondly, the HA was systematically adjusted to determine the hearing aid gain requirements to achieve equal loudness between electric and acoustic stimulation in the implanted ear and then for equal loudness between ears. Study 2. Performance was compared with three different cross-over frequency settings set according to audiometric criteria, following a four-week period of familiarisation with each setting. Speech perception in noise, localisation, functional performance in real life, and music perception were evaluated. On completion of all trials, participants selected their preferred cross-over frequency setting. The cochlear dead region edge frequency values were then determined for each participant using a fast-psychophysical tuning curves test. Results. Study 1. On average, participants’ preferred hearing aid gain settings were within 5 dB of the NAL-NL2 fitting formula’s prescribed gain in the implanted ear and non-implanted ear. In addition, the hearing aid gain to achieve equal loudness between electric and acoustic stimulation was higher on average than participants’ preferred loudness in the implanted ear. However, for the non-implanted ear, there was no significant difference between the gain preferred by participants and the gain required to achieve equal loudness between ears on average. Study 2. On average, cross-over frequency settings did not have a significant effect on performance scores. However, higher ratings on device usage were associated with the preferred cross-over frequency settings. All adults for whom a dead region edge frequency value could be determined using the fast-psychophysical tuning curves test preferred electric stimulation in their chosen cross-over frequency setting to start after the cochlear dead region edge frequency value in the implanted ear. Conclusions Study 1. The findings of this research suggest that the NAL-NL2 prescription provides an appropriate starting point for fitting hearing aids in adults who use a cochlear implant system in one ear together with acoustic stimulation in both ears. Secondly, the study findings suggest that there may be no benefit to performing a loudness balancing procedure between electric and acoustic stimulation in the implanted ear or non-implanted ear in a clinical setting. Using an empirically validated hearing aid prescription and real-ear measurements to verify that target gains are matched as closely as possible may be an important first step when clinically fitting electric and acoustic stimulation. Study 2. Changes in cross-over frequency settings, set according to audiometric criteria, did not affect performance. Identification of the cochlear dead region edge frequency value may potentially be helpful in determining the cross-over frequency setting for the implanted ear in routine clinical settings. Selecting a cross-over frequency setting where electric stimulation begins after the individually measured cochlear dead region edge frequency value in the implanted ear could provide an individualised approach to best fitting practices for devices that provide electric and acoustic stimulation. The findings should be interpreted with caution due to the limited number of participants in the current study. There is published literature on the potential benefit from avoiding the amplification of frequencies that incorporate cochlear dead regions for listeners with severe-profound steeply-sloping. This is the first direct examination of the relationship between the cochlear dead region edge frequency value in the implanted ear and the preferred cross-over frequency setting in adults who use electric and acoustic stimulation. Further investigation into the implications of this finding in fitting electric and acoustic stimulation is warranted. The evidence base created by this research may contribute to the development of guidelines and recommendations for clinical fitting of devices that provide electric and acoustic stimulation and may potentially lead to individualised treatment for optimal benefits in recipients of a cochlear implant with residual hearing in both ears.
KeywordsCochlear Implants; Hearing aids; Electric stimulation; Acoustic stimulation; Binaural hearing; Residual hearing; Fitting; Evaluating
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