Nearly three-fourths of adult cochlear implant (CI) recipients have aidable acoustic hearing in the non-implanted ear that could be used in a bimodal hearing configuration.1 For a typical CI candidate, the amount of residual acoustic hearing is not sufficient to allow high levels of daily communication; however, that residual acoustic hearing can provide significant speech recognition and sound quality benefit when paired with a CI in a bimodal hearing configuration. Bimodal hearing also provides significantly improved performance on various tasks of music perception, including chord, melody, melodic contour, and timbre recognition, compared with a CI-alone condition.2-7
Several bimodal hearing solutions are available, but there remains some confusion about optimal bimodal fittings, as the degree of bimodal benefit resulting from adding a hearing aid (HA) to a CI varies considerably among patients. Though unaided audiometric thresholds in a non-implanted ear and bimodal benefits have an inverse correlation,8-9 this correlation is driven by the extreme ends of the function, leaving little clinical guidance for the majority of patients with moderate to severe sensory losses in the non-implanted ear. While some studies provide guidance for the clinical fitting of bimodal listeners, there is a lack of prospective studies with large sample sizes that provide data-driven guidance for clinical fittings. Optimal bimodal fitting holds high clinical relevance to provide maximum bimodal benefit to patients with unilateral CI and accurately determine bilateral CI candidacy.
While we await the outcomes of prospective clinical trials systematically investigating the efficacy of various approaches to bimodal fitting in large populations, the following peer-reviewed papers provide clinical guidance to inform and improve current audiology practices.
How to Optimally Fit a Hearing Aid for Bimodal Cochlear Implant Users: A Systematic Review.
Vroegop JL, Godegebure A, and van der Schroeff MP. Ear Hear. 2018;39(6):1039-1045. Ear Hear. 2018;39(6):1039-1045.
Vroegop and colleagues10 described findings on bimodal fittings obtained from a systematic review of the peer-reviewed literature. They identified 1,165 records from which 17 were selected for review. The included articles primarily focused on the HA fitting component for bimodal listeners, with results summarized into four categories: (1) frequency response of the HA, (2) use of frequency lowering technology, (3) synchronization of automatic gain control (AGC) between HA and CI, and (4) interaural loudness balancing.
HA AUDIBILITY BANDWIDTH: LESS IS NOT MORE
Several studies have investigated the appropriateness of various iterations of the National Acoustic Laboratories (NAL) prescriptive fitting formula. Compared to the base NAL prescription, some patients reported subjective preference for more gain, while others in the same study reported subjective preference for less gain.11 In contrast, some studies have also reported that the NAL prescriptive formula yielded the best subjective bimodal benefit.12-14 These studies, however, did not compare speech recognition performance with different frequency-gain responses, and thus it is unclear whether subjective preference was correlated with patient performance in all cases. Further more, in some cases, the NAL fitting was compared to the subject's own HA fitting, though the audibility provided by the baseline fitting was not discussed.
Several studies have also investigated the usefulness of a broadband HA response compared with restricted high-frequency amplification. A majority reported significant bimodal benefit for broadband HA audibility compared with a restricted HA bandwidth.15,16 However, the referenced studies either did not include individuals with cochlear dead regions or did not assess for cochlear dead regions. In contrast, some evidence suggests that restricting high-frequency amplification may yield significantly higher speech recognition scores in some patients.17-18 One of these studies had tested for the presence of cochlear dead regions and found that for patients with confirmed dead regions, restriction of high-frequency amplification produced significantly higher bimodal speech recognition compared to full HA bandwidth.17-18
FREQUENCY LOWERING TECHNOLOGY: HOW LOW CAN YOU GO?
Many current HA systems make use of frequency lowering technologies, such as nonlinear frequency compression and frequency transposition, to deliver critical high-frequency information to a cochlear region where viable inner and/or outer hair cells are available to process the stimuli. Since many of these technologies are active in the default parameters of an HA fitting software, investigating its efficacy is warranted in the case of bimodal listeners for which the high-frequency stimuli are provided via the CI ear.
Bimodal studies on the efficacy of frequency lowering technology have focused on both speech recognition16,19-22 and horizontal plane localization.16,20 For speech recognition in quiet and noise, frequency-lowering technology provided either equivocal16,19-22 or significantly poorer20 outcomes compared with conventional amplification. For localization, this technology provided significant benefit in some cases16 or no benefit compared with conventional amplification.20 Overall, studies show little evidence to recommend the use of frequency lowering technology in a bimodal hearing configuration.
IS IT A PERFECT MATCH?
Most bimodal patients are fitted with a CI and HA that have entirely different automatic gain control (AGC) characteristics across ears, including different compression thresholds, compression ratios, and time constants. Thus, one can imagine how difficult it can be for a bimodal listener to integrate the signals from the different ears. A current CI system has an HA that matches AGC characteristics across the ears. One study investigated the effects of matched AGCs across the HA and CI in a group of 15 adult bimodal listeners.23 They evaluated the speech recognition in noise for various speech and noise spatial configurations. Compared with the unmatched HA and CI condition, the matched AGC resulted in statistically significant benefit at the group level for conditions in which stationary noise was presented at ±90 degrees as well as a single-talker distracter presented to the HA ear. They also reported subjective preference for the matched AGC in nine of 15 participants.
IT'S ALL ABOUT BALANCE—OR IS IT?
Loudness balancing across the HA and CI ears can be difficult given the different audible bandwidths across ears and the dramatically different hearing modalities. Several studies investigated various HA programming methods required to achieve interaural loudness balance. The trends were similar to that in HA frequency response. Specifically, some studies showed that listeners preferred less gain than the NAL prescriptive formula for interaural balance,11,23 while others reported that desired HA gain for interaural balance was roughly equivalent to NAL-R prescriptive gain settings.14 None of the studies, however, investigated whether the HA gain required to achieve subjective loudness balance yielded significantly higher outcomes compared with the base prescriptive formula.
Vroegop and colleagues10 observed significant bimodal benefits across studies; however, there were multiple inconsistencies with respect to the implementation of HA fitting. Despite inconsistencies in the literature, there are valuable data that we can use to guide our clinical practice for HA fittings in adult bimodal patients. Below are clinical recommendations from the studies discussed by Vroegop, et al.10
HA Audibility and Bandwidth. Though there were no studies that directly compared speech recognition performance across various validated prescriptive fitting formulae, there was one common thread among these studies: HA verification via real ear measures yielded significant bimodal and subjective benefits. Thus, it would follow that clinicians should be completing real ear verification of HA audibility as prescribed by a validated fitting formula. With respect to audibility bandwidth, the best way to apply the results to our clinical practice would be to implement a test for cochlear dead regions, such as the threshold equalizing noise (TEN) test. If there is evidence for dead regions, one could reasonably program an HA using full and restricted bandwidths. Using both subjective preference and aided speech recognition assessments with recorded stimuli will allow us to gauge the effectiveness of these two HA fitting strategies and provide an evidence base for our clinical practice.
Frequency Lowering Technology. Because most studies demonstrated no significant difference in performance between conventional amplification and frequency lowering amplification, a simple way to apply these data clinically would be to provide patients with both a conventional frequency response and frequency lowering response for the HA, gauge their subjective preference, and compare bimodal speech recognition performance with both HA responses.
AGC Match. Because the matched AGC system is only currently available in one CI/HA system, it is not possible to implement this clinically without the use of an Advanced Bionics Naida CI processor and Link HA. However, it is possible to match compression threshold and ratio manually for MED-EL cochlear implants if one considers the function of the device. Using the recommended sensitivity setting of 75 percent, the MED-EL AGC kneepoint is fixed at 53 dB SPL, beyond which the default compression ratio is 3:1 up to 100 dB SPL, at which point infinite compression is applied. Clinicians could match these settings in various HA systems with manual adjustments in the HA software. CI systems by Cochlear™ have a more complex AGC system with activation of autosensitivity control, making it a bit more complicated to manually program an HA to match the CI processor AGC. However, given Cochlear's partnership with GN Resound, it is highly likely that matched AGC systems will be available in the near future. Should we manually adjust HA settings to try to match the CI AGC settings, it is advised that both patient preference and bimodal aided speech recognition testing be completed to validate the fitting.
Loudness Balance. Most clinicians may already be assessing loudness balance across the CI and HA ears for bimodal patients. It is possible to adjust the HA ear, CI ear, or both to provide interaural balance for our patients. In the absence of data-driven guidance, the current systematic review would suggest that we start with the base HA prescriptive formula, verify the HA fitting via probe microphone measures, then adjust accordingly. As always, subjective preference and aided speech recognition assessment are both recommended to ensure that bimodal listeners are achieving their maximal hearing potential.
Hearing health care needs a large-scale, prospective study of bimodal fitting methods as well as criteria for identifying bilateral CI candidates following bimodal optimization. While this is no small task and one that will likely require years of experimentation and data dissemination, current research provides us with clinical guidance for HA fittings in bimodal listeners. Applying these data to our practice will allow us to offer patients the best chance to reach optimal bimodal benefit.
1. Vroegop JL, Godegebure A and van der Schroeff MP. (2018). How to Optimally Fit a Hearing Aid for Bimodal Cochlear Implant Users: A Systematic Review. Ear Hear
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10. Vroegop JL, Godegebure A and van der Schroeff MP. (2018). How to Optimally Fit a Hearing Aid for Bimodal Cochlear Implant Users: A Systematic Review. Ear Hear
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13. Morera, C., Cavalle, L., Manrique, M., et al. (2012). Contralateral hearing aid use in cochlear implanted patients: Multicenter study of bimodal benefit. Acta Otolaryngol
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14. English, R., Plant, K., Maciejczyk, M., et al. (2016). Fitting recommendations and clinical benefit associated with use of the NAL-NL2 hearing-aid prescription in Nucleus cochlear implant recipients. Int J Audiol
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15. Neuman, A. C., Svirsky, M. A. (2013). Effect of hearing aid bandwidth on speech recognition performance of listeners using a cochlear implant and contralateral hearing aid bimodal hearing. Ear Hear
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17. Zhang, T., Dorman, M. F., Gifford, R., et al. (2014). Cochlear dead regions constrain the benefit of combining acoustic stimulation with electric stimulation. Ear Hear
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18. Messersmith, J. J., Jorgensen, L. E., Hagg, J. A. (2015). Reduction in high-frequency hearing aid gain can improve performance in patients with contralateral cochlear implant: a pilot study. Am J Audiol
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19. Park, L. R., Teagle, H. F., Buss, E., et al. (2012). Effects of frequency compression hearing aids for unilaterally implanted children with acoustically amplified residual hearing in the nonimplanted ear. Ear Hear
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20. Perreau, A. E., Bentler, R. A., Tyler, R. S. (2013). The contribution of a frequency-compression hearing aid to contralateral cochlear implant performance. J Am Acad Audiol
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21. McDermott, H., & Henshall, K. (2010). The use of frequency compression by cochlear implant recipients with postoperative acoustic hearing. J Am Acad Audiol
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22. Hua, H., Johansson, B., Jonsson, R., et al. (2012). Cochlear implant combined with a linear frequency transposing hearing aid. J Am Acad Audiol
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23. Veugen, L. C., Chalupper, J., Snik, A. F., et al. (2016b). Matching automatic gain control across devices in bimodal cochlear implant users. Ear Hear
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