Abstract
Objectives:
Investigate the incidence of electrode contacts within the functional acoustic hearing region in cochlear implant (CI) recipients and assess its influence on speech recognition for electric-acoustic stimulation (EAS) users.
Study Design:
Retrospective review.
Setting:
Tertiary referral center.
Patients:
105 CI recipients with functional acoustic hearing preservation (≤80 dB HL at 250 Hz)
Interventions:
Cochlear implantation with a 24-, 28-, or 31.5-mm lateral wall electrode array.
Main Outcome Measures:
Angular insertion depth (AID) of individual contacts was determined from imaging. Unaided acoustic thresholds and AID was used to calculate the proximity of contacts to the functional acoustic hearing region. The association between proximity values and speech recognition in quiet and noise for EAS users at 6 months post-activation was reviewed.
Results:
60% of cases had one or more contacts within the functional acoustic hearing region. Proximity was not significantly associated with speech recognition in quiet. Better performance in noise was observed for cases with close correspondence between the most apical contact and the upper edge of residual hearing, with poorer results for increasing proximity values in either the basal or apical direction (r(14)=.48, p=0.043; r(18)=−.41, p=0.045, respectively).
Conclusion:
There was a high incidence of electrode contacts within the functional acoustic hearing region, which is not accounted for with default mapping procedures. The variability in outcomes across EAS users with default maps may be due in part to electric-on-acoustic interference, electric frequency-to-place mismatch, and/or failure to stimulate regions intermediate between the most apical electrode contact and functional acoustic hearing region.
Introduction
Cochlear implantation is indicated for patients with normal hearing or mild-to-moderate hearing loss in the low-to-mid frequencies, severe-to-profound hearing loss in the high-frequencies, and poor speech recognition with appropriately-fit amplification. Modified surgical approaches and flexible electrode array designs have supported postoperative hearing preservation. Patients with functional hearing preservation (e.g., unaided acoustic thresholds ≤80 dB HL) can be fit with the ipsilateral combination of amplification and electric stimulation, known as an electric-acoustic stimulation (EAS) device. EAS users experience better speech recognition with EAS as compared to listening with a cochlear implant (CI) alone device, though outcomes vary widely1–6. The variability in outcomes may be due in part to the placement of the electrode array relative to the functional acoustic hearing region, since electric stimulation may interfere with encoding of the acoustic signal, known as electric-on-acoustic interference7–9. The incidence of electrode contacts within the functional acoustic hearing region for CI recipients with hearing preservation is unknown.
There is increased variability in the lengths of electrode arrays that are used for hearing preservation cases. Lateral wall electrode arrays are typically preferred because they inflict less intracochlear trauma and allow for increased acoustic hearing preservation as compared to alternative designs10. Traditionally, short (e.g., <24 mm) lateral wall electrode arrays have been selected for CI candidates with functional low-to-mid frequency acoustic hearing in order to limit intraoperative trauma to the apical cochlear region. However, CI recipients of short lateral wall electrode arrays who lose functional acoustic hearing demonstrate poorer performance with a CI-alone device than CI recipients of longer electrode arrays11,12. Interestingly, functional hearing preservation has also been observed with long (e.g., 31.5 mm) electrode arrays13–17. The ultimate placement of an individual’s electrode array is influenced not only by the length of the array, but also the surgical approach and individual cochlear morphology. These combined variables result in wide variability in electrode array placement for recipients of the same electrode array18,19.
The default mapping procedures for EAS devices do not account for the placement of the electrode array relative to the functional acoustic hearing region. The default mapping procedures use the unaided acoustic hearing thresholds in the implanted ear to identify the frequency range to be fit with amplification and the crossover frequency between acoustic and electric stimulation. While the speech information provided acoustically and electrically with default maps is complementary, the place of stimulation may not be. Overlap in acoustic and electric stimulation from electrode contacts within the functional acoustic hearing region could produce electric-on-acoustic interference. Cases with electrode arrays placed more basal to the functional acoustic hearing region may have a gap between the cochlear regions receiving acoustic or electric stimulation, resulting in less efficient signal transmission. In both scenarios, EAS users may experience poorer speech recognition than cases with the most apical electrode contact placed at the upper edge of the functional acoustic hearing region.
The aims of the present study were to review the incidence of electrode contacts within the functional acoustic hearing region for patients with hearing preservation and to assess whether proximity to functional acoustic hearing was associated with speech recognition in quiet and noise for EAS users.
Materials and Methods
A retrospective review was conducted of the patient, device, and performance data for adult CI recipients with functional acoustic hearing preservation. Inclusion criteria were normal cochlear morphology, postlingual onset of hearing loss, implantation of a MED-EL lateral wall electrode array between January 2017 and August 2022, ≥18 years of age at implantation, and an unaided threshold of ≤80 dB HL at 250 Hz at device activation. Cases of revision surgery were excluded.
Patients were implanted with a Flex24, Flex28, or FlexSOFT electrode array (MED-EL Corporation, Innsbruck, Austria). These 12-channel lateral wall electrode arrays differ in array length (24-, 28-, and 31.5-mm, respectively) and electrode contact separation (1.9, 2.1, and 2.4-mm, respectively). Unaided acoustic thresholds for 125–8000 Hz were assessed behaviorally at the 4–6 week postoperative visit with pulsed pure tone stimuli that were presented via inserts. A value of 120 dB HL was entered in the absence of behavioral response.
Fitting of an EAS device was recommended for patients with an unaided threshold in the implanted ear of ≤65 dB HL at 125 Hz. Patients with poorer severities of hearing loss or who decided not to proceed with an EAS device were fit with a CI-alone device. At activation, most comfortable loudness (MCL) levels were measured behaviorally across all active channels, and the threshold levels were assigned as 10% of the MCL level for each channel. At the post-activation visits, MCL and threshold levels were measured behaviorally. For patients listening with an EAS device, the acoustic settings were adjusted to meet either the NAL-NL1 or NAL-NL2 prescriptive targets.
Aided speech recognition was evaluated in a soundbooth with consonant-nucleus-consonant (CNC) words in quiet20 and AzBio sentences in a 10-talker masker (10 dB SNR)21. Patients were seated approximately one meter from the loudspeaker and faced 0° azimuth. Recorded materials were presented at 60 dB SPL. Masking was presented via an insert to the contralateral ear when warranted.
The postoperative CT scan or intraoperative x-ray was used to calculate the angular insertion depth (AID) for each electrode contact. Postoperative CT scans were reviewed with the OTOPLAN software using previously described procedures22. If the CT scan was not available, the intraoperative x-ray image was reviewed using a rotating 3-D helical scala tympani model as previously described23. AID values were calculated by two reviewers for all cases. Cases were excluded from the present analysis if the estimated AID of the most apical electrode contact (E1) differed by >10° between the reviewers.
The primary aim was to review the incidence of electrode contacts within the functional acoustic hearing region. Proximity to the functional acoustic hearing region was determined from the unaided acoustic thresholds and the AID values for each electrode contact. Linear extrapolation of the unaided thresholds as a function of log-transformed frequency was used to determine the frequency at which the unaided acoustic hearing thresholds exceeded 80 dB HL. The 80 dB HL criterion was selected since thresholds at or below this level can be fit with the acoustic component. Frequencies were converted to AID values using the spiral ganglion frequency-to-place function described by Stakhovskaya et al.24. Proximity values were calculated as the difference in the AID for E1 and the AID at which the unaided acoustic hearing thresholds exceeded 80 dB HL. Negative values indicate E1 was placed basally to the functional acoustic hearing region; positive values indicate E1 was placed within the functional acoustic hearing region. Incidence was evaluated as the percentage of cases with positive proximity values.
The secondary aim was to determine whether there was an association between speech recognition with EAS and proximity of E1 to the functional acoustic hearing region. The database was queried for EAS users with an unaided threshold of ≤80 dB HL at 250 Hz at the 6-month post-activation visit. The data were limited to those with functional acoustic hearing at 250 Hz since significant benefit has been observed for acoustic stimuli with the low-pass filter of 250 Hz for bimodal users (hearing aid contralateral to the CI)25. Data from EAS users with place-based maps were excluded since the place-based mapping procedure used at the study site accounts for electrode array placement and our aim was to evaluate outcomes with default maps, which do not account for electrode array placement26. Percent correct data were converted to rationalized arcsine units27 and the associations with the proximity values were analyzed using one-tailed Pearson correlations computed in SPSS (version 27).
Results
One hundred twenty-five cases presented with an unaided threshold of ≤80 dB HL at 250 Hz at device activation. Of those, 105 had sufficient imaging for AID calculation. Table 1 lists the demographic information for the sample. The mean age at implantation was 64 years, with a range of 25 to 92 years (SD: 14 years). Forty-one cases were implanted with the Flex24, 43 with the Flex28, and 21 with the FlexSOFT electrode array. The AID of E1 ranged from 330 to 756° with a mean of 515° (SD: 82°). The upper frequency of functional acoustic hearing ranged from 250 to 4000 Hz, with a mean of 568 Hz (median: 420, SD: 537 Hz). The proximity of the E1 to the functional acoustic hearing region ranged from −183 to 356° with a mean of 40° (median: 22; SD: 111°).
Table 1:
Demographic information for adult CI recipients with functional acoustic hearing preservation. Low-frequency pure tone average (LFPTA) was the mean of unaided acoustic thresholds at 250 and 500 Hz.
| Demographic information for CI recipients with functional acoustic hearing preservation | ||
|---|---|---|
| sex | male (n=45) female (n=60) |
|
| age at implantation (years) | min: 25 mean: 64 max: 92 SD: 14 |
|
| electrode array | Flex24 (n=41) Flex28 (n=43) FlexSOFT (n=21) |
|
| angular insertion depth (degrees) | min: 330 mean: 515 max: 756 SD: 82 |
|
| Proximity to functional acoustic hearing region (degrees) | min: −183 mean: 40 max: 356 SD: 111 |
|
| Preop | 250 Hz unaided threshold (dB HL) | min: 10 mean: 42 max: 95 SD: 18 |
| LFPTA (dB HL) |
min: 10 mean: 49 max: 95 SD: 18 |
|
| Postop | 250 Hz unaided threshold (dB HL) | min: 0 mean: 50 max: 80 SD: 16 |
| LFPTA (dB HL) | min: 30 mean: 73 max: 100 SD: 15 |
|
SD: standard deviation; Preop: preoperative; Postop: postoperative; dB HL: decibel hearing level
Figure 1 plots the distribution of cases as a function of proximity of E1 to the functional acoustic hearing region, with positive values indicating placement within the functional acoustic hearing region and negative values indicating placement basal to this region. Sixty-three cases (60%) had one or more electrode contacts within the functional acoustic hearing region.
Figure 1:
Number of cases for each bin of proximity to the functional acoustic hearing region values. Proximity of the most apical electrode contact to the functional acoustic hearing region was calculated as the difference in the angular insertion depth (AID) of the most apical electrode contact (E1) and the AID of the unaided acoustic hearing threshold that exceeded 80 dB HL. Negative values indicate E1 was placed basally to the functional acoustic hearing region; positive values indicate E1 was placed within the functional acoustic hearing region. The shaded areas indicates cases with electrode contacts within the functional acoustic hearing region.
Figure 2 plots the AID of E1 for each case stratified by lateral wall electrode array. Open symbols indicate cases with E1 outside of the functional acoustic hearing region. Filled symbols indicate cases for which E1 was within the functional acoustic hearing region, with symbol color indicating the number of electrode contacts within this region (defined in the legend). These data show the variability in the number of electrode contacts within the functional acoustic hearing region across AID values and across electrode arrays. This variability is influenced by the amount of functional acoustic hearing available for each individual, illustrating the possibility for low-to-mid frequency hearing preservation with long (≥28 mm) lateral wall electrode arrays.
Figure 2:
Angular insertion depth of the most apical electrode contact for CI recipients with functional acoustic hearing preservation stratified by lateral wall electrode array: Flex24 (24 mm), Flex28 (28 mm), and FlexSOFT (31.5 mm). Open symbols indicate cases with electrode array placement outside of the functional acoustic hearing region. Filled symbols indicate cases with one or more electrode contacts within the functional acoustic hearing region. Symbol color indicates the number of electrode contacts within the functional acoustic hearing region, as defined in the legend.
Our second aim was to evaluate whether speech recognition for EAS users was associated with proximity of E1 to the functional acoustic hearing region. Sixty cases presented at the 6-month post-activation visit with an unaided threshold of ≤80 dB HL at 250 Hz and completed aided speech recognition assessment. Seven patients were removed because they were listening to place-based maps. Of the remaining 53 patients, 35 listened with an EAS device and 18 with a CI-alone device. Table 2 lists the demographic information for the EAS versus CI-alone users.
Table 2:
Demographic information of the sample of CI recipients with an unaided threshold of ≤80 dB HL at 250 Hz at the 6-month post-activation visit. Data are stratified by whether the patient selected to listen with a CI-alone or EAS device.
| Demographic information for CI versus EAS users | ||
|---|---|---|
| CI-alone users | EAS users | |
| sex | male (n=8) female (n=10) |
male (n=18) female (n=17) |
| age at implantation (years) | min: 48 mean: 64 max: 80 SD: 11 |
min: 26 mean: 64 max: 86 SD: 13 |
| electrode array | Flex24 (n=5) Flex28 (n=9) FlexSOFT (n=4) |
Flex24 (n=16) Flex28 (n=15) FlexSOFT (n=4) |
| angular insertion depth (degrees) | min: 370 mean: 516 max: 717 SD: 86 |
min: 330 mean: 501 max: 643 SD: 78 |
| Proximity of E1 to functional acoustic hearing region (degrees) | min: −33 mean: 47 max: 284 SD: 81 |
min: −131 mean: 32 max: 224 SD: 106 |
| Frequency at 80 dB HL (Hz) | min: 250 mean: 523 max: 1000 SD: 229 |
min: 250 mean: 538 max: 1414 SD: 277 |
CI: cochlear implant; dB HL: decibel hearing level; Hz: hertz; EAS: electric-acoustic stimulation
Figure 3 plots speech recognition for CNC words in quiet (Panel A; top) and AzBio sentences in noise (Panel B; bottom) by the proximity of E1 to the functional acoustic hearing region for EAS users. For CNC words in quiet, the association with the proximity values was non-significant for both negative and positive values of proximity (r(14)=.34, p=0.118; r(21)=−.14, p=0.274, respectively). For AzBio sentences in noise, there was a significant positive correlation with proximity values for cases with electrode array placement outside the functional acoustic hearing region (r(14)=.48, p=0.043), with better performance observed for cases with the electrode array closer to the upper edge of this region. Also, there was a significant negative correlation with proximity values for cases with one or more electrode contacts within the functional acoustic hearing region (r(18)=−.41, p=0.045). This indicates that more overlap of the electrode array within the functional acoustic hearing region may result in poorer speech recognition in noise.
Figure 3:
Association of aided speech recognition for EAS users at 6-months post-activation by the proximity of the most apical electrode contact to functional acoustic hearing region. Percent correct scores are plotted for CNC words in quiet (panel A; top) and AzBio sentences in a 10-talker masker at 10 dB SNR (panel B; bottom). The shaded areas indicates overlap with the functional acoustic hearing region. Plotting conventions for individual data are the same as in Figure 2. The relationship between proximity values and performance is indicated with a dashed line for cases with electrode contacts outside functional acoustic hearing region and a solid line for cases with one or more electrode contacts within this region.
Discussion
The aims of this study were to determine the incidence of electrode contacts within the functional acoustic hearing region in adult CI recipients and to evaluate whether proximity of E1 to the functional acoustic hearing region was associated with speech recognition for EAS users. Surprisingly, the majority (60%) of our sample had at least one electrode contact within the functional acoustic hearing region. Placement of the electrode contacts relative to the functional acoustic hearing region is not accounted for by the default mapping procedures, which could negatively influence performance with EAS. For the present sample, speech recognition in quiet was not significantly associated with proximity values. Importantly, speech recognition in noise was significantly associated with proximity values – with poorer performance observed for cases with more basal shifts and for cases with more overlap with the functional acoustic hearing region. These data suggest the benefit of aligning the most apical electrode contact with the upper edge of the functional acoustic hearing region.
A high incidence of one or more electrode contacts within the functional acoustic hearing region was found for the present sample – which was observed across recipients of 24, 28, and 31.5 mm lateral wall electrode arrays. These data are likely influenced by the clinical bias to implant a short (i.e., 24 mm) lateral wall electrode array for patients with preoperative normal hearing or mild hearing loss, and a longer array for patients with moderate loss. While short lateral wall arrays are thought to increase the probability of acoustic hearing preservation, other investigations have reported hearing preservation with long electrode arrays. For example, Hollis and colleagues reported that 84% of recipients of a 31.5 mm flexible lateral wall array experienced preserved acoustic hearing13. Interestingly, hearing preservation and one or more electrode contacts within the functional acoustic hearing region was observed for recipients of all of the electrode arrays reviewed in the present report. These data support the claim that low-to-mid frequency acoustic hearing can be preserved following insertion of a long lateral wall electrode array and suggest that the risk of intracochlear damage with longer arrays may be less than initially hypothesized.
The outcomes of EAS users with electrode contacts within the functional acoustic hearing range corroborate findings from studies of the influence of electric-on-acoustic interference. Lin et al (2011) demonstrated that stimulation of the most apical electrode contacts increased low-frequency acoustic detection thresholds in a subject with a fully-inserted 24-mm electrode array7. Krüger et al (2017) reported that acoustic thresholds increased exponentially as the difference between the electric and acoustic stimulation sites decreased8. Also, Imsiecke et al (2020) demonstrated a negative relationship between the speech reception threshold and the difference between place of electric and acoustic stimulation in patients with flexible lateral wall arrays9. For the present sample, the effects were likely observed for speech recognition in noise due to electric stimulation of apical electrode contacts interfering with the acoustic low-frequency cues that support recognition in challenging maskers. Taken together, these data suggest the utility of incorporating electrode array placement relative to the functional acoustic hearing region into the mapping of EAS devices in order to limit electric-on-acoustic interference and support better outcomes for EAS users.
The patterns of performance for the EAS users with the electrode array outside of the functional acoustic hearing region may be due in part to the distribution of frequency information across the cochlea. Electrode array placement outside of the functional acoustic hearing region may result in failure to stimulate regions intermediate between the most apical electrode contact and functional acoustic hearing region. This stimulation gap creates less efficient signal transimision, which may contribute to poorer outcomes, particulary in noise.
The performance of EAS users may also be influenced by electric frequency-to-place mismatches created by the default mapping procedures. Electric frequency-to-place mismatches are the discrepancies between the frequency information provided by a given channel and the cochlear place frequencies stimulated by the associated electrode contact. With default maps, patients with electrode arrays further away from the functional acoustic hearing region will have larger electric mismatches than patients with electrode arrays close to this region. Poorer speech recognition with spectrally-shifted maps as compared to maps that match the filter frequencies to the cochlear place frequencies has been reported in studies using EAS simulations and for EAS users26, 27–30. Larger sample sizes are needed to differentiate the effects of electric-on-acoustic interference, electric stimulation gaps, and/or electric frequency-to-place mismatches on the performance of EAS users.
The present data have clinical implications for electrode array selection and EAS mapping. If the best outcomes are achieved by patients with the most apical electrode contact at the upper edge of the functional acoustic hearing region, then one option would be to implant an electrode array that approximates this region. Electrode array selection could be based on the individual’s cochlear morphology using preoperative imaging and their unaided acoustic thresholds. A consideration is the reported variability in postoperative hearing preservation both initially after surgery and over time31. Another approach would be to incorporate electrode array placement into the mapping of EAS, such as with place-based mapping. Place-based mapping procedures account for electrode array placement to eliminate electric mismatches and limit electric-on-acoustic interference26,30,32. Ongoing work is investigating the effectiveness of place-based maps for EAS users and comparing outcomes to EAS users with default maps whose most apical electrode contact is at the upper edge of the functional acoustic hearing region.
There are limitations of the current study that are worth consideration. The sample was limited to recipients of MED-EL lateral wall electrode arrays to control for variables known to influence speech recognition, such as electrode number33 and signal coding strategy34. We also excluded cases of revision surgery to control for previous listening experience with EAS. Importantly, functional hearing preservation has been observed post-revision, though little is known about the influence of the differences in electrode array placement and the resulting proximity to functional acoustic hearing35. Also, our sample size of EAS users limited the ability to assess other variables that may contribute to speech recognition in quiet and noise, such as age, cognition, and available acoustic hearing2,6,36,37. At the study site, patients elect whether to be fit with a CI-alone or EAS device. Others have observed that less than half of patients with hearing preservation use an EAS device, with patients reporting limited perceived benefit of an EAS device over a CI-alone device as the reason for not using EAS38,39. Interestingly, our sample of EAS users was similar to those who selected at CI-alone device (see Table 2). It remains unclear for our sample whether selection of a CI-alone versus EAS device was driven by perceived benefit or other variables (e.g., out-of-pocket costs associated with the acoustic component).
In this sample of lateral wall electrode array recipients with functional hearing preservation, 60% had electrode contacts within the functional acoustic hearing region. For EAS users, better performance was observed when the most apical electrode contact was close to the region of functional acoustic hearing. Poorer performance was seen when the electrode array was placed more basal to this region or if overlapping with the functional acoustic hearing region. Ongoing work is evaluating how default mapping procedures for EAS devices, which can result in electric-on-acoustic interference and electric frequency-to-place mismatches, contribute to outcomes and the effectiveness of place-based mapping procedures that account for electrode array placement.
Acknowledgements:
The authors thank Connor Zimmerman, Darla McDonald, Jacqueline Eberhard, and Stephanie Panoncillo for their assistance with the data review and Emily Buss, PhD for her contributions to the data analysis and editing of the manuscript.
Disclosures of funding and conflicts of interest:
Author KDB serves on the surgical advisory board for MED-EL Corp and Advanced Bionics. MTD is supported by a research grant from MED-EL Corporation. ABO serves on the audiology advisory board for MED-EL Corporation and Advanced Bionics. She is also a consultant for Cochlear Corporation.
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