Neural Degeneration in Normal-Aging Human Cochleas: Machine-Learning Counts and 3D Mapping in Archival Sections

The relationships between cochlear nerve health and AzBio sentence scores in quiet and noise in postlingually deafened adult cochlear implant users

Objectives

This study investigated the relationships between the cochlear nerve (CN) health and sentence-level speech perception outcomes measured in quiet and noise in postlingually deafened adult cochlear implant (CI) users.

Design

Study participants included 24 postlingually deafened adult CI users with a Cochlear® Nucleus™ device. For each participant, only one ear was tested. Neural health of the CN was assessed at three or four electrode locations across the electrode array using two parameters derived from results of the electrically evoked compound action potential (eCAP). One parameter was the phase locking value (PLV) which estimated neural synchrony in the CN. The other parameter was the sensitivity of the eCAP amplitude growth function (AGF) slope to changes in the interphase gap (IPG) of biphasic electrical pulses (i.e., the IPGEslope). Speech perception was tested using AzBio sentences in both quiet and a ten-talker babble background noise with +5 dB and +10 dB signal-to-noise ratios (SNR). IPGEslope and PLV values were averaged across electrodes for each subject, both with and without weighting by the frequency importance function (FIF) of the AzBio sentences. Pearson and Spearman correlations were used to assess the pairwise relationships between the IPGEslope, the PLV, and age. Multiple linear regression models with AzBio score as the outcome and the PLV and the IPGEslope as predictors were used to evaluate the associations between the three variables while controlling for age.

Results

The correlation between the IPGEslope and the PLV was negligible and not statistically significant. The PLV, but not the IPGEslope, differed significantly across electrodes, where the apical electrodes had larger PLVs (better neural synchrony) than the basal electrodes. The IPGEslope, but not the PLV, was significantly correlated with participant's age, where smaller IPGEslope values (poorer CN health) were associated with more advanced age. The PLV, but not the IPGEslope, was significantly associated with AzBio scores in noise, where larger PLVs predicted better speech perception in noise. Neither the PLV nor the IPGEslope was significantly associated with AzBio score in quiet. The result patterns remained the same regardless of whether the mean values of the IPGEslope and the PLV were weighted by the AzBio FIF.

Conclusions

The IPGEslope and the PLV quantify different aspects of CN health. The positive association between the PLV and AzBio scores suggests that neural synchrony is important for speech perception in noise in adult CI users. The lack of association between age and the PLV indicates that reduced neural synchrony in the CN is unlikely the primary factor accounting for the greater deficits in understanding speech in noise observed in elderly, as compared to younger, CI users.

Increased listening effort and cochlear neural degeneration underlie behavioral deficits in speech perception in noise in normal hearing middle-aged adults

Middle-age is a critical period of rapid changes in brain function that presents an opportunity for early diagnostics and intervention for neurodegenerative conditions later in life. Hearing loss is one such early indicator linked to many comorbidities later in life. However, current clinical tests fail to capture hearing difficulties for ∼10% of middle-aged adults seeking help at hearing clinics. Cochlear neural degeneration (CND) could play a role in these hearing deficits, but our current understanding is limited by the lack of objective diagnostics and uncertainty regarding its perceptual consequences. Here, using a cross-species approach, we measured envelope following responses (EFRs) - neural ensemble responses to sound originating from the peripheral auditory pathway - in young and middle-aged adults with normal audiometric thresholds, and compared these responses to young and middle-aged Mongolian gerbils, where CND was histologically confirmed. We observed near identical changes in EFRs across species that were associated with CND. Perceptual effects measured as behavioral readouts showed deficits in the most challenging listening conditions and were associated with CND. Additionally, pupil-indexed listening effort increased even at moderate task difficulties where behavioral outcomes were matched. Our results reveal perceptual deficits in middle-aged adults driven by CND and increases in listening effort, which may result in increased listening fatigue and conversational disengagement.

Delayed hearing loss after cochlear implantation: Re-evaluating the role of hair cell degeneration

Delayed loss of residual acoustic hearing after cochlear implantation is a common but poorly understood phenomenon due to the scarcity of relevant temporal bone tissues. Prior histopathological analysis of one case of post-implantation hearing loss suggested there were no interaural differences in hair cell or neural degeneration to explain the profound loss of low-frequency hearing on the implanted side (Quesnel et al., 2016) and attributed the threshold elevation to neo-ossification and fibrosis around the implant. Here we re-evaluated the histopathology in this case, applying immunostaining and improved microscopic techniques for differentiating surviving hair cells from supporting cells. The new analysis revealed dramatic interaural differences, with a > 80 % loss of inner hair cells in the cochlear apex on the implanted side, which can account for the post-implantation loss of residual hearing. Apical degeneration of the stria further contributed to threshold elevation on the implanted side. In contrast, spiral ganglion cell survival was reduced in the region of the electrode on the implanted side, but apical counts in the two ears were similar to that seen in age-matched unimplanted control ears. Almost none of the surviving auditory neurons retained peripheral axons throughout the basal half of the cochlea. Relevance to cochlear implant performance is discussed.

On the Difficulty Predicting Word Recognition Performance After Cochlear Implantation

HYPOTHESIS

Preimplantation word scores cannot reliably predict postimplantation outcomes.

BACKGROUND

To date, there is no model based on preoperative data that can reliably predict the postoperative outcomes of cochlear implantation in the postlingually deafened adult patient.

METHODS

In a group of 228 patients who received a cochlear implant between 2002 and 2021, we tested the predictive power of nine variables (age, etiology, sex, laterality of implantation, preimplantation thresholds and word scores, as well as the design, insertion approach, and angular insertion depth of the electrode array) on postimplantation outcomes. Results of multivariable linear regression analyses were then interpreted in light of data obtained from histopathological analyses of human temporal bones.

RESULTS

Age and etiology were the only significant predictors of postimplantation outcomes. In agreement with many investigations, preimplantation word scores failed to significantly predict postimplantation outcomes. Analysis of temporal bone histopathology suggests that neuronal survival must fall below 40% before word scores in quiet begin to drop. Scores fall steeply with further neurodegeneration, such that only 20% survival can support acoustically driven word scores of 50%. Because almost all cochlear implant implantees have at least 20% of their spiral ganglion neurons (SGNs) surviving, it is expected that most cochlear implant users on average should improve to at least 50% word recognition score, as we observed, even if their preimplantation score was near zero as a result of widespread hair cell damage and the fact that ~50% of their SGNs have likely lost their peripheral axons. These "disconnected" SGNs would not contribute to acoustic hearing but likely remain electrically excitable.

CONCLUSION

The relationship between preimplantation word scores and data describing the survival of SGNs in humans can explain why preimplantation word scores obtained in unaided conditions fail to predict postimplantation outcomes.