A Cochlear Implant Performance Prognostic Test Based on Electrical Field Interactions Evaluated by eABR (Electrical Auditory Brainstem Responses)

Consonant and vowel confusions in well-performing adult cochlear implant users, measured with a nonsense syllable repetition test

OBJECTIVE

The study's objective was to identify consonant and vowel confusions in cochlear implant (CI) users, using a nonsense syllable repetition test.

DESIGN

In this cross-sectional study, participants repeated recorded mono- and bisyllabic nonsense words and real-word monosyllables in an open-set design.

STUDY SAMPLE

Twenty-eight Norwegian-speaking, well-performing adult CI users (13 unilateral and 15 bilateral), using implants from Cochlear, Med-El and Advanced Bionics, and a reference group of 20 listeners with normal hearing participated.

RESULTS

For the CI users, consonants were confused more often than vowels (58% versus 71% correct). Voiced consonants were confused more often than unvoiced (54% versus 64% correct). Voiced stops were often repeated as unvoiced, whereas unvoiced stops were never repeated as voiced. The nasals were repeated correctly in one third of the cases and confused with other nasals in one third of the cases. The real-word monosyllable score was significantly higher than the nonsense syllable score (76% versus 63% correct).

CONCLUSIONS

The study revealed a general devoicing bias for the stops and a high confusion rate of nasals with other nasals, which suggests that the low-frequency coding in CIs is insufficient. Furthermore, the nonsense syllable test exposed more perception errors than the real word test.

Electrical Field Interactions during Adjacent Electrode Stimulations: eABR Evaluation in Cochlear Implant Users

The present study investigates how electrically evoked Auditory Brainstem Responses (eABRs) can be used to measure local channel interactions along cochlear implant (CI) electrode arrays. eABRs were recorded from 16 experienced CI patients in response to electrical pulse trains delivered using three stimulation configurations: (1) single electrode stimulations (E11 or E13); (2) simultaneous stimulation from two electrodes separated by one (E_n and E_n+2, E11 and E13); and (3) stimulations from three consecutive electrodes (E11, E12, and E13). Stimulation level was kept constant at 70% electrical dynamic range (EDR) on the two flanking electrodes (E11 and E13) and was varied from 0 to 100% EDR on the middle electrode (E12). We hypothesized that increasing the middle electrode stimulation level would cause increasing local electrical interactions, reflected in characteristics of the evoked compound eABR. Results show that group averaged eABR wave III and V latency and amplitude were reduced when stimulation level at the middle electrode was increased, in particular when stimulation level on E12 reached 40, 70, and 100% EDR. Compound eABRs can provide a detailed individual quantification of electrical interactions occurring at specific electrodes along the CI electrode array. This approach allows a fine determination of interactions at the single electrode level potentially informing audiological decisions regarding mapping of CI systems.

Intra-Cochlear Current Spread Correlates with Speech Perception in Experienced Adult Cochlear Implant Users

Broader intra-cochlear current spread (ICCS) implies higher cochlear implant (CI) channel interactions. This study aimed to investigate the relationship between ICCS and speech intelligibility in experienced CI users. Using voltage matrices collected for impedance measurements, an individual exponential spread coefficient (ESC) was computed. Speech audiometry was performed to determine the intelligibility at 40 dB Sound Pressure Level (SPL) and the 50% speech reception threshold: I40 and SRT50 respectively. Correlations between ESC and either I40 or SRT50 were assessed. A total of 36 adults (mean age: 50 years) with more than 11 months (mean: 34 months) of CI experience were included. In the 21 subjects for whom all electrodes were active, ESC was moderately correlated with both I40 (r = −0.557, p = 0.009) and SRT50 (r = 0.569, p = 0.007). The results indicate that speech perception performance is negatively affected by the ICCS. Estimates of current spread at the closest vicinity of CI electrodes and prior to any activation of auditory neurons are indispensable to better characterize the relationship between CI stimulation and auditory perception in cochlear implantees.

Word Recognition and Frequency Selectivity in Cochlear Implant Simulation: Effect of Channel Interaction

In cochlear implants (CI), spread of neural excitation may produce channel interaction. Channel interaction disturbs the spectral resolution and, among other factors, seems to impair speech recognition, especially in noise. In this study, two tests were performed with 20 adult normal-hearing (NH) subjects under different vocoded simulations. First, there was a measurement of word recognition in noise while varying the number of selected channels (4, 8, 12 or 16 maxima out of 20) and the degree of simulated channel interaction (“Low”, “Medium” and “High”). Then, there was an evaluation of spectral resolution function of the degree of simulated channel interaction, reflected by the sharpness (Q10dB) of psychophysical tuning curves (PTCs). The results showed a significant effect of the simulated channel interaction on word recognition but did not find an effect of the number of selected channels. The intelligibility decreased significantly for the highest degree of channel interaction. Similarly, the highest simulated channel interaction impaired significantly the Q10dB. Additionally, a strong intra-individual correlation between frequency selectivity and word recognition in noise was observed. Lastly, the individual changes in frequency selectivity were positively correlated with the changes in word recognition when the degree of interaction went from “Low” to “High”. To conclude, the degradation seen for the highest degree of channel interaction suggests a threshold effect on frequency selectivity and word recognition. The correlation between frequency selectivity and intelligibility in noise supports the hypothesis that PTCs Q10dB can account for word recognition in certain conditions. Moreover, the individual variations of performances observed among subjects suggest that channel interaction does not have the same effect on each individual. Finally, these results highlight the importance of taking into account subjects’ individuality and to evaluate channel interaction through the speech processor.

Diagnostic Accuracy of Intracochlear Test Electrode for Acoustic Nerve Monitoring in Vestibular Schwannoma Surgery

OBJECTIVES

Cochlear implants (CIs) are a well-known hearing restoration option for patients with vestibular schwannoma (VS) in cases of neurofibromatosis type-2 and, more recently, for patients with sporadic VS. One of the main limitations when performing CI during VS surgery is the capability to preserve the acoustic nerve (AN) anatomically and functionally. Significant efforts have been directed toward developing an intraoperative testing method for monitoring the AN function to determine if, after tumor removal, it is suitable for conducting stimuli delivered by a CI. However, all these methods have significant limitations, and none of them have documented diagnostic efficacy. To overcome these limitations and to obtain reliable information before CI insertion, a minimally invasive intracochlear test electrode (TE) has been recently developed. This TE has demonstrated to be suitable to test the integrity of the AN before CI in patients without any residual hearing by recording electrically evoked auditory brainstem responses (EABR). The present study constitutes the next phase of this research, which was to determine the usefulness of EABR obtained intraoperatively with the intracochlear TE after the resection of a VS and to calculate its diagnostic accuracy to assess the functionality of the AN for CI.

DESIGN

This was a prospective, multicenter study of diagnostic accuracy. It was conducted in three tertiary referral centers between January 2015 and 2018. This study was designed following the Standards for Reporting of Diagnostic Accuracy Studies (STARD) statement guidelines. The STARD statement are guidelines to improve the completeness and transparency of reports of diagnostic accuracy studies. The diagnostic accuracy of the EABR evoked with the intracochlear TE after tumor removal was studied. Accuracy, sensitivity, specificity, positive predictive value, and negative predictive value were calculated. Patients eligible for the study were consecutive adults undergoing surgery for VS with simultaneous CI. The test under evaluation (index test) was the EABR obtained with the intracochlear TE after resection of the tumor. The reference test (gold standard) was the presence of auditory perception with the CI, defined as the presence of sound detection on an audiogram at 500, 1000, 2000, and 4000 Hz of no greater than 50 dB. In all the cases, auditory perception was verified by the presence of a positive EABR evoked with the CI.

RESULTS

Twenty-one patients were included during the study period; seven patients were excluded from the diagnostic efficacy analysis due to inconclusive EABR results or absence of the gold standard to compare (they did not finally receive the CI). Thus, the outcome of the gold standard was assessed in 14 cases: 9 cases had positive EABR, all of them obtained auditory perception with the CI, and 5 cases had negative EABR, only one case had auditory perception with the CI, which constitutes the only false negative of this study. Accuracy of the TE was 93% (95% confidence interval, 66 to 100%), sensitivity 90% (95% confidence interval, 71 to 100%), specificity 100% (95% confidence interval, 100 to 100%), positive predictive value 100% (95% confidence interval, 100 to 100%), and negative predictive value 80% (95% confidence interval, 45 to 100%).

CONCLUSIONS

EABR elicited with the intracochlear TE had a diagnostic accuracy of 93% for predicting auditory perception with CIs after VS removal. These results suggest that the intracochlear TE can be used intraoperatively after tumor removal to test the integrity of the AN as a useful tool to complement the surgeon's perception for decision-making regarding implantation.

Consonant and Vowel Identification in Cochlear Implant Users Measured by Nonsense Words: A Systematic Review and Meta-Analysis

PURPOSE

The purpose of this systematic review and meta-analysis was to establish a baseline of the vowel and consonant identification scores in prelingually and postlingually deaf users of multichannel cochlear implants (CIs) tested with consonant-vowel-consonant and vowel-consonant-vowel nonsense syllables.

METHOD

Six electronic databases were searched for peer-reviewed articles reporting consonant and vowel identification scores in CI users measured by nonsense words. Relevant studies were independently assessed and screened by 2 reviewers. Consonant and vowel identification scores were presented in forest plots and compared between studies in a meta-analysis.

RESULTS

Forty-seven articles with 50 studies, including 647 participants, thereof 581 postlingually deaf and 66 prelingually deaf, met the inclusion criteria of this study. The mean performance on vowel identification tasks for the postlingually deaf CI users was 76.8% (N = 5), which was higher than the mean performance for the prelingually deaf CI users (67.7%; N = 1). The mean performance on consonant identification tasks for the postlingually deaf CI users was higher (58.4%; N = 44) than for the prelingually deaf CI users (46.7%; N = 6). The most common consonant confusions were found between those with same manner of articulation (/k/ as /t/, /m/ as /n/, and /p/ as /t/).

CONCLUSIONS

The mean performance on consonant identification tasks for the prelingually and postlingually deaf CI users was found. There were no statistically significant differences between the scores for prelingually and postlingually deaf CI users. The consonants that were incorrectly identified were typically confused with other consonants with the same acoustic properties, namely, voicing, duration, nasality, and silent gaps. A univariate metaregression model, although not statistically significant, indicated that duration of implant use in postlingually deaf adults predict a substantial portion of their consonant identification ability. As there is no ceiling effect, a nonsense syllable identification test may be a useful addition to the standard test battery in audiology clinics when assessing the speech perception of CI users.