Round window electrocochleography just before cochlear implantation: relationship to word recognition outcomes in adults

Abstract

Hypotheses: Electrocochleography (ECoG) to acoustic stimuli can differentiate relative degrees of cochlear responsiveness across the population of cochlear implant recipients. The magnitude of the ongoing portion of the ECoG, which includes both hair cell and neural contributions, will correlate with speech outcomes as measured by results on CNC word score tests.

Background: Postoperative speech outcomes with cochlear implants vary from almost no benefit to near normal comprehension. A factor expected to have a high predictive value is the degree of neural survival. However, speech performance with the implant does not correlate with the number and distribution of surviving ganglion cells when measured postmortem. We will investigate whether ECoG can provide an estimate of cochlear function that helps predict postoperative speech outcomes.

Methods: An electrode was placed at the round window of the ear about to be implanted during implant surgery. Tone bursts were delivered through an insert earphone. Subjects included children (n = 52, 1-18 yr) and postlingually hearing impaired adults (n = 32). Word scores at 6 months were available from 21 adult subjects.

Results: Significant responses to sound were recorded from almost all subjects (80/84 or 95%). The ECoG magnitudes spanned more than 50 dB in both children and adults. The distributions of ECoG magnitudes and frequencies were similar between children and adults. The correlation between the ECoG magnitude and word score accounted for 47% of the variance.

Conclusion: ECoGs with high signal-to-noise ratios can be recorded from almost all implant candidates, including both adult and pediatric populations. In postlingual adults, the ECoG magnitude is more predictive of implant outcomes than other nonsurgical variables such as duration of deafness or degree of residual hearing.

Figure 1

Example of an ECoG recording from the round window during cochlear implantation. A: response to 500 Hz at 90 dB nHL. The top panel demonstrates the time series from the condensation starting phase. The lower panel is the spectrum of the response. Note that several harmonics are present. B: Response (time series and spectrum) to 250 Hz at 90 dB nHL. C: Response to 1000 Hz at 90 dB nHL.

Figure 2

Distribution of total response magnitudes for pediatric and adult cochlear implant recipients. The total response is defined as the sum of first and second harmonics across all frequencies where the response was significant. Arrows are the mean for each sample.

Figure 3

Responses according to frequency. A. Distribution of best frequencies for children and adults. The best frequency was defined as the frequency that contributed the most to the total response. B. The proportion of the total response contributed by each frequency. Data points represent the mean and error bars are the standard error. C. Proportion of significant responses to each frequency. Note that for all cases the responses across frequency are similar for children and adults.

Figure 4

Comparison of the ECoG magnitude (total response) with scores on the CNC words test. A. Scatterplot of results. The case with an asterisk is an outlier that was not included in the calculation of the line (see text and Fig. 5 for further consideration of this case). B. Box and whisker plot of the results for the sample divided into thirds.

Figure 5

An outlier case where the total response magnitude was large but the subject’s speech perception performance was less than expected (see asterisk in Figure 4). The stimulus was 500 Hz at 90 dB nHL. The small amount of harmonic distortion indicates that the ongoing response was primarily a cochlear microphonic with no contribution from neural phase-locking. Note also the lack of a compound action potential. This pattern of results suggests a limited neural response.

Figure 6

Comparisons with residual hearing. A. Pure tone average vs. total response. There was a moderate correlation between these factors. B. Pure tone average vs. CNC word score. There was a small but significant correlation between these factors.