Intracochlear Electrocochleography and Speech Perception Scores in Cochlear Implant Recipients

Abstract

Objectives/hypothesis: Previous studies have demonstrated that electrocochleography (ECochG) measurements made at the round window prior to cochlear implant (CI) electrode insertion can account for 47% of the variability in 6-month speech perception scores. Recent advances have made it possible to use the apical CI electrode to record intracochlear responses to acoustic stimuli. Study objectives were to determine 1) the relationship between intracochlear ECochG response amplitudes and 6-month speech perception scores and 2) to determine the relationship between behavioral auditory thresholds and ECochG threshold estimates. The hypothesis was that intracochlear ECochG response amplitudes made immediately after electrode insertion would be larger than historical controls (at the extracochlear site) and explain more variability in speech perception scores.

Study design: Prospective case series.

Methods: Twenty-two adult CI recipients with varying degrees of low-frequency hearing had intracochlear ECochG measurements made immediately after CI electrode insertion using 110 dB SPL tone bursts. Tone bursts were centered at five octave-spaced frequencies between 125 and 2,000 Hz.

Results: There was no association between intracochlear ECochG response amplitudes and speech perception scores. But, the data suggest a mild to moderate relationship between preoperative behavioral audiometric testing and intraoperative ECochG threshold estimates.

Conclusion: Performing intracochlear ECochG is highly feasible and results in larger response amplitudes, but performing ECochG before, rather than after, CI insertion may provide a more accurate assessment of a patient's speech perception potential.

Level of evidence: 4 Laryngoscope, 131:E2681-E2688, 2021.

Keywords: Electrocochleography; cochlear implant performance; intracochlear.

Figure 1.

An example of an intracochlear ECochG recording immediately following electrode insertion. Left panels (A,C,E,G and I) show Difference and Summation responses in the time domain; right panels (B,D,F,H and J) show Difference and Summation responses in the frequency domain for frequencies 125 Hz (A and B), 250 Hz (C and D), 500 Hz (E and F), 1000 Hz (G and H) and 2000 Hz (I and J). Significant peaks above the noise floor are marked with a closed Circle symbol. The sum of all significant peaks contributed to the Total Response. In this example, seven significant points contributed to the Total Response.

Figure 2.

Line graph demonstrating the change in CNC word scores (left) and AZBio sentences in quiet (right) over time, stratified by the magnitude of intraoperative ECochG TR (upper, middle, and lower third values). Clinical outliers are represented by an “X” and were excluded from the statistical analysis. Time explained most of the variance in speech perception scores (r² = 0.60, 95% CI 3.82 – 10.29 for CNC scores and r² = 0.57, 95% CI 0.99 – 17.01 for AZBio scores). The addition of ECochG TR accounted for an additional 3% of the variance.

Figure 3.

Scatterplot demonstrating the relationship between intraoperative ECochG TR and 6-month CNC word scores (left) and AZBio sentences in quiet (right). Patients > 80 years of age are represented by open black circles. Patients at risk for device failure are represented by gray circles. Clinical outliers are represented by an “X” and were excluded from the statistical analysis. Intraoperative ECochG TR explains 5.2% of the variability in 6-month CNC word scores (r²=0.052, 95% CI −0.27 – 0.69) and 4.0% of the variability in 6-month AZBio in quiet scores (r²=0.040, 95% CI −0.33 – 0.75).

Figure 4.

Box-whisker plots demonstrating the change in ECochG TR over time in all patients from immediately after CI insertion to beyond 12 months. The star symbols represent outliers, whose value is greater or lower than 1.5 times the interquartile range.

Figure 5.

Box-whisker plots demonstrating the change in ECochG TR over time in patients without residual hearing at 1-month (blue plots, n=13) and with residual hearing at 1-month (red plots, n=7). The star symbols represent extreme cases whose value is lower than 3 times the interquartile range. For those without residual hearing at 1 month, the number of cases with postoperative TR per time point were as follows: 1 month (n= 8), 3 months (n= 6), 6 months (n=10), 12 months (n=9), and greater than 12 months (n=3). For those with residual hearing at 1 month, the number of cases with postoperative TR per time point were as follows: 1 month (n= 5), 3 months (n= 5), 6 months (n=6), 12 months (n=3), and greater than 12 months (n=2).

Figure 6.

Scatterplots demonstrating the relationship between intraoperative ECochG predicted hearing thresholds and preoperative behavioral hearing thresholds (n=22) at 250 Hz to 2000 Hz. Intraoperative ECochG thresholds were estimated by using a 512-point FFT with a frequency resolution of 18 Hz. A Hanning window was used to analyze the CM responses and estimate the CM amplitude and noise floor. The CM amplitude is based on FFT amplitude at the stimulus frequency bin and the noise floor is estimated as the average of bins (−6,−5,−4,+4,+5, +6 where “0” bin corresponds to the stimulus frequency bin). An adaptive averaging between 8 to 40 repetitions was used to measure ECochG responses per stimulus frequency. An ECochG waveform with a signal-to-noise ratio of at least 24 dB was used for switching to the next stimulus frequency. The signal was treated as “No Response” if an ECochG signal could not be measured 6 dB above the noise floor after 40 repetitions. The CM amplitude measured at single stimulus presentation level was used to estimate CM thresholds using the following formula: Estimated CM (ECochG) threshold (dB HL) = Acoustic stimulus level (dB HL) – 20 * log10 (CM amplitude in μV/0.25 μV).” The scatterplot demonstrating hearing thresholds at 125 Hz was unable to be created as less than 5 participants had measurable thresholds at this frequency. At 250 Hz, Spearman’s rho = 0.175 (p=0.57), at 500 Hz, Spearman’s rho = 0.462 (p=0.13), at 1000 Hz, Spearman’s rho= 0.2 (p=0.51), and at 2000 Hz, Spearman’s rho = 0.06 (p=0.91).