Postoperative Electrocochleography from Hybrid Cochlear Implant users: An Alternative Analysis Procedure

Abstract

Objective: Shorter electrode arrays and soft surgical techniques allow for preservation of acoustic hearing in many cochlear implant (CI) users. Recently, we developed a method of using the Neural Response Telemetry (NRT) system built in Custom Sound EP clinical software to record acoustically evoked electrocochleography (ECoG) responses from an intracochlear electrode in Nucleus Hybrid CI users (Abbas et al., 2017). We recorded responses dominated by the hair cells (cochlear microphonic, CM/DIF) and the auditory nerve (auditory nerve neurophonic, ANN/SUM). Unfortunately, the recording procedure was time consuming, limiting potential clinical applications. This report describes a modified method to record the ECoG response more efficiently. We refer to this modified technique as the "short window" method, while our previous technique (Abbas et al., 2017) is referred as the "long window" method. In this report, our goal was to 1) evaluate the feasibility of the short window method to record the CM/DIF and ANN/SUM responses, 2) characterize the reliability and sensitivity of the measures recorded using the short window method, and 3) evaluate the relationship between the CM/DIF and ANN/SUM measures recorded using the modified method and audiometric thresholds.

Method: Thirty-four postlingually deafened adult Hybrid CI users participated in this study. Acoustic tone bursts were presented at four frequencies (250, 500, 750, and 1000 Hz) at various stimulation levels via an insert earphone in both condensation and rarefaction polarities. Acoustically evoked ECoG responses were recorded from the most apical electrode in the intracochlear array. These two responses were subtracted to emphasize the CM/DIF responses and added to emphasize the ANN/SUM responses. Response thresholds were determined based on visual inspection of time waveforms, and trough-to-peak analysis technique was used to quantify response amplitudes. Within-subject comparison of responses measured using both short and long window methods were obtained from seven subjects. We also assessed the reliability and sensitivity of the short window method by comparing repeated measures from 19 subjects at different times. Correlations between CM/DIF and ANN/SUM measures using the short window recording method and audiometric thresholds were also assessed.

Results: Regardless of the recording method, CM/DIF responses were larger than ANN/SUM responses. Responses obtained using the short window method were positively correlated to those obtained using the conventional long window method. Subjects who had stable acoustic hearing at two different time points had similar ECoG responses at those points, confirming high test-retest reliability of the short window method. Subjects who lost hearing between two different time points showed increases in ECoG thresholds, suggesting that physiologic ECoG responses are sensitive to audiometric changes. Correlations between CM/DIF and ANN/SUM thresholds and audiometric thresholds at all tested frequencies were significant.

Conclusion: This study compares two different recording methods. Intracochlear ECoG measures recorded using the short window technique were efficient, reliable, and repeatable. We were able to collect more frequency specific data with the short window method, and observed similar results between the long window and short window methods. Correlations between physiological thresholds and audiometric thresholds were similar to those reported previously using the long window method (Abbas et al., 2017). This is an important finding because it demonstrates that clinically-available software can be used to measure frequency-specific ECoG responses with enhanced efficiency, increasing the odds that this technique might move from the laboratory into clinical practice.

Keywords: Auditory nerve neurophonic; Cochlear implant; Cochlear microphonic; Electrocochleography; Hearing preservation; Hybrid; Neural response telemetry.

Figure 1.

Postoperative unaided audiogram of all study participants (n = 34). Solid black line indicates the grand mean with error bars of ± 1 standard deviation (SD). Light grey lines represent individual audiograms. No audiometric responses at the limits of the audiometer were coded as 125 dB HL.

Figure 2.

Recorded waveforms using the long window method (A and B) versus short window method (C and D) were compared. Recordings were obtained from a single subject (L58L) using 500Hz tone bursts presented at 95 dB SPL in positive polarity. Note the similarity in the recorded waveforms for the long and short window methods. Seven consecutive recordings (A) were made, then concatenated to produce a composite waveform (B) using the long window method. Similarly, two consecutive recordings (C) were made and the concatenated waveform (D) was obtained using the short window method.

Figure 3.

Example of the trough-to-peak analysis on CM/DIF and ANN/SUM waveforms. Responses obtained to 500Hz tone bursts presented at various stimulation levels from a single subject (L58L). By using a custom MATLAB peak-picking program, red markers were placed at the most robust troughs and peaks by visual inspection to identify thresholds and quantify amplitudes between the two points. Asterisks on the right indicate CM/DIF and ANN/SUM thresholds.

Figure 4.

(A) Raw time waveforms recorded in opposite polarities to 500 Hz tone bursts presented at 95 dB SPL from a single subject (L58L), (B) CM/DIF and ANN/SUM responses using the long versus short window methods. Responses using the long window method are marked in black, while overlaid waveforms in red show responses using the short window method.

Figure 5.

Amplitude growth functions of CM/DIF and ANN/SUM responses to 500Hz tone bursts from all participants. Solid black lines with filled symbols show the grand mean ± 1 SD. Light grey lines with empty symbols indicate individual subject data. Notice the different scales applied to ordinates of CM/DIF and ANN/SUM responses.

Figure 6.

Scatter plots of suprathreshold CM/DIF and ANN/SUM amplitudes recorded using the long and short window methods. The abscissa represents physiological responses using the short window method, while the ordinate shows those with the long window method. The dashed line is a reference line indicating equal amplitude of the short and long window results. Notice the different scales applied to ordinates of CM/DIF and ANN/SUM responses.

Figure 7.

Scatter plots of CM/DIF and ANN/SUM thresholds at test and retest for the stable hearing group. The abscissa represents physiological thresholds at test, while the ordinate shows those at retest session. The dashed line is a reference line indicating equal CM/DIF and ANN/SUM thresholds at test and retest.

Figure 8.

Examples from two subjects who experienced hearing loss over time. (A) Audiometric thresholds between test and retest. Note that the threshold values have been converted into dB SPL to facilitate comparison with physiologic thresholds, (B) CM/DIF and ANN/SUM thresholds changes at test and retest. Arrows indicate no response.

Figure 9.

Correlations between test – retest difference of physiologic and audiometric thresholds for the stable hearing group (in blue symbols) and delayed hearing loss group (in red symbols). Solid lines indicate linear regressions for each frequency. The dashed line is a reference line indicating equal difference of audiometric and physiologic thresholds between test and retest.

Figure 10.

Correlations between CM/DIF and ANN/SUM thresholds and audiometric thresholds across tested frequencies. Solid lines represent linear regressions for each frequency. The dashed line is a reference line, indicating the 1:1 relationship between audiometric and physiologic thresholds. The dashed line is a reference line indicating equal physiologic and audiometric thresholds.

Figure 11.

Box and whisker plots of differences between physiological thresholds and audiometric thresholds across tested frequencies, with medians (thin line) and means (thick line). Test statistics of the pairwise comparisons are presented under the box plots.