Clin Exp Otorhinolaryngol.  2024 May;17(2):99-108. 10.21053/ceo.2023.00034.

Electrically Evoked Auditory Brainstem Response Using Extracochlear Stimulation at Different Cochlear Sites: A Comparison With Intracochlear Stimulation

• Sunwoo W
• Goo W
• Oh SH

• KMID: 2556000
• DOI: http://doi.org/10.21053/ceo.2023.00034

Abstract

Objectives
. The distribution and extent of excitable spiral ganglion neurons (SGNs) have been investigated using the electrically evoked auditory brainstem response (EABR) during preoperative and perioperative periods. In this study, we investigated the EABR with extracochlear stimulation (eEABR) as a preoperative test to estimate these factors.
Methods
. Sixteen male Sprague-Dawley rats were used in this study. Experiments were conducted in nine rats with normal hearing and seven rats that were partially deafened with ouabain treatment. Each experiment involved the following steps: extracochlear stimulating electrode placement at three different sites along the axis of the cochlea and eEABR recordings; cochleostomy and four-channel intracochlear array implantation, followed by EABR recordings with various electrode pair combinations; and after electrophysiological measurements, harvest of the cochleae for histopathological evaluation. The slope characteristics of the amplitude growth function measured from eEABR and EABR, frequency-specific auditory thresholds, and the density of SGNs were compared.
Results
. Similar trends were observed in slope changes on different sites of stimulation with both types of stimulation in normal-hearing animals—specifically, a monotonically increasing slope with increasing distance between bipolar pairs. In addition, eEABR slopes showed significant correlations with EABR slopes when the expected cochlear regions of stimulation were similar in normal-hearing animals. In partially deaf animals, the auditory thresholds at several frequencies had a significant correlation with the eEABR slopes of each extracochlear electrode at the apical, middle, and basal cochlear positions. This indicated that increasing the regions of cochlear stimulation had a differential impact on eEABR slopes, depending on the neural conditions.
Conclusion
. Our results indicated that eEABR slopes showed significant spatial correlations with the functionality of the auditory nerve. Therefore, eEABR tests at various cochlear positions might be used for estimating the extent of excitable SGNs in cochlear implant candidates prior to implantation.

Keyword

Cochlear Implants; Evoked Potentials, Auditory Brain Stem; Electric Stimulation; Spiral Ganglion

Figure

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  Fig. 1. Placement of the bipolar electrodes for electrical stimulation. (A) The extracochlear implant consists of a raddle-shaped active electrode on the cochlear surface and a ball-type return electrode in the round window niche (dotted line). (B) For intracochlear stimulation, two electrode arrays were inserted into the basal and middle turn via cochleostomy. The inset image shows the intracochlear electrode array inserted into the scala, which has four contacts. The contacts are numbered from 1 (most basal) to 4 (most apical).

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  Fig. 2. Example of electrically evoked auditory brainstem responses for the various stimulation levels (μA) recorded in a normal-hearing animal. (A) Arrow and arrowheads indicate the minima and maxima detected as N3 and P4 components. (B) The amplitude growth function for the N3-P4 peak-to-peak amplitudes is plotted for the same example data. The dashed lines indicate the slope characteristics and the maximum amplitude computed from the Boltzmann sigmoidal equation.

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  Fig. 3. The normalized amplitude growth functions in response to intracochlear (A) and extracochlear stimulation (B) from a single normalhearing animal. The slope of the growth function is marked in the dashed line. The bipolar electrode pair and extracochlear electrode positions are shown in the legend of each panel.

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  Fig. 4. Correlation of normalized slope (normalized amplitude/mA) of the electrically evoked auditory brainstem response (EABR) and EABR with extracochlear stimulation (eEABR), recorded in the normal-hearing group. Depicted are individual data (points) and linear regression lines of EABR to eEABR slopes. The EABR slopes were obtained with the two most widely spaced intracochlear electrodes. Electrode pairs are indicated by different symbols, as the legend (color online) notes. For eEABR slopes, the two different positions were analyzed separately (A: middle position, B: apical position). Pearson correlation coefficients (r) and P-values for each comparison are shown in Table 1.

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  Fig. 5. Tone-pip auditory brainstem response (ABR) audiograms and percent spiral ganglion neuron (SGN) loss as a function of cochlear frequency place (kHz) related to each cochlear region for individual partially deafened animals. Hearing degeneration (dB SPL, ●) is on the primary left axis, and the corresponding degree of SGN loss (%, ■) is plotted on the right axis.

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  Fig. 6. Scatter plots showing the relation between electrically evoked auditory brainstem response with extracochlear stimulation (eEABR) slope (normalized amplitude/mA) and auditory brainstem response (ABR) thresholds (dB SPL) at 20 kHz for the seven animals in the partially deaf group. Electrode regions are indicated by different symbols, as noted in the legend (color online). Pearson correlation coefficients (r) and P-values for each comparison are shown in Table 2.

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