Edited by: Simone Dalla Bella, University of Montpellier 1, France
Reviewed by: Aravindakshan Parthasarathy, Mass Eye and Ear, Harvard Medical School, United States; Ingrid Johnsrude, University of Western Ontario, Canada
*Correspondence: Douglas C. Fitzpatrick
This article was submitted to Auditory Cognitive Neuroscience, a section of the journal Frontiers in Neuroscience
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Auditory neuropathy spectrum disorder (ANSD) is characterized by an apparent discrepancy between measures of cochlear and neural function based on auditory brainstem response (ABR) testing. Clinical indicators of ANSD are a present cochlear microphonic (CM) with small or absent wave V. Many identified ANSD patients have speech impairment severe enough that cochlear implantation (CI) is indicated. To better understand the cochleae identified with ANSD that lead to a CI, we performed intraoperative round window electrocochleography (ECochG) to tone bursts in children (
Auditory neuropathy spectrum disorder (ANSD) is a hearing dysfunction characterized by an apparent discrepancy between the measures of cochlear and neural function when viewed by surface electrode auditory brainstem response (ABR) testing. Relatively healthy hair cells are identified by the presence of otoacoustic emissions (OAEs) and/or cochlear microphonic (CM) in ABR testing, coupled with small or absent wave V (Kaga et al.,
Speech perception outcomes with cochlear implantation, including those with ANSD, demonstrate wide variations from patient to patient (Cohen et al.,
When using low frequency tones, the “on-going response” (continuous steady state response to tones) of the ECochG signal, which is used to calculate the ECochG-TR, is typically composed of the cochlear microphonic (CM) and the auditory nerve neurophonic (ANN). The CM is derived from currents through mechano-sensitive transduction channels in the stereocilia of hair cells (Dallos,
Data in this study include 296 ears from 267 subjects (29 were second sides). Of these, 285 ears were studied under the approval of the Institutional Review Board (IRB) at the University of North Carolina at Chapel Hill (#05-2616) and 11 ears from the Ohio State University (OSU) and Nationwide Children's Hospital (Ohio State University IRB approval #2015H0045). Adults and pediatric (<18 years of age) CI recipients who were English speaking or whose parents were English speaking, and whose ear for implantation was not atretic, were offered enrollment in the study. Written informed consent was obtained from all adults, and parental/guardian consent was obtained for all pediatric subjects. Children who had attained 7 years of age were also asked to assent to participate in the study. In the situation where both ears were implanted and recorded, each ear was considered separately.
A total of 48 ears (39 subjects) were in the ANSD group, 45 ears from children and 3 from adults. The evaluation and management paradigm for children with ANSD is the same between participating study institutions, which for UNC has been published previously (Buchman et al.,
All ECochG recordings were made to acoustic stimulation from the round window (RW) intraoperatively during CI surgery. For the purposes of this study, a foam insert earphone was placed and secured in a manner to prevent occlusion of the sound tubing. The inverting and common electrodes were placed behind the contralateral mastoid and on the glabella, respectively. A standard transmastoid facial recess approach was employed. The anterosuperior portion of the RW overhang was drilled to provide better access to the RW niche. A monopolar electrode (Neurosign, Magstim Co., Wales, UK or Neuro-Kartush raspatory probe instrument, Integra, Plainsboro, NJ, U.S.A.) was then placed with the tip situated immediately within the RW niche. Impedance of the RW and surface electrodes were measured and recordings were terminated if any had impedances of greater than 16 kilo-ohms (kΩ) that could not be reduced below this point. Saline was introduced into the RW niche if the monopolar electrode impedance was high; this was typically enough to bring the impedance measurement to an acceptable level. The Bio-logic Navigator Pro (Natus Medical Inc., San Carlos, CA) system was used to generate acoustic stimuli and record responses. Acoustic stimuli were delivered from Etymotic speaker (ER-3b) through sound tubing and insert earphones. Responses to a frequency series were performed in all subjects, and in most subjects a level series was then performed at the frequency which elicited the strongest response during the prior sweep (typically 500 Hz). The frequency series consisted of 250, 500, 750, 1,000, 2,000, and 4,000 Hz tone bursts presented in alternating phase at 90 dB nHL (101–112 dB SPL). A Blackman window was used to shape the tone bursts which had 1–4 ms rise and fall times with plateaus ranging from 5 to 20 ms (lower frequencies 250–750 Hz had shorter rise and fall times with longer plateaus compared to higher frequencies). Next the level series began at 90 dB nHL and was typically performed in 10 dB decrements until no response was seen during the recordings. Condensation, rarefaction, as well as the difference and sums of pairs of these were stored as averages in separate buffers. A final trial was included where the sound tubing was occluded with a surgical clamp to ensure the recorded responses were not speaker artifact.
The ECochG results were processed and analyzed using custom software routines written in MATLAB. The condensation and rarefaction traces were extracted and used to calculate the sum and difference waveforms. To evaluate the overall residual response magnitude from each cochlea we measured the “total response,” or ECochG-TR, from the ongoing, steady-state part of the response to the tones (Fitzpatrick et al.,
For each frequency a window (4–12 cycles per window dependent on frequency with bin widths ranging from 62 Hz at 250–331 Hz at 4,000 Hz) that isolated the ongoing portion, which occurs after the CAP and prior to the end of the stimulus, was selected for a fast Fourier transform (FFT) to analyze the spectral characteristics of the response. A significant response at a given stimulus frequency or harmonic was present if it exceeded the noise level by three standard deviations. The noise and its variance were determined from up to 6 bins, 3 on each side of the peak that were outside the ranges of response to the stimulus frequency. Responses that were not significant were given a value of 0.02 μV, which is the limit of our detection threshold, when included in summary data. The ECochG-TR was calculated as the sum of the magnitudes of the significant responses at the first, second and third harmonics across all 6 stimulus frequencies, all presented at 90 dB nHL. The first and third harmonics were measured from the difference of the two phases, and the second harmonic from the sum.
In the Introduction, we described some of the issues related to measuring and separating the different potentials in the ECochG. Here, we will describe the presumed sources for each potential and describe in more detail the issues with more quantitative measurements that lead to the development of the nerve score. In Figures
Schematics of the sources of the CM, ANN, and CAP
The ANN is the evoked potential correlate of neural phase-locking to low frequency stimuli, which is the firing of action potentials over restricted portions of a stimulus cycle. Like the CAP (Goldstein and Kiang,
As mentioned, the CAP is produced by the convolution of the unit potential and the population PSTH of auditory nerve fibers (Figure
The SP (not shown) is produced by multiple sources capable of producing a DC response to tones. These include the asymmetry in hair cell transduction, which is likely to different between inner and outer hair cells, which also differ in their membrane properties (Kros,
With these features of the ECochG signal in mind, the goal of this study is to subjectively characterize the presence of neural compared to hair cells components in the responses to tones of children with and without ANSD who are receiving CIs. The neural components are the ANN and the CAP, with some neural contribution to the SP a possibility as well. The descriptions of the sources of these potential provided above helps to explain why the ANN and CAP are difficult to quantify, such that only a qualitative method was used. That is, the ANN is always mixed with the CM in the ongoing part of the response to the low frequency tone. However, the ANN is generally the more distorted signal, because the shape of the unit potential is unrelated to the stimulus, and the cycle histogram is roughly half-wave rectified. So the presence of strong harmonics, both even and odd, is evidence of the presence of the ANN. However, due to its periodicity the ANN's magnitude cannot be known because some or most will be in the first harmonic, where the largest part of the CM also resides. Furthermore, some of the CM can be in the higher harmonics due to the asymmetric and symmetric distortions described in Figure
In many CI subjects the CAP is obvious and can be measured using accepted methods. However, in a recent study (Scott et al.,
The SP is relatively easily quantified as a sustained shift in the baseline. Here the problem is one of interpretation, since the sources of the SP are less than fully clear. However, we will present data from ANSD subjects that suggests there is a neural contribution to the SP.
For these reasons we have not yet found an acceptable objective means of identifying neural activity in each case. That is, although many features, such as large CAP or large harmonic distortions clearly correlate with neural activity, each metric has issues with false positive or negatives. In most cases, the reasons for the results can be observed in the responses themselves when further examined. In Figures
The example in D is a case with both a strong ANN and a strong CAP. A strong ANN is suggested by the prominent response at twice the stimulus frequency seen in the summed curve to the 250 Hz stimulus, and is clearly seen in the “average cycle” (lower left, solid curve). This curve is the average of all cycles from condensation phase stimuli in a window after the CAP, and from rarefaction stimuli after shifting the response in time to match that of the condensation phase. The average curve in this case is highly distorted compared to a sinusoid representing the stimulus (dashed line) that has been shifted in phase to have the best fit to the response. The lack of an ANN to the high frequency stimulus is shown by the lack of an AC component in the summed curve, and to a purely sinusoidal average cycle. The CAP (arrows) is most clearly seen to the high frequency stimulus in the summed curve, but is also readily visible in the response to condensation phase stimuli. However, it is embedded in an SP that is rising during the same time period, making its measurement problematic.
The example in E is a case where the ANN and CAP were small relative to the CM. To the low frequency stimulus there was still an AC component in the summed curve, but inspection of the average cycle showed a peak-flattened shape that is consistent with rectification of the CM as much as the presence of an ANN. There is also some AC response to twice the stimulus frequency in the summed curve to the high frequency stimulus, representing asymmetry in the CM rather than the ANN. To both frequencies a CAP is present but small CAP (arrows).
Because of these difficulties in measurement of the ANN and CAP we devised a subjective scale termed the “nerve score.” To classify the presence and strength of the ANN we examine the average cycle of the ongoing response to low frequency tones (1,000 Hz and less). An ANN was considered present when the response appeared as a distorted version of a sinusoid, and the distortion was not compatible with a simple rectification or saturation of the CM. Our previous animal experiments where the neural responses were removed with kainic acid (Forgues et al.,
To tones, the SP is a baseline shift that persists for the duration of the tone (Figure
The ECochG-TR magnitudes as a function of age for the entire cohort are depicted in Figure
RW ECochG-TR for all CI ears.
The proportion of significant responses obtained at each frequency was also different among the groups (Figure
Most of the ANSD cases were children, who have a distinct mix of hearing loss etiologies that leads to CI use. As expected from the results in Figure
Distributions of ECochG-TR by etiology in children. ANSD subjects had the highest ECochG-TR magnitudes. Outliers: o, Close extreme; *, Far extreme.
It might be expected that the large responses seen in the ANSD cohort would be associated with relatively low nerve activity. However, we found a wide spectrum of neural responses, which spanned the full range of “nerve scores.” Examples of ANSD cases with nerve scores demonstrating a high degree of neural activity, in the form of a CAP and/or ANN, are shown in Figure
ECochG examples of ANSD subjects with considerable evidence of neural activity. For each case the response to condensation and rarefaction phase of a low frequency stimulus is shown on the left. The middle panels show the individual (dotted lines) and average cycles (thick line) to condensation phase stimuli taken from a window (8–20 ms) intended to isolate the ongoing, or steady state portion of the response. The solid black line is the best fit sinusoid. The right panels show the sum of responses to the two phases, for the frequencies as shown, which isolates the CAP and SP.
The case in Figure
Examples of cases with nerve scores demonstrating a low degree of neural activity (nerve score ≤ 1) are shown in Figure
ECochG examples with minimal neural activity.
Two additional cases in ANSD subjects help to show the sensitivity of the method to identify neural activity even in cases where it is expected to be small. One of the cases was a 1 year old with a mutation in the gene for otoferlin, a protein required for docking of vesicles containing neurotransmitter. This was the only one of our sample with this etiology. This presynaptic site of lesion should block the ANN but not affect transduction, so the phenotype expected is a large CM with no ANN. The case did show a very large CM to all frequencies as expected. However, there was also evidence for neural activity in the average cycle to a 250 Hz tone (Figure
Average 500 Hz cycle for two different known etiologies of the ANSD group.
The distributions of the different patterns of ANN and CAP are shown in Table
Distribution of nerve scores in ANSD subjects.
2 | 2 | 4 | 6 | ||
1 | 2 | 3 | 12 | ||
2 | 1 | 3 | 0 | ||
0 | 2 | 2 | 2 | ||
2 | 0 | 2 | 2 | ||
1 | 1 | 2 | 7 | ||
1 | 0 | 1 | 7 | ||
0 | 1 | 1 | 9 | ||
0 | 0 | 0 | 3 | ||
Total | 48 |
To compare the nerve scores among the different groups, only those where the ECochG-TR was >0.5 μV were used for the non-ANSD groups. The nerve score for responses smaller than this were always 0 because components other than the CM could not be visually distinguished. All of the ANSD subjects had and ECochG-TR >0.5 μV. Nerve scores were not significantly different among subjects with different etiologies of hearing loss (Figure
Nerve score distributions for children with different etiologies of hearing loss. All groups showed the full range of nerve scores, and the distribution in the ANSD group was not significantly different from the others.
To frequencies of 1,000 Hz or greater the SP could be prominent, but to lower frequencies it was typically small. In response to the higher frequencies, three morphologies of the SP were observed, as illustrated in Figures
The distributions of SP polarity and magnitude differed among ANSD and non-ANSD groups. The frequency where these differences were most clearly seen was 2,000 Hz as in Figures
SP morphologies to 2,000 Hz at 90 dB nHL in ANSD and non-ANSD subjects.
No SP | 7 | 5 | 8 | 20 |
CAP, small SP | 3 | 3 | 10 | 16 |
Negative SP, no CAP | 17 | 1 | 5 | 23 |
Total | 27 | 9 | 23 | 59 |
Illustration of the differences in the values of SP for ANSD and non-ANSD subjects is shown in Figure
Distributions of the SP to 2,000 Hz at 90 dB nHL among the three groups.
In addition to the SP, a number of ANSD cases (
Overshoot potential in some ANSD children reflected in the “sum” tracing.
Three adults with ANSD were identified by the presence of CM on ABR, after audiological testing had suggested ANSD. As with the children, the ECochG-TRs in adults were large (Figure
Our expectation was that ANSD subjects would have a large cochlear response and relatively little neural activity compared to other CI subjects. The results were that ANSD cases had on average a larger ECochG-TR; the responses extended more often to high frequencies; and responses to each frequency were on average larger in ANSD compared to non-ANSD cases. However, in the ANSD cases there was a full range of “nerve scores,” derived from CAPs and the ANN, with the scores dominated by the presence of the ANN. Thus, to low frequencies there was little difference in neural activity in ANSD compared to non-ANSD cases. In contrast, to high frequencies the majority of ANSD cases showed no CAP and a strongly negative SP, while this pattern was rare in the non-ANSD groups. Thus, the hallmark of ECochG in subjects with a clinical report of ANSD is of large responses with a lack of neural activity to high frequencies, combined with responses to low frequencies that have the same distribution of neural activity as found in non-ANSD cases. In the following, we will describe how these attributes are fully compatible with the main clinical findings of a CM with small or absent wave V in ABR results.
To high frequency stimuli the cyclic response to tones consists purely of the CM, since it is above the range of phase-locking in the auditory nerve. The main distinction of ANSD compared to other CI subjects is the large CM to high frequencies, which accounts for the appearance of the CM in ABR recordings from these subjects. We did not fully explore the upper end of the frequency range, since in most subjects the highest frequency used was 4 kHz, where most ANSD subjects still had robust responses, in contrast to the non-ANSD groups, where responses to 2 and 4 kHz were relatively rare.
To low frequencies the responses in ECochG are still primarily the CM, even though they can be mixed with the ANN, when present. Thus, the larger overall responses to low frequencies in ANSD compared to non-ANSD subjects could indicate greater CM from the apex than in the non-ANSD groups. However, a more likely cause is the additional CM from higher CF regions of the cochlea that respond to low frequency stimuli as well.
The presence of the CM indicates the integrity of hair cells, but it cannot be specifically localized to outer hair cells, as is generally understood to be the case in studies of normal hearing animals (Dallos,
The CAP is a highly variable feature in CI subjects (Scott et al.,
The finding of a large degree of neural activity to low frequencies seems at odds with the clinical understanding of ANSD as representing an underlying etiology that affects the chain from IHCs to the CNS differently than in non-ANSD cases. However, the main difference between the clinical definitions of ANSD used here is the presence of a CM; both groups are receiving a CI and thus have a small or absent wave V. So, as previously discussed, the presence of a CM is well accounted for by the ECochG results showing greater hair cell activity to high frequencies, and the small but measurable neural activity primarily to low frequencies across all groups accounts for the reduced magnitudes of later waves in the ABR.
Despite its first description in the 1950s (Davis et al.,
Two cases with cochlear nerve deficiency, an extreme example of ANSD (Figures
In some cases, a large transient potential was observed to the stimulus offset to high frequencies. There was no CAP at stimulus onset in these cases, so the offset potential is unlikely to be a CAP to the offset. These responses were scored as “no SP” but a small or absent SP can also indicate a balance of contributions from outer hair cells, inner hair cells, and the auditory nerve. That is, different sources of sustained potentials can sum to be near zero at the steady state, while different time courses for each source allow them to be revealed when the stimulus changes.
Most previous studies of ECochG in ANSD children undergoing cochlear implantation used 8 kHz tone pips or clicks as stimuli (Gibson and Sanli,
The focus of much of the previous work with ECochG in ANSD subjects has been to identify phenotypes showing different sites of lesion that may result in different speech perception outcomes (Gibson and Sanli,
In both adult and pediatric non-ANSD cases the ECochG-TR was on average lower than in ANSD cases. In children, those with inflammatory reactions including CMV or meningitis had the lowest ECochG-TR. For those with the smallest responses there were no detectable CAPs, ANNs or SPs that could be distinguished from a sinusoidal CM. However, to the majority of cases where these additional potentials could be detected, the neural involvement covered the full spectrum of nerve scores, similar to the ANSD group. Previously, it was noted that adults and children had similar ranges of ECochG-TR, and a similar distribution of frequencies that contributed to the responses, as also reported here in Figure
The difference between ANSD and non-ANSD subjects lies primarily in the high frequency regions of the cochlea. These regions produce a larger CM and SP, and are less likely to produce a CAP, compared to non-ANSD subjects. These features are consistent with a large hair cell response combined with a limited neural response expected for ANSD. In contrast, for responses to low frequencies the neural components, primarily in the form of the ANN, are similar between ANSD and non-ANSD subjects, and vary from no evidence of neural contributions to clear evidence of CAP and/or ANN. Therefore, responses from low frequency parts of the cochlea produce a similarly wide distribution of evidence for neural activity between ANSD and non-ANSD subjects. It remains to be determined if the levels of neural activity seen using acoustic stimuli by ECochG are important in speech perception outcomes with the CIs.
This study was carried out in accordance with the approved protocols and recommendations of the University of North Carolina at Chapel Hill's Institutional Review Board and the Ohio State University's Institutional Review Board (#05-2616 and #2015H0045) with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki.
WR, JR, CG, MH, ZB, TF, CB, KB, DF, OA contributed to data collection and manuscript preparation. WR, JR, DF, and OA contributed to analysis of data.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.