Audiological Profile of Adult Persons with Auditory Neuropathy Spectrum Disorders

From hidden hearing loss to supranormal auditory processing by neurotrophin 3-mediated modulation of inner hair cell synapse density

Loss of synapses between spiral ganglion neurons and inner hair cells (IHC synaptopathy) leads to an auditory neuropathy called hidden hearing loss (HHL) characterized by normal auditory thresholds but reduced amplitude of sound-evoked auditory potentials. It has been proposed that synaptopathy and HHL result in poor performance in challenging hearing tasks despite a normal audiogram. However, this has only been tested in animals after exposure to noise or ototoxic drugs, which can cause deficits beyond synaptopathy. Furthermore, the impact of supernumerary synapses on auditory processing has not been evaluated. Here, we studied mice in which IHC synapse counts were increased or decreased by altering neurotrophin 3 (Ntf3) expression in IHC supporting cells. As we previously showed, postnatal Ntf3 knockdown or overexpression reduces or increases, respectively, IHC synapse density and suprathreshold amplitude of sound-evoked auditory potentials without changing cochlear thresholds. We now show that IHC synapse density does not influence the magnitude of the acoustic startle reflex or its prepulse inhibition. In contrast, gap-prepulse inhibition, a behavioral test for auditory temporal processing, is reduced or enhanced according to Ntf3 expression levels. These results indicate that IHC synaptopathy causes temporal processing deficits predicted in HHL. Furthermore, the improvement in temporal acuity achieved by increasing Ntf3 expression and synapse density suggests a therapeutic strategy for improving hearing in noise for individuals with synaptopathy of various etiologies.

[The issue of auditory neuropathy: from origins to the present]

The issue of auditory neuropathy spectrum disorders (ANSD) has been in a focus of specialists attention for a relatively short time, but during this time a huge amount of scientific and practical knowledge about this hearing disorder has been accumulated. ANSD is a specific auditory deficit caused by dysfunction of periphery part of the auditory system, which may affect the inner hair cells, the spiral ganglion neurons and the auditory nerve, as well as the area of synaptic contact between them, while the outer hair cells, as a rule, remain intact. As a result, a specific condition is formed, in which a patient's otoacoustic emissions and/or cochlear microphonics are present, auditory brainstem responses are abnormal or absent, electrophysiological data may not correlate with hearing level, the discrepancy between pure tone audiometry and speech discrimination is observed. ANSD prevalence, epidemiology, contemporary views on its etiology, including detailed information on hereditary forms of the disorder and its risk factors are considered in the review. The data on the basic rungs of the ANSD pathogenesis, which underlie the development of various forms of the disorder and mainly determine the rehabilitation approach, are presented. The detailed clinical and audiological characteristics of ANSD are presented; contemporary approach to ANSD diagnosis and rehabilitation, including indications for surgical treatment, are considered.

Acoustical and Perceptual Analysis of Noise Reduction Strategies in Individuals With Auditory Neuropathy Spectrum Disorders

Purpose The conventional amplification devices render minimal or no benefit at abating the speech perception problems of individuals with auditory neuropathy spectrum disorder (ANSD). This study was undertaken to evaluate the effect of noise reduction strategies (multiband spectral subtraction, Wiener-as, Karhunen-Loeve transform [Subspace], and ideal binary mask [IdBM] algorithm) on speech using speech perception measures and acoustic measure among individuals with ANSD. Method Two groups of participants (age: 17-43 years) were recruited in the study. Group I comprised 12 individuals with a confirmed diagnosis of ANSD and not exceeding moderate degree of hearing loss and Group II of 10 individuals with normal hearing in both ears. The signal-to-noise required for 50% speech recognition (SNR-50) was measured for the participants in five conditions, that is, unprocessed speech and speech processed with four noise reduction strategies. Additionally, an acoustic objective measure Extended Short-Time Objective Intelligibility algorithm was employed to estimate the intelligibility index across the conditions. Results Significant difference was found across conditions in both the groups. Pairwise comparison revealed significantly better speech perception on SNR-50 measure with IdBM strategy, for both the groups. No significant difference in SNR-50 was observed with other noise reduction strategies. IdBM condition also gave the highest intelligibility index (d) values using Extended Short-Time Objective Intelligibility algorithm. This finding needs to be verified on a larger group of individuals with ANSD. Conclusions IdBM noise reduction strategy rendered significantly lower SNR-50 compared to other noise reduction strategies for individuals with ANSD in this study. This provides clinical evidence for the same and also recommends trying on a larger group of participants before its implementation in hearing devices. Apart from this, the current strategies used in hearing aids provide no improvement in speech identification in noise for this population. Hence, though the present hearing aids may show benefit in quiet condition, chances of its rejection are high in noisy backgrounds.

Behavioural and Electrophysiological Correlates of Aided Performance in Individuals with Late Onset Auditory Neuropathy Spectrum Disorder: A Review

Hearing aids are one of the rehabilitative options for individuals with late onset auditory neuropathy spectrum disorder (ANSD). There is a growing body of evidence that supports the effectiveness of hearing aids among these individuals. The objective of the study was to review the behavioural and electrophysiological factors that determine the effectiveness of amplification in individuals with late onset ANSD. An intensive search was carried out using the PubMed, SpringerLink and Google Scholar databases using the following keywords: auditory neuropathy, late onset ANSD, hearing aids and amplification. There were both retrospective and experimental research studies that showed aided improvement in individuals with late onset ANSD. The aided improvement in these studies has been attributed to preserved neural synchrony. Hence, it was concluded that, hearing aids are effective in improving speech perception in a sub-group of individuals with late onset ANSD. Measures that evaluate neural synchrony are promising tools to identify the sub-group of such individuals. Speech identification scores at higher sensation levels and cortical potentials are the two clinical measures that indicate aided performance in individuals with late onset ANSD.

Auditory Deficits in Patients With Mild and Moderate Obstructive Sleep Apnea Syndrome: A Speech Syllable Evoked Auditory Brainstem Response Study

Objectives

The energy consumption process of cochlea and neural signal transduction along the auditory pathway are highly dependent on blood oxygen supply. At present, it is under debate on whether the obstructive sleep apnea syndrome (OSAS) would affect the auditory function since the patients suffer from low oxygen saturation. Moreover, it is difficult to detect the functional state of auditory in less severe stage of OSAS. Recently, speech-evoked auditory brainstem response (speech-ABR) has been reported to be a new electrophysiological tool in characterizing the auditory dysfunction. The aim of the present study is to evaluate the auditory processes in adult patients with mild and moderate OSAS by speech-ABR.

Methods

An experimental group of 31 patients with mild to moderate OSAS, and a control group without OSAS diagnosed by apnea hypopnea index in polysomnogram were recruited. All participants underwent otologic examinations and tests of pure-tone audiogram, distortion product otoacoustic emissions, click-evoked auditory brainstem response (click-ABR) and speech-ABR, respectively.

Results

The results of pure-tone audiogram, distortion product otoacoustic emissions, and click-ABR in OSAS group showed no significant differences compared with the control group (P>0.05). Speech-ABRs for OSAS participants and controls showed similar morphological waveforms and typical peak structures. There were significant group differences for the onset and offset transient peaks (P<0.05), where OSAS group had longer latencies for peak V (6.69± 0.33 ms vs. 6.39±0.23 ms), peak C (13.48±0.30 ms vs. 13.31±0.23 ms), and peak O (48.27±0.39 ms vs. 47.60± 0.40 ms) compared to the control group. The latency of these peaks showed significant correlations with apnea hypopnea index for peak V (r=0.37, P=0.040), peak C (r=0.36, P=0.045), as well as peak O (r=0.55, P=0.001).

Conclusion

These findings indicate that some auditory dysfunctions may be present in patients with mild and moderate OSAS, and the damages were aggravated with the severity of OSAS, which suggests that speech-ABR may be a potential biomarker in the diagnosis and evaluation at early stage of OSAS.