Prevalence of auditory neuropathy/synaptopathy in a population of children with profound hearing loss

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Summary

Objective

To examine the prevalence of auditory neuropathy/synaptopathy (AN/AS) in a cohort of children with profound hearing loss.

Methods

From 1997 until 2004, 5190 children, aged 1–15 years, whose hearing ability was uncertain or who had risk factors for hearing impairment were investigated with subjective and objective hearing tests. Three thousand four hundred and fifteen from these children were screened for AN/AS using pure-tone audiometry, impedance measurement, transient evoked otoacoustic emissions (TEOAE) and click-evoked auditory brainstem responses (ABR).

Results

From 3415 patients who participated in an ABR and TEOAE assessment, 379 children showed absent or elevated (≥80 dB nHL) ABR thresholds. Within this group we found 32 cases with evidence of AN/AS via visible TEOAE and/or cochlear microphonics (CM) coupled with absent ABR. In the remaining 3036 children, AN/AS, could be ruled out by means of detectable ABR-thresholds and coherent findings in pure-tone audiometry and TEOAE assessment. This results in a prevalence of AN/AS of 0.94% within the group at risk for hearing loss, compared to 8.44% among profoundly hearing impaired children.

Conclusion

This study shows that AN/AS is a common finding in the population of hearing impaired infants. In the majority of our AN/AS children (50%, n = 16), an early audiological diagnosis was made under the age of 12 months. Therefore, clinicians and other health care professionals should generally be sensitised for AN/AS in infants, so that an appropriate treatment can promptly be initiated. Further research on clinical and pathophysiological aspects is necessary to better identify and manage patients suffering from AN/AS.

Introduction

The term auditory neuropathy (AN) describes a hearing disorder, where a combination of normal otoacoustic emissions (TEOAE) and/or cochlear microphonics (CM) with absent or severely abnormal click-evoked auditory brainstem responses (ABR) at high stimulus levels can be found. The patients with AN present varying pure-tone thresholds ranging from mild to severe uni- or bilateral hearing loss. In quiet they complain a loss of speech comprehension that is out of proportion with their pure-tone hearing thresholds [1]. The term auditory neuropathy was initially defined in 1996 by Starr et al. [2]. He introduced 10 patients where the AN was combined with a general peripheral neuropathy and presumed that the auditory nerve was impaired in a similar way. Nevertheless, other causal mechanisms should be taken into account, resulting in the same conflicting audiological findings. Moreover, Rapin and Gravel [3] discussed these issues in excruciating detail and suggested that the term auditory neuropathy should be restricted to cases in which the locus of pathology is limited to the spiral ganglion cells, their processes or the eighth nerve. The diagnostic term AN is anatomically inappropriate for pathologies from the brainstem to the auditory cortex. The site of lesion could also be localized at the level of the inner hair cells (IHC) as well as the IHC afferent synapse. Recently, a mouse model of auditory neuropathy was described by Khimich et al. [4]. In this mouse mutant for the IHC presynaptic scaffolding protein Bassoon, the lack of active-zone-anchored synaptic ribbons impaired synchronous auditory signalling consistent with “auditory neuropathy”. As long as the specific locus of pathology is not particularly known and an adequate technique to prove the function of the inner hair cells is missing we would favour the expression auditory neuropathy/synaptopathy (AN/AS) to describe patients with normal TEOAE and/or CMs but absent ABR.

The prevalence of AN/AS in children with hearing loss is not precisely known. There are mainly case-studies and small cohorts with 10 or less affected patients with AN/AS described in the literature [2], [5], [6], [7], [8], [9], [10], [11]. However, initial estimates on the prevalence of AN/AS in patients at risk for hearing loss by Davis and Hirsh [12] and Kraus et al. [13] were 0.5% and 1.3%, respectively. They described patients with pure-tone thresholds between normal hearing and moderate hearing loss but absent or abnormal ABR responses and poor speech understanding. Conclusions drawn from these series are limited, as these papers were published well before the TEOAEs were in use and normal TEOAEs or CMs are a precondition of the emerged current AN/AS definition. Furthermore, estimating the occurrence of AN/AS from these studies is difficult as valid results cannot be obtained from studies with small numbers of subjects. Results from studying a large group of 72 patients with AN/AS [14] are restricted to identification of underlying mechanisms, e.g. genetic defects as well as the course of this condition, but predictions about prevalence rates cannot be made.

A systematic Australian study of AN/AS is provided by Rance et al. [15]. This series describes a group of 5199 neonates at risk for hearing impairment, 109 children were found to have elevated ABR-thresholds ≥40 dB nHL and 12 with audiological findings consistent with AN/AS. This data set indicates a prevalence of AN/AS of 11.01% in a population of children with hearing loss, and 0.23% within the at-risk population. However, the value of such surveys is highly dependent on the selection of the participating subjects and needs to be confirmed by other institutions. The presented study provides an insight and discussion of the prevalence of AN/AS in a group of children at-risk for hearing impairment in an urban population in Germany.

Section snippets

Subjects

This retrospective study was performed at the Department of Otolaryngology, Head and Neck surgery at the University of Cologne, Germany (tertiary referral centre). It describes an unselected group of 5190 children with suspicion of hearing loss (Fig. 1, column A). They were either referred to us from children's hospitals or were introduced by ENT-specialists. The infants often show inadequate results in newborn hearing screenings, have risk factors for hearing loss in their medical or family

Results

In the last 8 years, we assessed 5190 children at risk for hearing loss with subjective and objective audiological tests (Fig. 1, column A). In 1775 children assessed with pure-tone audiometry, tympanometry and TEOAE we found no evidence of hearing impairment. Even if these children did not undergo ABR testing, they showed clear and compatible findings in audiological assessment and had no signs to point at AN/AS (Fig. 1, column B). However, we cannot completely rule out an AN/AS in this group

Discussion

With the widespread implementation of otoacoustic emissions in the clinical routine, the early diagnose of AN/AS in children is getting more common. Nevertheless, up to now the prevalence of the disorder has not been precisely defined. The frequency of occurrence of AN/AS is estimated between 0.23% and 1.3% for the at risk clinical population [13], [15] and 11–14.6% in the selected group of patients with severe to profound hearing loss as proofed by absent ABRs [8], [12], [13].

The results of

Acknowledgment

This work was supported by a scholarship of the “Novartis-Stiftung für therapeutische Forschung” to Dirk Beutner.

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