Editorial: ‘Auditory neuropathy’ and cochlear implantation – myths and facts

Is auditory neuropathy an appropriate term? A systematic literature review on its aetiology and pathogenesis

To clarify the aetio-pathogenesis of Auditory Neuropathy Spectrum Disorder (ANSD), a total of 845 papers were divided into four categories: Review, Audiology, Treatment and Aetiology. Aetiology was the topic analysed categorising papers as: Genetics, Histopathology, Imaging and Medical diseases. Isolated ANs were in relation to Otoferlin, Pejvakin and DIAPH3 deficiency, and the syndromes were mainly Charcot Marie Tooth, Friedreich Ataxia, mitochondrial disorders and those associated with optic neuropathies. In histopathology papers, important information was available from analyses on human premature newborns and on some syndromic neuropathies. From cochlear dysmorphism to cerebral tumours associated with ANs, these are described in what is identified as the Imaging area. Finally, the prevalent clinical pathology was bilirubinopathy, followed by diabetes. In conclusion, AN/ANSDs do not refer to a clear pathological condition, but to an instrumental pattern without any evidence of auditory nerve involvement, except in a few conditions. The terms AN/ANSD are misleading and should be avoided, including terms such as “synaptopathy” or “dis-synchrony”.

New criteria of indication and selection of patients to cochlear implant

Numerous changes continue to occur in cochlear implant candidacy. In general, these have been accompanied by concomitant and satisfactory changes in surgical techniques. Together, this has advanced the utility and safety of cochlear implantation. Most devices are now approved for use in patients with severe to profound unilateral hearing loss rather then the prior requirement of a bilateral profound loss. Furthermore, studies have begun utilizing short electrode arrays for shallow insertion in patients with considerable low-frequency residual hearing. This technique will allow the recipient to continue to use acoustically amplified hearing for the low frequencies simultaneously with a cochlear implant for the high frequencies. The advances in design of, and indications for, cochlear implants have been matched by improvements in surgical techniques and decrease in complications. The resulting improvements in safety and efficacy have further encouraged the use of these devices. This paper will review the new concepts in the candidacy of cochlear implant. Medline data base was used to search articles dealing with the following topics: cochlear implant in younger children, cochlear implant and hearing preservation, cochlear implant for unilateral deafness and tinnitus, genetic hearing loss and cochlear implant, bilateral cochlear implant, neuropathy and cochlear implant and neural plasticity, and the selection of patients for cochlear implant.

The mitochondrial connection in auditory neuropathy

'Auditory neuropathy' (AN), the term used to codify a primary degeneration of the auditory nerve, can be linked directly or indirectly to mitochondrial dysfunction. These observations are based on the expression of AN in known mitochondrial-based neurological diseases (Friedreich's ataxia, Mohr-Tranebjærg syndrome), in conditions where defects in axonal transport, protein trafficking, and fusion processes perturb and/or disrupt mitochondrial dynamics (Charcot-Marie-Tooth disease, autosomal dominant optic atrophy), in a common neonatal condition known to be toxic to mitochondria (hyperbilirubinemia), and where respiratory chain deficiencies produce reductions in oxidative phosphorylation that adversely affect peripheral auditory mechanisms. This body of evidence is solidified by data derived from temporal bone and genetic studies, biochemical, molecular biologic, behavioral, electroacoustic, and electrophysiological investigations.

Newborn hearing screening and strategy for early detection of hearing loss in infants

OBJECTIVE

More than 80% of permanent hearing losses (HL) in children are congenital. Newborn hearing screening (NHS) is the best method for early detection of suspected hearing loss. If the NHS is not universal more than 30% permanent hearing losses are not identified. There are various methods of NHS: otoacoustic emissions (TEOAE, DPOAE) and automatic auditory brainstem response (AABR). After hearing screening, and when hearing loss is suspected, tympanometry and audiological methods then used for determination of hearing threshold; these include ABR, ASSR or/and behavioral methods. The goal of this study is to evaluate the influence of UNHS on the early detection of hearing loss in children before and after the implementation of obligatory universal newborn hearing screening in Slovakia, and also on the etiologic evaluation of hearing impaired infants identified by screening.

METHOD

In Slovakia NHS started in 1998 and was provided in ENT departments. From May 1, 2006 UNHS has been mandatory in Slovakia, using two stages TEOAE in all newborn departments in Slovakia (64 newborn departments). In year 2005--42% of newborns in Slovakia were screened, in 2006--66% newborns and in 2007--94, 99% (three small newborn departments do not yet have equipment for OAE screening). For determination of hearing thresholds ASSR are used in two ENT departments and ABR in the other four ENT departments.

RESULTS

Comparing the number of identified cases with bilateral severe permanent HL or deafness before and after UNHS, 22.8% more cases of PHL were identified in the first year of UNHS. Also the average age of diagnosis of PHL was lower. In the year 2007, 94% of newborns were screened. We found 0.947/1000 newborns with bilateral severe PHL (35.9%) more than before UNHS). After audiologic and etiologic assessment of the 76 infants who failed screening, 5 (6.58%) were found to have normal hearing, 16 (22.54%) had unilateral and 55 (77.46%) had bilateral SNHL. A non-syndromic genetic cause was present in 25.45% of cases, syndromic in 9%, perinatal cause (31%), congenital CMV infection in 7.27%, bilateral cochlear anomalies without other abnormality in 1.83% and unknown etiology in 25.45%.

Auditory neuropathy: endocochlear lesion or temporal processing impairment? Implications for diagnosis and management

BACKGROUND/OBJECTIVE

Auditory neuropathy/dys-synchrony, characterized by absent auditory brainstem responses, normal otoacoustic emissions or cochlear microphonics, and word discrimination disproportional to the pure-tone audiogram, may be accompanied by perceptual consequences that could jeopardize language acquisition in affected children. However, the related evidence is constantly changing leading to a serious debate. The aim of the present paper is to review the current knowledge on auditory neuropathy/dys-synchrony, and to present the therapeutic strategies that can be employed in its management, taking into account the potentially underlying pathophysiology.

MATERIALS/METHODS

Literature review from Medline and database sources. Related books were also included.

STUDY SELECTION

Controlled clinical trials, prospective and retrospective cohort studies, nested-based case-control and analytical family studies, laboratory and electrophysiological studies, animal models, case-reports, joint statements and review articles.

DATA SYNTHESIS

Auditory neuropathy/dys-synchrony, in contrast to what is widely believed, is a very frequent disease, responsible for approximately 8% of newly diagnosed cases of hearing loss in children per year. Hyperbilirubinemia and hypoxia represent major risk factors, whereas generalized neuropathic disorders, or a genetic substrate involving the otoferlin gene, are responsible for the phenotype of auditory neuropathy/dys-synchrony in certain cases. Auditory nerve myelinopathy and/or desynchrony of neural discharges are the most probable underlying pathophysiologic mechanisms. Genetic testing may be helpful in cases of non-syndromic prelingual children. Auditory neuropathy/dys-synchrony management aims at restoring the compromised processing of auditory information, either through conventional amplification and/or alternative forms of communication, or by cochlear implantation (combined with intensive speech and language therapy).

CONCLUSION

Auditory neuropathy/dys-synchrony is more frequent than considered in the past, especially amongst hearing-impaired children. Accurate diagnosis, based on subjective and objective hearing assessment techniques (including the various electrophysiological assessment measures), and timely treatment of the affected children is of paramount importance, with hearing aids, intensive speech and language therapy (and sign language when indicated) providing the mainstay of habilitation, and cochlear implantation representing a valid therapeutic alternative.