Auditory neuropathy is a disorder in which hearing thresholds do not match with other measures of auditory function such as ABR data and speech understanding scores.
In some cases there may be the presence of otoacoustic emissions or cochlear microphonics (CM) that do not correlate with ABR or audiometric thresholds. Diagnostic testing in these initial reports pointed to a disorder of the cochlear nerve as opposed to sensory loss of cochlear origin.
Over time it became clear that auditory neuropathy was manifest in different ways and degrees, and as the result of numerous causes. This is one reason why other terms such as auditory dyssynchrony and peri-synaptic audiopathy were used to describe the disorder. The recognition that there was a wide range of hearing loss types and etiologies led to the adoption of the term auditory neuropathy spectrum disorder (ANSD)
In some cases, a patient will show some evidence of Oto-Acoustic Emissions or robust Cochlear Microphonics but otherwise have poor auditory thresholds. It is often suspected that if there is residual cochlear haircell function, that it could be jeopardized by electrode insertion. There is also the possibility of some recovery of neural connections (to the surviving haircells) that could improve hearing function. In this regard we should note that the presence of OAEs or of a large microphonic is indicative of outer haircell function.
In our overall population of ANSD patients (i.e. not just the ones entering the CI program) one in five showed some threshold recovery to a level of hearing that allowed adequate speech understanding and language development without a hearing prosthesis. It appears that ANSD subjects showing such good unaided outcomes are often those that start with relatively good audiometric thresholds at their first behavioral test
In other cases, there are ANSD patients who do not have severe to profound hearing loss on behavioral testing, i.e. do not meet normal audiometric criteria for CI, but who also do not progress with speech and language development as would be expected from their (aided) hearing threshold levels.
The figure below shows the pre-implant, unaided audiograms of ANSD children (implanted ear) compared with the audiograms of 37 matched, non-ANSD children that were implanted. This matched group consisted of randomly selected congenitally deaf children with sensorineural hearing loss of cochlear origin, and with similar age and gender demographics. Note that the audiograms of many of the ANSD children show hearing thresholds that are better than the profound levels normally accepted as the CI criterion. The lower plot shows speech awareness thresholds (SAT) for the ANSD children (blue) and non-ANSD patients (red).
In ANSD patients, the prospect of improving hearing function, or relatively good audiometric thresholds means that there is a delay in proceeding to cochlear implantation. As illustrated in the figure on the left, in our pediatric program in Toronto, over the past 12 year period the average age of implant for ANSD children (n=17) is 3.3 years compared to 1.9 years for a randomly selected group (n=39) of non-ANSD, congenitally deaf implanted children. This “wait and see” period is required because audiometric thresholds suggest that conventional hearing aids should be sufficient for speech and language development.
References
David R. Schramm and Robert V. Harrison. Performance after cochlear implantation in children with auditory neuropathy. Cochlear implants inter. (2010) 11(supp.1): 207-212.