What is auditory neuropathy/auditory dys-synchrony?
AN/AD most likely is not a single disorder with a single underlying cause. A distinguishing characteristic of an AN/AD is the presence of auditory responses associated with normal function of the outer hair cells of the cochlea and poor neural synchrony of the auditory (VIIIth cranial) nerve. Furthermore, we believe that the term auditory dys-synchrony may provide a more comprehensive view of auditory neuropathy that connects logically to viable management options (Berlin, Hood and Rose, 2001).
What symptoms characterize people with AN/AD?
Our experience with AN/AD patients is that they have great difficulty understanding even simple sentences in competing noise-despite the fact that they can understand some words or sentences in quiet. Learning speech and language through the auditory channel exclusively is very difficult for patients with AN/AD. This is most likely due to difficulty in achieving a clear and consistent auditory signal in a dys-synchronous auditory system. Transmission of auditory information to the cortex depends on the synchronous (simultaneous) firing of many neurons to move the signal along the neural pathway.
What causes AN/AD?
The variability observed in patients suggests several possible sources of AN/AD, each of which could result in normal OAEs and dys-synchronous ABR. AN/AD may originate in the cochlea (specifically the inner hair cells) in some patients, while the presence of other peripheral neural disorders in other patients suggests a neural site. Some patients have risk factors related to hearing loss in their history (such as hyperbilirubinemia, exchange transfusion, premature birth, or perinatal asphyxia), but there are also a significant number of patients with no risk factors. Heredity is another possible underlying factor since a number of families have been identified with two or more members with AN/AD.
How is AN/AD diagnosed?
Clinical methods are available to accurately identify patient with AN/AD and, when applied properly, can distinguish AN/AD from other types of hearing disorders. Otoacoustic emissions (OAEs) and/or cochlear microphonics (CM) are compared to measures of neural function. Absent middle-ear muscle reflexes and auditory brainstem responses (ABR) are consistent with dys-synchrony of the VIIIth nerve and, in the presence of normal outer hair cell function, demonstrate AN/AD.
What treatment options are effective for AN/AD?
AN/AD continues to present management challenges. However, experience with these patients shows that some management approaches are more useful than others. Hearing aids improve detection of sound but, in our experience, have been of minimal benefit in improving discrimination sufficient to facilitate speech and language development. Cochlear implants are a management option. AN/AD children with cochlear implants demonstrate synchronous neural responses and performance on behavioral tests comparable to non-AN/AD children with cochlear implants. We recommend visual communication methods (such as Cued Speech, sign language, or signed English) as a necessary component for language development. Auditory-Verbal (AV) therapy is useful post-implant, but has not worked in our practice as the sole method of teaching language prior to obtaining a cochlear implant.
This article was originally submitted by Linda Hood, Ph.D. who is Professor, Kresge Hearing Research Laboratory, Department of Otolaryngology and Biocommunication, Louisiana State University Health Sciences Center, New Orleans and subsequently edited by AAC. Literature cited: Berlin,C, Hood L, Rose K. 2001 On renaming auditory neuropathy as auditory dys-synchrony. Audiology Today 13:15-17. Acknowledgements: Research at Kresge Hearing Research Laboratory is supported by NIH National Institute on Deafness and Other Communication Disorders, Oberkotter Foundation, Kam's Fund for Hearing Research, American Hearing Research Foundation, National Organization for Hearing Research, Deafness Research Foundation, Marriott Foundation, Kleberg Foundation, and Louisiana Lions Eye Foundation.