Patients with auditory neuropathy/auditory dys-synchrony (AN/AD) continue to present clinical challenges, particularly related to management. Clinical methods are available to accurately identify patients with AN/AD though understanding of the specific underlying mechanisms and etiologies continues to evolve. Experience with these patients shows that some management methods are more useful than others.
1. How does one diagnose auditory neuropathy?
Normal outer hair cell (OHC) function and dys-synchronous neural responses characterize AN/AD. A combination of measures of OHC function and neural synchrony are necessary to correctly identify patients. Otoacoustic emissions (OAEs) and cochlear microphonics (CM) reflect outer hair cell function and results on these tests, in the absence of middle-ear problems, are normal. Either transient or distortion product OAEs are appropriate. When measuring CM, it is important to compare responses obtained with condensation stimuli to those obtained with rarefaction stimuli (Berlin et al., 1998). CM reverses direction with polarity changes, while neural (such as auditory brainstem) responses do not.
The results of OAEs and/or CM are compared to measures of neural function. Absent middle-ear muscle reflexes (MEMR) and auditory brainstem responses (ABR) are consistent with dys-synchrony of the VIIIth nerve and, in the presence of OHC responses, consistent with AN/AD. Patients with AN/AD are distinguished from patients with space-occupying lesions, such as VIIIth nerve tumors, or multiple sclerosis, in that results of radiological evaluation are normal in AN/AD patients.
2. What is the etiology?
The variation of characteristics among patients and the fact that several possible underlying mechanisms can result in normal OHC function and poor neural synchrony suggest that AN/AD is not a single entity with a single underlying etiology (e.g., Starr et al., 2000). Since some of the possible mechanisms and etiologies may not be specifically neural in nature, 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).
Possible sites include the inner hair cells (IHC), the synaptic juncture between the IHC and auditory nerve, or the auditory nerve itself. Each of these could result in normal OAEs and a dys-synchronous ABR. Several sources of information support the possible involvement of IHC, including animal models (Deol and Kocher, 1958; Bussoli, Kelly, and Steel, 2001) and recent human histological data (Amatuzzi et al., 2001). Some patients with AN/AD have demyelinating conditions such as hereditary motor sensory neuropathy, Charcot-Marie-Tooth disease or other neural conditions.
Some patients have risk factors related to hearing loss in their history, but there are also a significant number of patients with no risk factors. Factors observed in infants include hyperbilirubinemia, exchange transfusion, premature birth, and perinatal asphyxia (e.g., Deltenre et al., 1997; Berlin et al., 1998; Rance et al., 1999; Simmons and Beauchaine, 2000). Heredity is another possible underlying factor since a number of families have been identified with two or more members with AN/AD.
What are the best management options for children diagnosed with auditory neuropathy?
Our experience with patients with AN/AD is that even those who can discriminate some words or sentences in quiet are unable to discriminate even simple sentences in competing noise. Hearing aids improve detection of sound through amplification but, in our experience, have been of minimal benefit in improving discrimination sufficient to facilitate speech and language development (Berlin et al., 2000).
Cochlear implants are a viable management option for patients with AN/AD. Some patients may have absent or malfunctioning IHC with intact neural function, while in those with impaired neural elements a cochlear implant may improve synchrony via electrical stimulation. AN/AD children with cochlear implants demonstrate synchronous neural responses and performance on behavioral tests comparable to non-AN/AD children with cochlear implants (Shallop et al., 2001; Trautwein et al., 2000).
Learning speech and language through the auditory channel exclusively is very difficult for non-implanted 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. We recommend visual communication methods (such as Cued speech, sign language, or signed English) as a necessary component for language development in children. 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.
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Acknowledgments: 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.