Auditory Neuropathy/Auditory Dyssynchrony
What is Auditory Neuropathy/Auditory Dyssynchrony (AN/AD)?
Auditory Neuropathy/Auditory Dyssynchrony (AN/AD) is present when patients have normal outer hair cell function in the cochlea, but the VIIIth nerve that carries electrical signals to the brain has responses that are dys-synchronous. This means that instead of a smooth transition of information from the cochlea to the brain, the signals are not synchronized so information is not relayed to the brain in a consistent manner. The amount of dys-synchrony can vary from person to person and can fluctuate in an individual over time.
How is AN/AD diagnosed?
Most hospitals screen hearing of newborns using OAE equipment. The otoacoustic emissions (OAE) tests for a response to sound by the outer hair cells of the cochlea but does not test how the brain responds to sound. Therefore, a typical newborn hearing screening program may not identify a child with AN/AD because the outer hair cells can function normally. There are some children that will have some abnormal cochlear function causing reduced OAE responses in addition to AN/AD that can be identified during newborn hearing screening.
Patients with auditory neuropathy/dys-synchrony exhibit no auditory brain stem response (ABR), no middle ear muscle response, and both normal otoacoustic emissions or normal cochlear microphonics. An absent or grossly abnormal ABR is not always associated with deafness. In contrast, a hearing loss of 30 dB or more usually predicts absent otoacoustic emissions, but normal emissions can be seen in some patients whose behavioral audiograms imply total deafness.
To summarize, in a diagnosis of AN/AD a normal OAE will be present but ABR will be abnormal.
Are there risk factors or case history information that can help in identifying children who potentially have AN/AD?
Yes, but many children who have AN/AD do not have any risk factors. The risk factors that have been associated with AN/AD are (1) hyperbilirubinemia ranging from just above normal to quite high, including infants that have gone through exchange transfusions, (2) premature (25 – 36 weeks gestational age), and (3) perinatal asphyxia. AN/AD can also run in families, following both recessive and dominant inheritance patterns. AN/AD can be suspected when a child has residual hearing and has been fit with good high-quality hearing aids and still does not progress in speech and language development, despite consistent amplification wear and parents who are involved communicators.
How can children with AN/AD vary in their ability to use hearing?
Most people who have AN/AD have both ears affected however there are some that have unilateral auditory dys-synchrony. Hearing thresholds vary from near normal hearing to profound hearing loss. A few people are able to recognize speech when it is quiet – all have difficulty if any background noise is present. Some children have progressive hearing loss and have hearing thresholds that are indistinguishable from children who are deaf. Such children have been successfully implanted and can perform well with a cochlear implant. Some children have stable hearing but do not successfully progress in speech and language development despite consistent amplification wear and good intervention.
Learning language through visual means (i.e., sign language) is necessary. Some of these children have been successful cochlear implant users and have moved away from visual language systems post-implant. Some children actually show a worsening of dys-synchrony symptoms over time whereas others seem to recover their awareness of sound, but continue to have difficulty in noise and delayed language development.
Although hearing aids have been found to assist persons with AN/AD in quiet, it is apparent that even if a little background noise is present that they have great difficulty understanding. In a group of 100 children with AN/AD that have been followed through the Louisiana State University Health Sciences Center in New Orleans, there has been no observed facilitation of speech and language development with hearing aids alone.
How can cochlear implants benefit people with AN/AD?
Researchers believe that it is possible that electrical stimulation, like that from a cochlear implant, can help to synchronize activity of the impaired portions of the brain and allow use of auditory information. Even for children who receive implants, it is important to allow them to gradually move from a visual language system to an auditory language. Abrupt removal of the communication system that a child has depended upon prior to receiving a cochlear implant is not recommended and could interrupt smooth continuation of progress.
Are there any communication methods that facilitate development of speech and language in children with AN/AD?
Learning speech and language through the auditory channel exclusively is very difficult for children with AN/AD. This is because it is difficult or impossible to achieve a clear and consistent auditory signal (unless the person has a cochlear implant) in a dys-synchronous auditory system. Use of a visual communication system, such as English-based sign systems or Cued speech are recommended methods to develop language through visual communication assuming that the parents desire the child to eventually use spoken language. ASL is appropriate in addition to the other visual communication means as a viable option for developing language if spoken language is not a goal. Auditory Verbal therapy by itself, before cochlear implantation, has not been observed to work as the sole method of teaching language to AN/AD children.
Information was excerpted by Karen Anderson , 12/04 from : Hood, L. J. (2002). Auditory neuropathy/auditory dys-synchrony: New insights. The Hearing Journal (March).