Auditory neuropathy/auditory dys-synchrony (AN/AD) is a hearing disorder in which the inner ear or auditory nerve is unable to produce a synchronous response to incoming sound. AN/AD results in degraded processing of temporal cues in speech. Consequently, persons with AN/AD frequently experience inordinate difficulty understanding speech, especially in the presence of competing noise.

From an audiological perspective, AN/AD manifests in the form of abnormal measures that require a synchronous response from the auditory nerve. Particularly, the auditory brainstem response (ABR) is absent in most cases or present with significant abnormalities. In spite of the abnormal ABR, evidence of normal or near-normal outer hair cell (OHC) function exists as indicated by present otoacoustic emissions (OAE) and/or cochlear microphonic (CM).

Recent reports have suggested that AN/AD is a relatively common cause of childhood hearing loss, with as many as 10%-15% of congenital hearing losses attributed to AN/AD (Rance, 2005). Consequently, it is likely that most pediatric audiologists have provided or will provide audiological services for a child with AN/AD. There is a contemporary assessment battery appropriate for identifying children with AN/AD.

Contemporary Protocol

The incidence of AN/AD is higher for children who have had an extended stay in the neonatal intensive care unit (NICU), with AN/AD accounting for as much as 40% of hearing loss in the NICU (Rea and Gibson, 2003). Consequently, universal newborn hearing screening in the NICU must be conducted with an ABR—rather than OAE—screening. An OAE screening may provide a passing result and fail to identify the significant impairment of auditory function. Furthermore, if a child who has spent five or more days in the NICU fails an ABR screening, a referral should be made for immediate diagnostic audiologic assessment rather than conducting a repeat screening (JCIH, 2007).

If a child's ABR is abnormal, AN/AD must be ruled out by examining for evidence of OHC function. OAE testing should be conducted to rule out the presence of robust cochlear activity. Cochlear function should be further examined by evaluating whether a CM response exists. Several reports have described cases in which OAEs are absent, but the diagnosis of AN/AD is made on the basis of a present CM in conjunction with an abnormal neural response (Berlin et al., 1999; Rance et al., 1999).

The clinician must select appropriate stimulus and acquisition parameters to measure the CM. Specifically, a click stimulus at a level between 75–90 dB nHL should be used to measure the CM. The click stimulus elicits a more robust OHC response relative to tone bursts, but a high stimulus level is still needed to generate a response that can be recorded with surface electrodes. The ABR to a click stimulus with both rarefaction and condensation polarities should also be recorded in order to distinguish the CM that is produced by OHC and inverts with changes in stimulus polarity (see figure 1 online [PDF]) from a neural response that does not invert with changes in stimulus polarity. Also, the non-inverting electrode should be placed at the vertex or very high forehead, and the inverting electrode placed near the mastoid process or at the ear lobe. Placement of the inverting electrode at the nape will substantially reduce the likelihood of recording a CM. To distinguish the CM from electromagnetic artifact that may be present at high stimulus levels needed to elicit the CM, the tubing of the insert earphones may be clamped. If the response is eliminated, then it can be assumed that it was physiologic in nature rather than a stimulus-related artifact.

AR Assessment

Acoustic reflex (AR) assessment may also be used to evaluate the neural integrity of the peripheral auditory system. Berlin et al. (2005) showed that 133 of 136 persons with AN/AD had abnormal AR. The AR response requires that the auditory nerve possess neural synchrony, so an abnormal response in conjunction with present OAE is indicative of AN/AD. Tympanometry and otoscopy must be conducted to determine whether a middle ear disorder is responsible for the absence of the AR response.

Behavioral audiometric assessment should also be conducted to determine the impact of the disorder on the child's hearing sensitivity. Behavioral observation audiometry may be conducted during the first few months of life to look for evidence of responsiveness to sound, and visual reinforcement audiometry should be attempted at 6 months of age to determine behavioral thresholds. For children with AN/AD, behavioral thresholds may fall anywhere within the normal to profound range. The configuration of the hearing loss is also variable, but low-frequency rising configurations are more frequently observed for children with AN/AD relative to children with sensory hearing loss. Further, AN/AD is often associated with fluctuating hearing sensitivity, so behavioral assessment should be conducted frequently to ensure that the degree and configuration of the loss have been aptly characterized.

Cochlear Nerve MRI

Another assessment that should be considered for children with AN/AD is magnetic resonance imaging (MRI) of the cochlear nerve. Buchman et al. (2006) reported that in 51 children with AN/AD, MRI testing revealed 11 to have cochlear nerve deficiencies. Some of the children had unilateral cochlear nerve deficiency, and the MRI findings were used to determine the ear receiving a cochlear implant (CI). If a child has a bilateral cochlear nerve deficiency, the CI team should counsel the family regarding the possibility of a poorer post-CI outcome.

It would be remiss to fail to mention the importance of a comprehensive speech and language assessment. Given the wide variability in performance for children with AN/AD, the most important determinant of whether intervention is appropriate for a child with AN/AD is the development of the child's speech, language, and auditory skills. Speech-language pathologists with experience in providing services for children with hearing loss are the most appropriate professionals to conduct speech and language assessments, and age-appropriate development of speech and language should be the desired goal.

Future Directions

Advances in diagnostic practices will undoubtedly improve the ability to localize the site of lesion (i.e. inner hair cells, auditory nerve, etc.) and determine the most appropriate intervention strategy for individuals with AN/AD. Rance et al. (2005) demonstrated that some children with AN/AD received good benefit from the use of hearing aids while others did not. Interestingly, he also showed that children who performed well with hearing aids were more likely to possess present cortical auditory evoked potentials (CAEP) than poor hearing aid users.

In contrast, Kumar and Jayaram (2005) measured speech recognition, psychophysical temporal processing abilities, and CAEP in a group of adults with AN/AD. They showed a strong correlation between temporal processing abilities and speech recognition but no relationship between speech recognition and CAEP. Finally, the National Acoustic Laboratories in Australia have reported on the use of CAEP to fit hearing aids for infants with AN/AD (see King et al., 2005, for more information). Their approach appears to hold promise as a method to fit hearing aids early for young children with AN/AD. Further research is needed to investigate the role of CAEP in the assessment of AN/AD along with the subsequent determination of optimal intervention.

Additionally, some reports have indicated that children with AN/AD do well with a CI, while other reports have suggested that benefit from cochlear implantation is variable for children with AN/AD. Gibson and colleagues (2007) have shown that the electrocochleography and ABR evoked with electrical stimulation at the round window (RWECochG/ABR) may explain this variability in benefit and serve as a prognostic indicator for success with a CI. For 39 children diagnosed with AN/AD, normal and robust RWECochG/ABR were typically recorded for children who acquired good speech recognition with use of a CI, while children with poor speech recognition typically possessed abnormal RWECochG/ABR. Additional research is needed to determine the utility of using RWECochG/ABR to determine appropriate intervention for children with AN/AD.

To summarize, pediatric audiologists should be familiar with contemporary protocols for the assessment of AN/AD in children. Also, clinicians have a responsibility to stay abreast of future advances in assessment procedures that may improve the effectiveness of intervention for children with AN/AD.

The authors thank James Hall for his helpful comments during the preparation of the manuscript.