The clinical entity now known as auditory neuropathy/auditory dys-synchrony (AN/AD) is relatively new and has only become identifiable since the advent of otoacoustic emissions (OAEs). Individuals with this type of hearing impairment exhibit evidence of normal outer hair cell (OHC) function, but neural conduction along the auditory pathway appears to be disordered.
Although it has been formally labeled only since 1995, the disorder itself is not new. Past reports in the literature described patients with paradoxical audiological test findings (Davis & Hirsch, 1979). These patients presented with auditory brainstem response (ABR) findings that were inconsistent with behavioral test findings or with subjective reports of auditory perception. These cases are now assumed to have involved AN/AD. To date, prevalence of the disorder remains questionable, with the reported rates of occurrence ranging from as low at 0.5% of the hearing impaired population to as high as 15% (Rance, 2005).
A constellation of test findings characterizes this population and distinguishes them from those with sensorineural hearing loss. Typically, OHC function is preserved, evidenced by either present OAEs or cochlear microphonic responses. Results from ABR testing, however, indicate either impaired function of the auditory nerve or of the inner hair cells (IHCs). Acoustic stapedial reflexes are absent or abnormal, further indicating impaired neural conduction of the auditory signal.
Auditory sensitivity thresholds are generally elevated, but there are cases in which the thresholds are normal or near-normal in range. Initially, it was commonly held that the clinician should expect word- recognition abilities disproportionately poorer than behavioral thresholds would predict. Continued investigation has shown that this is often not the case for speech understanding in quiet, although it continues to be impossible to predict word recognition in any given case of AN/AD with the behavioral audiogram alone (Rance, 2005). The literature also shows that patients with the disorder exhibit impaired abilities on temporally based psychoacoustic tasks (Zeng & Liu, 2006). In combination with other findings, this has led to the supposition that in at least some cases, AN/AD is a disorder of neural timing in the auditory system.
Despite the common test findings, the population of patients with AN/AD is quite heterogeneous. Numerous and varied etiologies may be responsible for this disorder. Multiple environmental factors have been linked to AN/AD, and a number of different genetic mutations may be involved. Pathogenesis of the disorder has not been definitively established, but multiple sites of lesion appear to be likely. These lesion sites include IHCs, the synapse between the IHCs and the auditory nerve, and various structures on the auditory nerve itself.
The diversity of etiologies, sites of lesion, and auditory abilities presents challenges in making remediation decisions for individuals with this auditory disorder. In essence, no single treatment strategy will be uniformly successful, and clinicians should explore multiple options for intervention without routinely ruling out any particular intervention. Mode of communication, amplification, and cochlear implantation are among the areas that can be explored.
A Protocol for Assessment and Management
Like all children with hearing loss, children with AN/AD should be referred to otology, medical genetics, and ophthalmology to evaluate the etiology of their condition and potential underlying associated medical conditions. Referral to early intervention services immediately following the identification of hearing impairment is also essential.
The initial diagnosis of AN/AD should be based on comprehensive electrophysiological assessment including, at a minimum, ABR testing, immittance measurements including tympanometry and acoustic reflex thresholds, and OAEs. Developmentally appropriate behavioral assessment of children with AN/AD should be completed at frequent intervals until behavioral thresholds can be quantified. Given the high likelihood of developmental delays and significant medical conditions with these children, the age at which behavioral audiometry can be reliably attempted may vary widely. The frequency of behavioral evaluation will depend on the developmental status and cooperation of the child, but should minimally be completed every three months until 6 years of age.
Once behavioral hearing sensitivity has been established, a trial with amplification can be initiated using a prescriptive approach that assures the audibility of speech (such as Desired Sensation Level m[i/o]). Gain and maximum output should be verified using probe microphone measurements. Whenever possible, loaned hearing aids can provide an opportunity to evaluate the benefits of amplification without significant financial implications to the family.
Counseling parents that hearing aids may or may not provide adequate access to speech and language is essential. Questionnaires such as the Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) and the Early Listening Function (ELF), to name only two, help to provide a framework to evaluate the benefits of amplification for these young children. Ideally, children with AN/AD who utilize amplification should be monitored monthly for changes in hearing sensitivity to determine the need for adjustments in amplification and to evaluate progress.
The length of time required to evaluate the benefit or lack of benefit from amplification will vary based on the developmental level of the child, consistency of amplification use, and the clinical judgment of the professionals working with the family. In some cases, a child's lack of improved responsiveness with amplification will be immediately apparent. A variety of data including audiological results, parental reports, and progress in speech and language development will assist in determining the length of a trial for amplification.
If a lack of hearing aid benefit is documented, further evaluation to determine candidacy for cochlear implantation can be undertaken. Given the significant negative impact of background noise on the speech understanding of individuals with AN/AD, personal FM technology should be considered regardless of whether a child utilizes a hearing aid or a cochlear implant.
The variability in speech perception skills for adults with AN/AD has been reported in a number of studies (Starr et al., 1998; Mason et al., 2003). Speech recognition data obtained from adults with AN/AD cannot simply be applied to children with the disorder. Factors such as age of onset, etiology, duration, and degree of functional hearing will differ between the two groups.
A retrospective analysis of word recognition scores for children with AN/AD collected in previous studies was compiled by Rance (2005). The analysis included open-set word recognition data for 46 children with three-frequency pure tone average (PTA) thresholds ranging from 18 dB to 80 dB. Word recognition was examined as a function of PTA and was compared to predicted values (Yellin et al., 1989) for adults with sensorineural hearing loss.
Results indicated that 44% of ears with AN/AD had word recognition scores that were better than or equal to the predicted range for adults with sensorineural hearing loss, while 56% of the ears demonstrated speech perception scores that were poorer than would be expected for individuals with the same degree of sensorineural hearing loss. This analysis demonstrates that PTA was found to be a poor predictor of speech understanding, and highlights the wide range of performance on word recognition tasks for children with AN/AD. Unfortunately, assessment of speech understanding is often not possible in very young children and may be confounded by other factors such as developmental or speech and language delays, which are common in children with AN/AD.
Despites these limitations, the available data on speech recognition for children with AN/AD underscores that no single management strategy is effective for this heterogeneous group of children. With potential word recognition scores in the same range as patients with sensorineural hearing loss, hearing aids are an effective strategy to provide speech audibility. Cochlear implants may be considered for patients who do not show improved responsiveness with the use of amplification.
An area of significant controversy regarding the management of AN/AD in children is related to the effectiveness of hearing aids in providing access to speech and language. The research literature provides abundant examples of case studies of adults and children with varying degrees of benefit from the use of amplification, but the question regarding whether hearing aids are beneficial for children with AN/AD has not been systematically evaluated. The limited prevalence of the disorder combined with the previously discussed heterogeneity make formal analyses difficult.
Nonetheless, Rance and colleagues (2002) evaluated the word recognition scores of 15 children diagnosed with AN/AD in both unaided and aided conditions in quiet. All of the participants in this study used hearing aids for a minimum of 12 months prior to participating in the study. Of the 15 subjects, seven demonstrated no significant improvement in word recognition scores in the aided condition compared with the unaided condition. The remaining eight subjects, however, showed a mean improvement in monosyllabic word recognition of 52%, suggesting that a subset of children in this study did show significant improvement in speech understanding in quiet with amplification.
Arguments have been made against hearing aid use in children with AN/AD. Given the finding of normal cochlear OHC function in these patients, some have argued that hearing aid use should be avoided as long as OAEs are present to preserve OHC function. While the idea of conservation of OAEs seems rational, there are numerous examples in the literature of spontaneous disappearance of OAEs without hearing aid use and also examples of children who have extensive amplification experience who have preserved OAEs. Interestingly, the argument for the preservation of OAEs has not been applied to the use of cochlear implants with children who have AN/AD.
Providing hearing aids with gain and output limited to levels that would be prescribed for a patient with mild hearing loss has also been suggested as a way to provide an opportunity to evaluate hearing aid benefit without substantial risk to the child. However, children with a greater hearing loss will not have access to important acoustic information for decoding speech and language. Although providing speech audibility through a hearing aid for a child with AN/AD is not a guarantee that an undistorted speech signal will be delivered, insufficient audibility guarantees that the child will not have access to critical speech information. Acceptance of mild gain strategies for fitting hearing for children with AN/AD has gained clinical acceptance, but it has limited the ability to evaluate whether appropriately fit hearing aids are beneficial for these children.
In light of the fact that the majority of the adult patients initially diagnosed with AN/AD also had indicators of peripheral neuropathy, cochlear implantation was not at first considered a viable avenue for remediation (Starr et al., 1996). It was suspected that the hearing loss in AN/AD was due to poor function of the VIII cranial nerve, which seemed to contraindicate use of a cochlear implant (CI).
Among the first reported cases of an individual with AN/AD with a CI was a child who underwent surgery before she was actually diagnosed with the disorder. She was identified with a severe-to-profound hearing loss and received a CI at age 2. It was not until her brother was being evaluated for CI candidacy and was found to have present OAEs that she underwent OAE testing herself and present OAEs were detected in her non-implanted ear (Trautwein et.al., 2001). The fact that she had shown rapid progress in her auditory and speech-language skills during the first year of CI use led some cochlear implant teams to re-evaluate whether cochlear implantation might be a viable treatment option for patients with AN/AD when the goal is development of effective auditory/oral communication skills.
A few years after the disorder was first described, some reports of single cases began to appear in the research literature that suggested that CIs may benefit this population. Sininger and Trautwein (2000) described a case with normal electrical ABR findings following CI surgery. Similarly, Fabry (2000) reported on a patient with improved auditory function after receiving a CI. There are now numerous reports in the research literature of successful outcomes for cochlear implants in individuals diagnosed with AN/AD (Peterson et. al., 2003). This is evidenced by synchronous neural responses during post-implant electrically evoked auditory brainstem response (EABR) testing, presence of electrically evoked stapedial reflexes, and speech-language development comparable to controls with severe-to-profound cochlear hearing loss.
Why does the cochlear implant work for patients with this type of hearing impairment? Obviously, in some cases of AN/AD the CI may bypass the site of lesion (i.e., the IHCs or synaptic junction) just as in traditional CI recipients with sensorineural hearing loss. In addition, it appears that the electrical stimulation provided by the device helps restore synchronous firing of the cochlear nerve. Research has shown that electrical stimulation is more effective at producing a synchronous neural response than is acoustic stimulation. Further, the biphasic pulsatile stimulation produced by the implant's electrodes in most processing strategies may by its nature increase the synchrony of activity in the auditory nerve (Rance et. al., 2005).
Despite the apparent success of this means of intervention, however, the recommendation for cochlear implantation in cases of AN/AD should not occur automatically. As mentioned previously, some individuals with the disorder derive considerable benefit from hearing aids. In these cases, CI surgery could be deemed unnecessary if the desired outcome could be achieved through amplification. In addition, there is a chance, albeit small, that auditory function may spontaneously improve during the first one to two years of life (Madden, et al., 2002). Finally, there are reported cases of patients with AN/AD who have received a CI and experienced limited outcomes (Miyamoto et al., 1999).
The original supposition that the site of lesion for this hearing impairment was the auditory nerve still holds true for some patients diagnosed with AN/AD, and neural degenerative conditions associated with AN/AD appear to be present in 30%–40% of the patients with the disorder. In addition, Buchman et al. (2006) reported that 9 of 51 subjects with AN/AD (18%) were identified through magnetic resonance imaging (MRI) as having abnormally small or absent cochlear nerves. Naturally, this would be expected to significantly impact transmission of stimuli from the CI to the auditory centers of the brain.
The heterogeneity in etiology, site of lesion, and auditory abilities in patients with AN/AD leads to the contention that a remediation protocol with respect to this disorder should include multiple options for mode of communication and access to auditory input. This includes recommendation of a trial with amplification. In addition, we agree with the recommendation of others that MRI results should be obtained routinely, especially when CI candidacy is being evaluated.