Auditory neuropathy spectrum disorder: Evaluation and management

Do children with “auditory neuropathy” benefit from amplification? Is cochlear implantation a preferred option? What advice should we give to the parents of an infant with this disorder?

The answers to these questions are more complicated than we once thought. Over the past decade, as universal newborn hearing screening has expanded and as audiologists have become more familiar with the disorder and its diagnosis, a growing number of infants and young children with auditory neuropathy have been identified. Indeed, auditory neuropathy presents many challenges for the professionals responsible for providing a diagnosis and recommendations for clinical management. Controversy exists in almost every aspect of the disorder, including etiology, site of lesion, management recommendations, and the terminology used to describe the disorder.^1–5

At a recent international conference in Como, Italy, a panel was charged with developing guidelines for identification and management of infants with this disorder. The panel adopted the term “auditory neuropathy spectrum disorder” (ANSD) as a way of describing its heterogeneous and multifaceted nature. (Guidelines Development Conference on the Identification and Management of Infants with Auditory Neuropathy, International Newborn Hearing Screening Conference, Como, Italy, June 19-21, 2008).

Briefly, ANSD is thought to be a hearing impairment in which outer hair cell function is normal but afferent neural conduction in the auditory pathway is disordered.⁶ As more patients, particularly young children, have been diagnosed, studies have shown that patients with ANSD are a heterogeneous group despite similar audiologic test findings. Characteristics of ANSD have been reported in patients with histories of prematurity, neonatal insult, hyperbilirubinemia, perinatal asphyxia, artificial ventilation, and various infectious processes.^7,8 Genetic abnormalities have also been identified, including OTOF, PMP22, MPZ, and NDRGI.

Speculation regarding underlying mechanisms includes selective inner hair cell loss, a synaptic or myelinization disorder, or an auditory nerve disorder with other peripheral neuropathies.^1,9In 2006, Buchman and colleagues described a group of children who presented with electrophysiologic test results typical of ANSD who were subsequently diagnosed by magnetic resonance imaging (MRI) as having cochlear nerve deficiency i.e., absent or small cochlear nerves.¹⁰ While the disorder was initially thought to be quite rare, current estimates of ANSD prevalence range from 7%-10% of children with permanent hearing loss.¹¹

EVALUATION

Diagnosis of ANSD is accomplished using electrophysiologic measures. Specifically, auditory brainstem response (ABR) testing is completed using insert earphones with high-intensity click stimuli. Both rarefaction and condensation polarities are evaluated to determine if a cochlear microphonic is present. A diagnosis of ANSD is made if the ABR conducted in this manner shows an absent or grossly abnormal ABR with only the presence of a cochlear microphonic.⁷ (See Figure 1.) Insert earphones must be used and care taken to complete a “no sound run” by clamping or disconnecting the sound tube from the transducer to eliminate the possibility of stimulus artifact. If testing is limited to click stimuli with alternating polarity, an incorrect diagnosis of profound sensory hearing loss may result. Otoacoustic emissions may be present initially but disappear over time; thus, absence of emissions does not preclude the diagnosis of ANSD. Middle ear muscle reflexes are almost always absent.¹²

Figure 1: Diagnostic test results from a child with auditory neuropathy spectrum disorder. Note abnormal ABR tracings accompanied by normal OAE results.

Unlike “typical sensorineural hearing loss,” threshold estimation is impossible using ABR or ASSR evaluation. Therefore, determination of thresholds for infants and young children with ANSD must be made using developmentally appropriate behavioral measures such as visual reinforcement audiometry or play audiometry. For the very young infant or for children with multiple disabilities unable to perform reliable conditioned responses, behavioral observation audiometry may be used to obtain information about the child's residual auditory capacity. Cortical evoked potentials may also be useful in cases where behavioral methods are inconclusive.^13,14

EVIDENCE TO GUIDEMANAGEMENT DECISIONS

Studies of children with ANSD show widely varying auditory capacity. Among the clinical characteristics reported are: pure-tone thresholds that vary from normal to profound, disproportionately poor speech-recognition abilities for the degree of hearing loss, difficulty hearing in noise, and impaired temporal processing.^6,13,15–18

While some children with ANSD have disproportionately poor speech-recognition ability for their degree of hearing loss, the performance of others is similar to peers with typical sensorineural hearing loss. For example, in 2002, Rance and colleagues in Australia compared unaided and aided speech-perception abilities for a group of 15 children with ANSD to a group with typical sensory hearing loss matched for age and hearing level.¹³ Their results showed that approximately 50% of the children with ANSD had speech-recognition scores similar to those of the children with sensory hearing loss; the other 50% showed essentially no open-set speech-perception ability. Interestingly, the children who showed no open-set speech perception had absent cortical evoked potentials, while the children who had measurable speech-recognition scores had present CAEPs.

Similarly, although it has been reported that children with ANSD have particular difficulty hearing in the presence of background noise,^16,19 a 2007 study by Rance et al. showed that while children with typical sensorineural hearing loss and those with ANSD had more difficulty in noise than children with normal hearing, the effects were not consistent across subjects and some children with ANSD showed relatively good speech-perception abilities even at low signal-to-noise ratios.²⁰

The original report by Starr and colleagues described a group of 10 patients who showed abnormal or absent auditory brainstem responses (ABR) in the presence of outer hair cell function as indicated by measurable cochlear microphonics or otoacoustic emissions.⁶ The patients in their study presented without neurologic involvement at the time of diagnosis; however, eight of the ten patients subsequently developed other peripheral neuropathies. For this group of patients, little or no benefit was obtained from the use of hearing aids.

Following the report by Starr and colleagues describing what appeared to be “neural” hearing loss, a number of early recommendations were made regarding clinical management for this population including: use of low-gain hearing aids or FM systems, use of low-gain amplification in one ear only, or the avoidance of hearing aid use altogether.^4,21–23 Further, because the early reports described what appeared to be pathology of the auditory nerve, it was initially thought that cochlear implantation would not be beneficial.^24,25 Over time, as more young children diagnosed with ANSD have been evaluated, investigators have shown that both hearing aid use and cochlear implantation can be of benefit to some children with ANSD.^{7,13,17,19,26–28}

MANAGEMENT RECOMMENDATIONS

The variable nature of ANSD and the differences inherent in various habilitative strategies create challenges for pediatric audiologists and intervention specialists. Professionals who see these children clinically are called upon to make management recommendations based on limited diagnostic information, e.g., abnormal ABR. In most cases, the clinical tools currently available provide only limited information regarding site of lesion and essentially no ability to predict if a given child will benefit from amplification.

Consequently, a prudent approach, in cases where there is residual hearing and once reliable threshold estimates can be obtained, is to provide an adequate trial of amplification using an evidence-based hearing aid fitting protocol.^29,30 These established protocols include measurement of real-ear-to-coupler difference (RECD) measures with use of a prescriptive hearing aid fitting method (e.g., DSLv5, NAL) to ensure that speech at conversational levels is audible and comfortable. Once hearing aids have been provided, speech and language development must be carefully evaluated to monitor communication milestones.

Although evaluation of speech-recognition abilities in pediatric populations is often challenging, closed-set speech-recognition tests, e.g,. Early Speech Perception test (ESP)³¹ may be useful when children are too young for open-set testing. Parent questionnaires such as the Infant-Toddler: Meaningful Auditory Integration Scale (IT-MAIS)³² may also be helpful in evaluating progress. For an overview of speech-recognition testing for children less than 3 years of age see Eisenberg, Johnson, and Martinez.³³

Because the behavioral pure-tone audiogram is of limited prognostic value in predicting aided benefit in children with ANSD, cochlear implantation should be considered even if audiometric thresholds are better than what would typically be considered when progress with conventional amplification is inadequate, but only after an adequate trial with amplification, appropriate early-intervention services, and a comprehensive evaluation by a team of professionals. The length of time needed to determine benefit from hearing aid use will vary depending on at what age the child is diagnosed and provided with amplification, the consistency of hearing aid use, and presence of other disabilities or medical conditions.

Considering the increased risk of difficulty hearing in the presence of background noise, personal FM use by parents and other caregivers is likely to be beneficial; FM use in the classroom is especially important.

As with any infant diagnosed with sensorineural hearing loss, a comprehensive medical evaluation is essential. In addition to a complete otologic examination, referral should be made to medical specialists in ophthalmology, genetics, and neurology. At our center, all children with a diagnosis of ANSD receive magnetic resonance imaging to determine the status of the auditory nerve.

Because of the complexity of the disorder and the increased likelihood of additional disabilities in children with a diagnosis of ANSD, it is essential that evaluation and management occur in a setting where a specialized team approach includes pediatric audiologists, speech-language pathologists, and early-intervention specialists experienced in the management of ANSD and physicians familiar with both typical sensorineural hearing loss and the unique needs of children with ANSD. It is important to make sure families understand the heterogeneous nature of the disorder and its underlying etiologies, the controversies that exist regarding treatment options, and the difficulty predicting outcomes during infancy.

In summary, ANSD affects a substantial number of young children with congenital hearing loss. The heterogeneous nature of this disorder, the wide range of clinical findings, and the variability seen in outcomes associated with various treatment options require the use of a full range of intervention and habilitation strategies. At the University of North Carolina at Chapel Hill, we are engaged in a prospective longitudinal study of approximately 150 infants and young children with ANSD. The families of all newly identified infants are counseled that decisions regarding amplification, cochlear implantation, and habilitation strategies, including auditory/oral, cued speech, or manual communication, may be considered, depending on family preference and the individual needs of each child. Although controversy surrounds the management of ANSD and further research is needed, appropriate audiological assessment combined with comprehensive medical assessment and early intervention is enabling many children with ANSD to achieve functional communication skills and successful developmental outcomes.