Yes, ANSD is quite common, affecting seven to 10 percent of children with permanent hearing loss (Int J Audiol. 2010;49:30 http://bit.ly/2v8ro2X; Int J Audiol. 2013;52 Suppl 2:S55 http://bit.ly/2v8RaEi; Trends Amplif. 2005;9:1 http://bit.ly/2v8w7lf). Among children who have been confined in the neonatal intensive care unit (NICU) and eventually diagnosed with permanent hearing loss, about one in four will have ANSD. Other disorders have also been associated with progressive forms of ANSD such as Leber's hereditary optic neuropathy, Brown-Vialetto-Van Laere syndrome, Charcot-Marie-Tooth disease, and Friedrich's ataxia. So, if your caseload includes more than a few children with hearing loss, you will most likely serve a child with ANSD.
9. How do I diagnose ANSD?
There are several valuable resources for clinicians to diagnose ANSD, including the following:
* Ontario Infant Hearing Program's Audiologic Assessment Protocol
* British Columbia Early Hearing Program's Audiology Assessment Protocol
* England's National Health Service's Guidelines for the Assessment and Management of Auditory Neuropathy Spectrum Disorder in Young Infants
* Guidelines for Identification and Management of Infants and Young Children with Auditory Neuropathy Spectrum Disorder
These guidelines have minor differences, but the essentials are the same. ANSD is diagnosed when the neural components of the auditory brainstem response (ABR) are absent or severely abnormal, with signs of functional outer hair cells in the form of present otoacoustic emissions (OAEs) or a present cochlear microphonic (CM). Here are some relevant (but not comprehensive) highlights of the ANSD assessment battery:
* The presence of normal tone burst ABR waveforms rules out ANSD. Thus, it is unnecessary to complete a click-evoked ABR to determine whether ANSD exists. If tone burst ABR is normal, ANSD does not exist.
* A definitive diagnosis of ANSD is made when OAEs are present and the ABR is absent. However, for several reasons, OAEs are absent in many children diagnosed with ANSD. In these cases, ANSD is diagnosed based upon the presence of a cochlear microphonic and an absent ABR (Fig. 1). Of note, the Ontario Infant Hearing assessment protocol states that ANSD is a possible diagnosis when the click-evoked “CM is twice the size of any neurogenic waves.”
* The acoustic reflex test is quite valuable in the assessment of children at risk of ANSD. Berlin and colleagues reported absent or abnormal acoustic reflexes in 260 patients diagnosed with ANSD (Int J Audiol. 2010 http://bit.ly/2v8ro2X). Stated differently, the presence of normal acoustic reflexes essentially rules out a diagnosis of ANSD.
8. Are there other variations to consider for ABR assessment?
Yes, without a doubt. People with ANSD make up a heterogeneous group as the site-of-lesion varies across patients and can include the inner hair cells, cochlear synapse, cochlear nerve, and auditory brainstem. The conventional ABR assessment battery can be adjusted to find the exact site of lesion. Incorporating electrocochleography (eCochG) provides a more effective assessment of peripheral auditory function (Int J Pediatr Otorhinolaryngol. 2003;67:707 http://bit.ly/2uakY6d). The eCochG enhances the clinician's ability to record the CM and wave I of the ABR, and allows for the assessment of the summating potential. Careful measurement of these responses can help delineate the inner hair cell, auditory synapse, and cochlear nerve dysfunction.
An electrically-evoked ABR (eABR) may also be recorded with trans-tympanic stimulation at the promontory or round window. Trans-tympanic eABR may be used to determine if electrical stimulation can elicit an ABR waveform. Gibson and colleagues have shown that cochlear implant outcomes are better for patients with robust pre-operative eABR (Ear Hear. 2007;28[2 Suppl]:102S http://bit.ly/2uNBO8N).
7. Will all children with ANSD have poorer outcomes than children with cochlear hearing loss?
The jury is still out for this question, but preliminary outcomes of the Australian Hearing Services Longitudinal Outcomes of Children with Hearing Loss (LOCHI) study suggest that the tentative answer is no. The LOCHI study is prospectively evaluating the outcomes of 460 children diagnosed with permanent hearing loss. In a 2013 report, Ching and colleagues found no statistically significant difference in the language outcomes of the children with ANSD when compared with the outcomes of children with cochlear hearing loss (Int J Audiol. 2013 http://bit.ly/2v8RaEi). In addition, no significant difference was found in the language outcomes of children with ANSD who used hearing aids when compared with children with ANSD who used cochlear implants.
6. If children with ANSD can have the same outcome as children with cochlear hearing loss, then why are clinicians losing sleep over ANSD management?
Great question! In the LOCHI study, the reasonably good language outcomes of children with ANSD were attributed to the early intervention received by this patient group. Other studies suggesting poorer outcomes for children with ANSD often included children who were fitted with hearing aids and/or received cochlear implants at a relatively late age. In contrast, the children with ANSD in the LOCHI study were either fitted with hearing aids by six months of age or given cochlear implants by 12 months of age. Of note, the auditory nervous system may not develop normally when children with ANSD receive cochlear implants beyond the critical period of language development (Int J Audiol. 2011;50:98 http://bit.ly/2u9Un99).
5. While early intervention is vital, how was the LOCHI group able to fit hearing aids and identify cochlear implant candidacy among very young children?
The LOCHI group used several measures to guide management decisions for children with ANSD. Audiologists and early interventionists (e.g., auditory-verbal therapists) worked together to evaluate the functional auditory abilities of all children in the study. Clinicians used standardized questionnaires (e.g., Parent's Evaluation of Aural/Oral Performance of Children), behavioral audiometry, and parent observations to determine the child's responsiveness to auditory stimuli. The LOCHI group also routinely measured the cortical auditory evoked response (CAER) to see if speech evoked an auditory response in the brain with and without hearing aids, determine hearing aid candidacy, and guide the early fitting of hearing aids. Furthermore, the absence of aided CAERs indicated the potential need for cochlear implants. CAER may also be measured post-implantation to evaluate whether electrical stimulation is evoking an auditory response in the auditory cortex (for more on the benefits of CAER, see Hearing Journal. 2017;7038 http://bit.ly/2u1g5Ny). The Frye Electronics HEARLab system, for example, allows for simple and expeditious CAER assessment using the LOCHI approach.
4. Sweet! So, if the CAER is present, I should expect all to be well, right?
Not exactly. Although research suggests better speech recognition and/or language outcomes when the CAER is present in children with ANSD, clinicians should continue to expect the child to experience difficulties in understanding speech in noise. (Ear Hear. 2002;23:239 http://bit.ly/2p0KLuy; Int J Audiol. 2011 http://bit.ly/2u9Un99). Significant research shows that virtually all people with ANSD who use hearing aids will have poorer speech recognition in noise than those with cochlear hearing loss (particularly when matched in degree of pure-tone hearing threshold loss). The disruption in temporal processing inherent in ANSD impairs the processing of the fine temporal structure and enhances the deleterious effects of masking.
Electrical stimulation of the cochlear nerve via cochlear implantation may partially overcome the dyssynchronous auditory responses in acoustic stimulation. However, considerable peer-reviewed studies also show that many cochlear implant recipients understand speech very well in quiet but not in noise. As such, clinicians should strongly recommend the use of hearing assistive technology designed to improve hearing in challenging acoustic situations. Using remote microphones can also be considered for children with ANSD.
3. For children with ANSD who do not benefit from hearing aids, will cochlear implants be successful?
In most cases, cochlear implants are very successful for children with ANSD, particularly if the site of lesion includes the inner hair cells or cochlear synapse. Additionally, many children will benefit from a cochlear implant when the site of lesion resides at the cochlear nerve. Often, these cases can be attributed to a demyelinating disorder, which results in the reduced ability of the cochlear nerve to respond synchronously to acoustic stimuli. The electrical stimulation from a cochlear implant over-synchronizes the response of the cochlear nerve, thereby improving synchronicity. However, the outcome is limited when the cochlear nerve is deficient or absent. The presence of a cochlear-implant-elicited, electrically-evoked compound action potential (e.g., Auditory Response Telemetry, Neural Response Imaging, and Neural Response Telemetry) indicates that the cochlear implant has restored synchrony to the cochlear nerve. Of course, if the site of lesion is within the auditory nervous system (e.g., brainstem, auditory cortex), good cochlear implant outcomes will likely be diminished.
2. What other tests can be done to determine the outcome of cochlear implants for a child with ANSD?
When considering cochlear implants, children with ANSD should undergo magnetic resonance imaging (MRI) to evaluate the physical integrity of their cochlear nerve and auditory nervous system. MRI is an excellent tool to identify deficient and absent cochlear nerves (Otol Neurotol. 2016;37:438 http://bit.ly/2uar6eK; Otol Neurotol. 2010;31:780 http://bit.ly/2uahO2i). Studies have shown that cochlear nerve deficiency is often asymmetric, an important finding because MRI identifies an ear that is a viable candidate for cochlear implantation. MRI should also be completed for children with single-sided deafness (SSD), as research shows that as many as 50 percent of children born with SSD have a deficient/absent cochlear nerve in the affected ear (Otol Neurotol. 2013;34:554 http://bit.ly/2uNudqy). Furthermore, the MRI may be used to evaluate the physical status of the auditory brainstem structures and the cerebrum, as well as the myelination present in the nervous system.
1. Finally, how can an interdisciplinary team be maximized for clinical ANSD management?
Optimal management of children with ANSD should be administered by an interdisciplinary team. A pediatric audiologist provides comprehensive assessment of auditory function. A speech-language pathologist/auditory-verbal therapist also evaluates functional auditory and spoken language development. An otologist/neurotologist provides a thorough medical evaluation of the auditory system including imaging. A neurologist may be included because ANSD can occasionally be associated with another neurological dysfunction. A geneticist should also be included, as up to 40 percent of ANSD diagnoses have a genetic cause (Sem Hear. 2002;23:193). A pediatrician may manage the child's overall care. Psychologists, social workers, and educators may also provide valuable input and services. Finally, the family is the most important member of the team. As children with ANSD are diverse, the family is almost always best-suited to provide the most useful insight into the unique abilities, limitations, and needs of the child.
While managing children with ANSD can be challenging, pediatric hearing health care professionals can incorporate contemporary knowledge into basic pediatric audiology services and work with an interdisciplinary team to successfully address the unique needs of these young patients.
Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.