Auditory Neuropathy
The term “auditory neuropathy” was coined by Starr and colleagues in 1996, but was criticised by others who felt that the pathology may be misleading and instead preferred terms such as “auditory dys-synchrony”, “auditory de-synchrony” or “auditory mismatch” – terms they believe could better identify or reflect what is happening in the auditory system (Sutton, 2004; Berlin, Li, Hood, Morlet, Rose, & Brashears, 2002).
The absent or abnormal morphology at high stimulus levels evident in Auditory Brainstem Response (ABR) diagnostic testing demonstrate abnormal neural responses. While on the other hand, the otoacoustic emission testing using (distortion product otoacoustic emissions) DPOAEs demonstrate present pre-neural responses, suggesting at least a moderate hearing loss present (in DPOAE testing, one would expect to get emissions up to at least 60dB HL) (Gorga, Neely, Ohlrich, 1997).
These conflicting results clearly reflect the pattern of “auditory neuropathy”, a label for a pattern of test results that is typically classified to individuals whose test results show an abnormal ABR and present OAEs. Commonly, most auditory neuropathy individuals would reflect having the presence of OAEs and/or cochlear microphonics (CM) revealing normal outer hair cell function, in conjunction with absent ABRs (Starr, Picton, Sininger, Hood, Berlin, 1996).
From the literature, it appears that the site of lesion for auditory neuropathy or auditory dys-synchrony (AN/AD) has not been fully understood. On the one hand, Abdala, Sininger, & Starr (2000) mentioned that many patients with AN/AD often display a hearing disorder associated with peripheral neuropathy, denoting that the rational site of lesion, would therefore be the auditory nerve. Patients with suspecting AN/AD would also display poor speech discrimination results (Hood, et. al., 2002).
Hood (1998) meanwhile, mentions that the characteristics of AN/AD seem to reflect more than a single etiology and that a number of pathologies can also produce AN/AD. Some of these pathologies, according to Rance, McKay, & Grayden (2004), include: genetics, poor status of inner hair cells (IHCs), an abnormality of the synapse between the IHCs and auditory nerve fibres, a spiral ganglion cell disorder, depleted neuronal populations in the auditory brainstem, and disruption of the synchronicity of the auditory nerve fibres or demyelisation of the auditory nerve. There has also been a noted association between hypoxia and the characteristics of AN/AD.
Harrison (1998) proposed that one of the causes of scattered loss of IHCs and its central cochlear afferent connections has been associated with the extended periods of mild hypoxia, which is greatly prevalent in high risk births. Thumak (2002) further states that hypoxia occurs at the IHCs and slowly progresses to include the spiral ganglion cells, with possible continued degeneration of more central neurons in the midbrain.
All patients with suspecting AN/AD should undergo cochlear microphonics (CM) assessment. As stated by Berlin, Morlet, & Hood (2003), the CM or cochlear hair cell potentials reflect the electrical activity (or external stimulus) originating from the outer and inner hair cells, and would not affect the CM in AN/AD patients. The CM can also be mistaken as the ABR, however by assessing the effects of polarity, intensity, and masking, the CM (cochlear responses) can be clearly differentiated from the ABR (neural responses) (Berlin, Morlet, & Hood, 2003).
Immittance testing (tympanometry and reflexes) is another test that should be undertaken as it can rule out conductive pathologies and can be a useful tool in evaluating middle ear status where ABR is not available. Immittance testing together with OAE testing can be a useful test battery for screening children suspected of having AN/AD. If there are conflicting results in any of these tests, children would be candidates for further evaluation of AN/AD (Berlin, Morlet, & Hood, 2003).
In particular, acoustic reflex testing will be useful in identifying a conductive impairment and can differentiate between cochlear and retrocochlear pathology (Hood, 2002). In patients with AN/AD, you would often see normal middle status when conducting tympanometry, but abnormal/absent acoustic reflexes on the ipsilateral or contralateral side (Hood, 2002). It is worth noting that other types of lesions such as VIII nerve tumors or multiple sclerosis can also result in absent/abnormal responses (Hood, 2002).
Electrocochleography (EcochG) can also be useful identifying AN/AD. Although the procedure can be invasive, the EcochG proves to be valuable in providing a more detailed test of the ear and hearing. It can help us check what level of hearing loss there is and obtain more detailed information about the type of hearing loss (can provide information about the hair cells and the neural response from the auditory nerve) that cannot always be seen on other types of audiology tests (Abbas & Brown, 2009).
McMahon, Patuzzi, Gibson, & Sanli (2008) also state that the measurable waveforms of the EcochG pre- and post- cochlear implantation can be useful in evaluating both the presynaptic and postsynaptic sites of AN/AD, which can have implications for actually fitting cochlear implants. In cases of neural function, the CAP seen in EcochG, in conjunction with the ABR would be grossly abnormal or absent (Abbas & Brown, 2009).
Prognosis
There is certainly great uncertainty surrounding the AN/AD prognosis. The combination of test results and ongoing audiological, medical, or neurological assessments can assist in communicating to parents what AN/AD could possibly mean for their child and what strategy options are available. Sutton (2004) mentions that it would be helpful to relay to parents that the term AN/AD is a label for a pattern of test results and not a label for their child; an absent ABR would not necessarily imply a profound hearing loss; AN/AD will need to be monitored closely as their child may not respond to sound in a typical way; many children with AN/AD are able to make good use of their hearing; and that although we are not be able to predict the impact of AN/AD on their child at this early stage (Charlotte is only 7 weeks), with ongoing assessments and observations, we will be able to do as much as possible for their child.
Prior to informing the parents of a child who is 7 weeks and suspecting of having AN/AD, behavioural testing such as behavioural observation audiometry (BOA) could be performed to possibly rule out a profound hearing loss. BOA could be helpful when integrating with the other test results and when communicating the information to parents on AN/AD – that their child does not necessarily have a profound hearing loss but that there is some neural deficit.
Treatment/Management Plan
Instigating a treatment/management plan would prove challenging for parents, audiologists and other professionals due to the great variation among individuals presenting with AN/AD. At the age of 7 weeks, early intervention is vital and communication options should be implemented as soon as possible. Cone-Wesson (2003) states that “if a spoken or signed language is not introduced early in life, cognitive, emotional, and social development may also be delayed”.
Aural-oral and visual-manual methods of communication for children with significant hearing impairment have been under constant debate. For instance, the aural-oral approach has been seen to improve speech production in hearing impaired children (Geers, 2002), yet other studies measuring language competence found that when a child uses their preferred mode of communication, they have shown to also benefit from both the aural-oral approaches as well as those that incorporate sign language (Connor, Hieber, Arts, & Zwolan, 2000). To date, there has not been any study to suggest the use of one method over the other.
Many authors, nevertheless, have agreed on cued speech as the recommended rehabilitation tool and communication option for children exhibiting AN/AD (Cone-Wesson, 2003; Berlin, 1999). Cone-Wesson (2003) defines cued speech as a communication method which acts as a visual manual aid to lip-reading and can provide important visual information for speech understanding. Moreover, parents report that this method helps their child develop receptive language (Hood, et. al., 2002). This method should however only be recommended for families who do not use sign language. For those families who use sign language and their child has been immersed in the Deaf culture, the use of sign language would be recommended (Berlin, Morlet, & Hood, 2003).
There has been considerable controversy surrounding fitting children with AN/AD conventional amplification devices such as hearing aids or a personal FM). According to Starr, Picton, Sininger, Hood, & Berlin (1996), amplification was of little/no benefit (only 50% of children with AN/AD reported little/no benefit) and in some cases, there were detrimental effects. Their argument was that there was a risk of obtaining a noise-induced hearing loss when amplification is prescribed for children as hearing thresholds fluctuate and the gain for these amplification devices is often estimated. Starr, Picton, Sininger, Hood, & Berlin (1996) also mentioned that the use of amplification can cause damage to the existing OHCs, causing hearing loss that was not present in patients with AN/AD.
Deltenre, et. al. (1999) further mentioned that OAEs may deteriorate in these children as part of the natural course of AN/AD even when hearing aids are not in use. Therefore, it is difficult to conclude or provide a prognosis for hearing aid benefit in children with AN/AD. On the other hand, a few other authors assessed children with AN/AD with use of amplification and found that there were notably improved speech perception abilities (Cone-Wesson, Rance, & Sininger, 2001; Madden, Hilbert, Rutter, Greinwald, & Choo, 2002).
Should a child not benefit from hearing aid amplification, the next step would be to conduct further assessments and evaluate if they are a candidate for cochlear implantation. If they fit the candidacy selection criteria for a cochlear implant, they would then be referred on to a cochlear implant centre for discussion of cochlear implantation. Although there have been varied outcomes in children with cochlear implantation, there have been a lot of positive findings on children with AN/AD who were implanted – these children remarkably demonstrated improvement in their speech awareness thresholds and improved abilities in speech perception (Buss, et. al., 2002; Madden, Hillbert, Rutter, Greinwald, & Choo, 2002; Shallop, Peterson, Facer, Fabry, & Driscoll, 2001).
According to Cone-Wesson (2003), this is perhaps due to the electrical stimulation provided by the cochlear implant which produces highly synchronous firing of auditory neurons. One should however, note that the diagnosis of AN/AD should not be an immediate referral for cochlear implantation and some patients with AN/AD may benefit from amplification (Rance, et. al., 1999). In particular, for children who have little or no residual hearing, the decision with fitting amplification or proceeding with cochlear implantation should only be made after evidence of a cochlear nerve on an MRI (Buchman, et. al, 2006).
Clearly, the treatment, rehabilitation and management plan for the child with AN/AD should be carefully considered, as each child is different and may exhibit different outcomes. The management plan should essential account for early identification, and include a family-focused intervention; a discussion on hearing aids or cochlear implants as well as communication strategies; and careful monitoring of functional hearing, emerging language, and speech perception abilities. Cone-Wesson (2003) states that these multimodal and longitudinal evaluation protocols used should help families determine which rehabilitation approach would be most suitable for their child.
Summary
Due to the large variation in a number of individuals displaying similar or vastly different AN/AD characteristics, fluctuating hearing loss and some patients losing their OAEs over some time, ongoing assessments and close monitoring are essential (Rance et. al., 1999). At the age of 9 months, there are other tests that could be undertaken to provide a greater understanding about whether a child could develop spoken language.
First, with gross motor function and normal IQ, a child at 9 months would be developmentally ready to take visual reinforcement observation audiometry (VROA) in the soundfield. This test is used to condition the baby to respond to sound, and would be helpful in enabling clinicians to denote at this stage whether the child has at least one good ear to be able to develop speech and language.
Cortical auditory evoked potentials (CAEPs) is another test that can be used to evaluate speech perception abilities and can be helpful for assessing children with AN/AD. Cone-Wesson (2003) states that the timing and amplitude of CAEPs in children with AN/AD did not appear different to responses obtained from control groups of children with normal hearing and those who had a sensorineural hearing loss. Cone-Wesson (2003) also stated that some children with AN/AD had present CAEPs, however, some had absent CAEPs. She mentioned that those who had present CAEPs reflected the presence of neural synchrony and has significantly better speech perception scores than those who did not have obtainable CAEPs. However, she further stated that whether or not CAEPs can be used as a prognostic test for amplification benefit has yet to be demonstrated.
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