The management of cochlear nerve deficiency S R Freeman1 , S M Stivaros 2,6, R T Ramsden 1, M P O’Driscoll 1,4, J R Nichani 1, I A Bruce 1,5, K M Green 1,4, L A Henderson 1, S A Rutherford 3, A T King 3,7, S K Lloyd 1,4 1

Manchester Auditory Implant Centre, Central Manchester Foundation NHS Trust, Manchester Academic Health Science Centre, Manchester, UK, 2Department of Radiology, Royal Manchester Children's Hospital, Central Manchester Foundation NHS Trust, Manchester Academic Health Science Centre, Manchester, UK, 3 Department of Neurosurgery, Salford Royal Hospital NHS Foundation Trust, Salford, UK, 4School of Cancer and Enabling Services, University of Manchester, UK, 5Respiratory and Allergy Centre, Institute of Inflammation and Repair, University of Manchester, UK, 6Centre for Imaging Sciences, School of Population Science, University of Manchester, UK, 7Faculty of Medicine and Life Sciences, Institute of Cardiovascular Sciences, University of Manchester, UK The assessment process is critical in deciding whether a profoundly deaf child with cochlear nerve deficiency (CND) will be suitable for a cochlear or auditory brainstem implant (ABI). Magnetic resonance imaging (MRI) using submillimetric T2 weighted gradient echo or turbo spin echo sequences is mandatory for all profoundly deaf children to diagnose CND. Evidence of audition on behavioural or electrophysiological tests following both auditory and electrical stimulation sometimes allows identification of significant auditory tissue not visible on MRI. In particular electric auditory brainstem response (EABR) testing may allow some quantification of auditory tissue and help decide whether a cochlear implant will be beneficial. Age and cognitive development are the most critical factors in determining ABI benefit. Hearing outcomes from both cochlear implants and ABIs are variable and likely to be limited in children with CND. A proportion of children will get no benefit. Usually the implants would be expected to provide recognition of environmental sounds and understanding of simple phonetics. Most children will not develop normal speech and they will often need to learn to communicate with sign language. The ABI involves a major neurosurgical procedure and at present the long term outcomes are unknown. It is therefore essential that parents who are considering this intervention have plenty of time to consider all aspects and the opportunity for in depth discussion. Keywords: Auditory neuropathy, Prelingual deafness, Cochlear implantation, Auditory brainstem implantation

Introduction Deciding on the optimal management of a congenitally profoundly deaf child with cochlear nerve deficiency (CND) presents a highly complex challenge. This paper presents the approach currently taken by the Manchester Auditory Implant Centre (MAIC).

availability of an auditory brainstem implant (ABI); 13 were deemed unsuitable for ABI (although 1 then had an ABI elsewhere); the parents of 13 children declined an ABI; 1 had an ABI elsewhere and returned to Manchester for rehabilitation; 6 had an ABI in Manchester.

The Manchester experience

Assessment process

Between 1996 and March 2012, 44 children with CND have been assessed at MAIC. Two had unilateral CND and had a cochlear implant (CI) in the other ear; 5 had bilateral CIs, 1 of whom had no response in either ear and 7 had a unilateral CI, 4 of whom had no response; 30 were deemed unsuitable for a CI. Out of the 35 children who were either unsuitable or gained no benefit from a CI: 2 were seen prior to the

A number of factors are considered when assessing whether a profoundly deaf child with CND may be suitable for implantation.

Correspondence to: S. R. Freeman, University Department of Otolaryngology - Head and Neck Surgery, Manchester Royal Infirmary, Oxford Road, Manchester M13 9WL, UK. Email: [email protected]

© W. S. Maney & Son Ltd 2013 DOI 10.1179/1467010013Z.000000000129

Magnetic resonance imaging Detailed imaging is essential in the investigation of any child with profound deafness. Computed tomography (CT) will identify bony abnormalities but is unable to recognize nerves (Adunka et al., 2007). As CND can occur with normal bony anatomy, magnetic resonance imaging (MRI) is mandatory for direct visualization of these nerves. Casselman et al. (1997)

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Figure 1 (A–C) Demonstrates high resolution (0.5 mm slice thickness), heavily T2-weighted axial turbo spin echo acquisition MRI images through the IAMs in a profoundly deaf child diagnosed with CND. These images highlight some of the problems encountered. He was referred at 21 months age with no response on visual reinforcement audiometry but a single response on auditory brainstem response testing in the right ear at 95 dB with a 1 kHz tone pip only. (A) Axial image demonstrates some pulsation artefact running across the image from the basilar artery. There is some cerebellar crowding, obscuring the porus acousticus and cerebello-pontine angle. Only a single hypoplastic nerve (white arrow) is seen travelling towards the right IAM (no nerve could be identified within the IAM or on parasagittal reconstructions). (B) Axial image more superiorly demonstrating a probable facial nerve (white arrow) passing to an aberrant right facial canal. (C) Reconstructed sagittal image (A – anterior, P – posterior) showing the facial nerve (white arrow) travelling just inferiorly to the trigeminal nerve (black arrow).

first defined the generic MRI protocols for identifying abnormalities of the auditory nerves. This can be achieved using heavily T2-weighted gradient echo or turbo spin echo sequences, e.g. DRIVE, FSE, or FIESTA. The important factor is to acquire the imaging using submillimetric slice thicknesses and inter-slice distances. This allows for an axial acquisition in high resolution with excellent tissue/fluid contrast. Review of the imaging in the parasagittal plane is often required and the imaging protocol described above allows for the multi-planar reconstruction of parasagittal images from the axially acquired dataset (Fig. 1C). This reduces the necessity for the increased scan times that would be required if direct parasagittal scan acquisitions were undertaken. It must be noted however that in some complex cases the best imaging will be obtained as a direct parasagittal acquisition and this must be considered the gold standard for resolution when such parasagittal views are required. The diameter of the normal cochlear nerve (CN) is similar to the facial nerve whereas the vestibulocochlear nerve (VCN) should be 1 1/2–2 times this size. Unfortunately, difficulties arise which can degrade the MRI imaging. Poor resolution can arise

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if a thicker-section imaging protocol is used, which means that adequate parasagittal views cannot be reconstructed. Movement artefact can degrade the imaging (Fig. 1A), a particular problem in the paediatric cohort, meaning that repeat imaging sometimes with sedation or general anaesthesia is required. Children with CND can have associated abnormalities that reduce visualization of the nerves such as cerebellar crowding, masking the porus acousticus, and the VCN within the cerebello-pontine angle (Fig. 1A) or a narrow internal auditory canal (IAC) or cochlear nerve canal (CNC) masking the CN (Adunka et al., 2007; Komatsubara et al., 2007; Miyasaka et al., 2010). The facial nerve can travel in unusual positions such as an early exit of the internal auditory meati (IAM), a separate anterior facial canal, traversing the petrous apex (Fig. 1B and C) or traversing Meckel’s cave alongside the trigeminal nerve (Casselman et al., 2001; Giesemann et al., 2011). If only a single nerve is seen in the IAM, it is important to exclude an aberrant facial nerve as the nerve in the IAM could be the VCN (Fig. 1A–C). A single nerve carrying both auditory and facial fibres has also been reported (Thai-Van et al., 2000).

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Computed Tomography CT can be very helpful in delineating important abnormal bony landmarks such as a narrowed IAC or CNC or an aberrant facial nerve canal. The width of the CNC in particular has a high association with CND and the higher resolution of CT over MRI allows more precise measurements. CT can also aid surgical planning for transtympanic electrically evoked auditory brainstem response (ttEABR) testing or CI as it is the only modality that can identify bony abnormalities of outer and middle ears or mastoids.

Audiological assessment Once a deaf child has been diagnosed with (or suspected of having) CND based on imaging, further more detailed audiometric and electrophysiological tests are still essential. MRI cannot always differentiate CN aplasia from hypoplasia. Any child with complete CN aplasia cannot have auditory function, so both auditory brainstem responses (ABRs) and behavioural testing are used to confirm this. Where there is audition, then auditory nerve tissue must be present even if it cannot be seen on MRI and a CI may give some benefit (Bradley et al., 2008). Clearly, the absence of audition does not exclude the presence of auditory nerve tissue as deafness may occur at the cochlear level. Behavioural tests also help determine a child’s cognitive ability by determining whether they can condition to a tactile stimulus. This information is helpful for ABI assessment.

Transtympanic round window electricallyevoked auditory brainstem response In cases where MRI is equivocal or where MRI or audiological tests suggest a hypoplastic CN, ttEABR can help to quantify auditory nerve tissue (Kim et al., 2008; Warren et al., 2010). We have found that, using a biphasic current pulse stimulus with 200 μs per phase pulse widths, children who go on to perform well with a CI usually have a clear EABR waveform of two or three peaks and a response threshold of around 0.5 mA. In two children who underwent unilateral CI but subsequently had no benefit, the EABR response threshold was at 1.75 mA and waveform morphology was poor, suggesting that the threshold and morphology of the EABR can be a prognostic indicator for outcomes with a CI (Fig. 2).

Age A prelingually deafened child will lose auditory plasticity progressively during the first few years of life necessitating early implantation (Govaerts et al., 2003; Manrique et al., 1999). This is particularly important for an ABI, where outcomes are more variable. Currently we do not offer an ABI to children

Figure 2 Demonstrates the ttEABR for the right ear shown in Fig. 1. Stimuli consisted of biphasic current pulses with 200 μs per phase pulse widths and an amplitude of 1.75 mA. The EABR recorded is a single wave with a latency of 4.8 ms. No response was seen with smaller stimulation parameters. Based on the ABR and EABR responses he was implanted with a right cochlear implant but gained no benefit.

aged over 4 years. The lower limit for ABI surgery was considered to be 18 months of age at the European consensus meeting in order to reduce the risk of surgical complications (Sennoroglu et al., 2011).

Parental understanding and expectation Both CIs and ABIs have potential risks and require parental engagement in postoperative programming and rehabilitation in order to maximize benefit. In particular, an ABI has the possibility of extremely serious complications (see ABI surgery below) with variable and uncertain benefit and there is an intensive postoperative programme of appointments. It is therefore essential that parents can demonstrate understanding and a commitment to this programme.

Cognitive development of the child We only offer an ABI if, preoperatively, a child is developmentally able to carry out behavioural testing to a tactile or visual stimulus. This level of cognitive development is necessary to optimize the fitting of the ABI sound processor which requires feedback from the child in order to identify electrodes that elicit a hearing sensation. Outcomes to date would suggest that ABI benefit appears to be limited to little more than environmental detection in children with cognitive delay.

Cochlear implant outcomes in CND Following assessment, some children may be deemed suitable for CI. We aim to implant between 12 and 18 months of age, so that an ABI could still be carried out by 2 years if no benefit is seen. When a

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child has CND, outcomes from CI are unreliable and it is rare to get good outcomes (Buchman et al., 2011; Valero et al., 2012). Many studies have shown that children with CND frequently get no benefit at all with their CI, hence there is a need for a rigorous assessment process. For most children with CND who do gain benefit from a CI, this will be limited and they may require sign language either as their main mode of communication or to assist their hearing.

Issues surrounding ABI in children ABI programming difficulties Although the cochlear nucleus is frequency specific, the anatomy does not facilitate pitch ranking in the same way as the cochlea, so accurate speech programming is much more difficult. Adults who have learned speech prior to becoming deaf can give feedback to give an idea of the pitch given by different electrodes; even then, pitch ranking does not follow the expected course along the electrode. For children who have no concept of pitch perception, it is even more difficult to reproduce speech patterns. An additional problem is that of non-auditory stimulation, by which neighbouring neural tissue such as the vestibular nuclei or the facial or glossopharyngeal nerves are stimulated by the implant. This can produce uncomfortable sensations and some electrodes on the device will inevitably need to be turned off to reduce this. While adults are able to communicate the fact that they are getting non-auditory sensations, young children find this much harder. Programming the ABI sound processor is a challenging and time-consuming process for the child, the family, and the audiologist. There is a theoretical risk that the ABI could cause cardiac stimulation, although this has never been reported in any ABI patient. Nevertheless the initial programming is performed in a paediatric hospital setting with cardiac monitoring.

ABI risks Colletti et al. (2010) have suggested that the risks of ABI in non-tumour patients are similar to those of microvascular decompression and thus far this seems to be the case. This equates to a risk of serious intracranial complications occurring, such as meningitis, stroke, or bleeding, of approximately 1%. There is probably a similar risk to the surrounding nerves, in particular the facial, glossopharyngeal, and vagal. Temporary dysphagia can occur requiring nasogastric feeding for a period of days to weeks. The ABI is unlikely to last for the child’s lifetime and being able to replace it is uncertain. Very few revision procedures have been carried out on adult ABI patients although there are reports of it having been

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done successfully (Lenarz, 2001; Behr, 2007). As technology advances it may be that in the future there will be alternatives available.

ABI outcomes Outcomes with the ABI are notoriously variable in the adult population, although they may be better in nontumour patients (Nevison, 2002; Otto, 2002; Behr, 2007; Colletti, 2009). In adult patients undergoing ABI because of vestibular schwannoma removal in NF2, up to 20% will become non-users because so little benefit is gained. For most they will not be able to understand speech with the ABI alone but gain a significant improvement in communication when using lip reading in combination with the ABI. A small proportion achieve outcomes similar to a CI and are able to communicate with the ABI alone. In children with CND, evidence is only starting to emerge but the results may be fairly similar (Colletti and Zoccante, 2008; Eisenberg et al., 2008; Sennaroglu et al., 2009; Choi et al., 2011). We counsel parents that, both from our experience and the experience of other centres, a small proportion will get no benefit and the majority will continue to use sign language as their main form of communication but with some environmental discrimination and recognition of phonetics; development of speech intelligibility is likely to be limited. There are anecdotal reports of children around the world who are able to communicate with their ABI alone and have developed good speech. At the present time it is difficult to know if a child will achieve functional speech and language through their ABI; however, the likelihood of this good outcome is small.

Conclusion Hearing outcomes for children with CND are both variable and likely to be limited. With careful assessment it is sometimes possible to recognize those who will benefit from a CI but, if not, then an ABI may also be an option. Nevertheless, a proportion of children will get no benefit. Usually the implants would be expected to provide recognition of environmental sounds and understanding of simple phonetics. Most children will not develop normal speech and they will often need to learn to communicate with sign language. The issue of neural plasticity means that early diagnosis and referral are vital if children are to have the best chance of gaining benefit. Owing to the difficulties in programming and the limited outcomes, at present we only offer an ABI to children who are cognitively normal. The ABI involves a major neurosurgical procedure and at present we do not know what the long-term outcomes are. It is therefore essential that parents who are considering this intervention have plenty of time to

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consider all aspects and the opportunity for in depth discussion.

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The management of cochlear nerve deficiency.

The assessment process is critical in deciding whether a profoundly deaf child with cochlear nerve deficiency (CND) will be suitable for a cochlear or...
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