9.1 Introduction

The surgical rehabilitation options for children with prelingual deafness include both the cochlear implant (CI) and the auditory brainstem implant (ABI). Continued studies are needed to assess the long-term benefit in auditory perception for prelingually deaf children fitted with these devices. ^{1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14}

Indeed, the population of children with no functional auditory nerve and who are not candidates for CIs constitutes a challenge. Their central auditory cortices may have never received input from the auditory periphery, and the remaining peripheral auditory system may be insufficient to support sound input from a prosthesis. Studies have shown encouraging results from children receiving an ABI. ^{15 , 16 , 17 , 18 , 19} This chapter discusses the outcomes obtained in a 74-child study group followed up to 15 years following ABI insertion. Additionally, we discuss our experience with bilateral ABI implantation in children, as well as cranioplasty techniques with resorbable mesh that may reduce the risk of cerebrospinal fluid (CSF) leak. We have learned through detailed review of our surgical video library that the nervus intermedius provides an important landmark leading to the foramen of Luschka (FL) and may assist in ABI placement. The use of near-field potentials in addition to traditional far-field evoked potentials has given us additional insight into improved positioning of the electrode paddles, and we have also gained some experience with revision ABI surgery in the case of device failure.

9.2 Total Experience and Selected Patient Study Group

From 2000 to 2014, 103 children (14 children with prior hearing and 89 children with congenital deafness), ranging in age from 8 months to 16 years, were implanted with ABIs via retrosigmoid approach, either Cochlear or MED-EL, at our institution or at other centers following our personal protocol. A thorough medical evaluation was performed before the decision for implantation, and patients were evaluated with computed tomography (CT) scan and magnetic resonance imaging (MRI). ²⁰ All parents were informed of the risks and potential benefits of the ABI and provided informed consent as approved by the local hospital human subjects review board. Intraoperative and postoperative electrically evoked auditory brainstem responses (EABRs) were performed in all children. ²¹

From the 103-patient surgical group, a 74-child study group implanted at our center for which full records were available included 57 cochlear nerve deficiency, 1 auditory neuropathy, 10 cochlear malformations, 3 bilateral cochlear post-meningitic ossification, and 2 neurofibromatosis type 2 and 1 bilateral cochlear fractures due to head injury. Among these children, 22 had been previously fitted elsewhere with CIs. The follow-up period ranged from 6 months to 15 years. Five children had less than 1 year of follow-up, 69 reached the 1-year postimplantation stage, 56 reached the 5-year follow-up stage, and 23 reached the 10-year follow-up mark, with 2 reaching 15 years of follow-up.

The assessment of auditory perception was performed in all children with the Categories of Auditory Perception (CAP) test, ^{22 , 23} an eight-point hierarchical scale of auditory performance.

9.3 Results

Seventy-four patients who underwent ABI at our institution were included in the study. The mean age was 3.8 ± 2.9 years. There were 40 males and 34 females. Clinical and demographical data of the study population are reported in Table 9.1. There were 38 patients with associated disabilities: 2 attention deficit hyperactivity disorders, 4 autistic spectrum disorders, 7 mild-moderate cognitive delays, 6 mild cognitive delays associated with motor deficits, 1 mild cognitive delay associated with visual impairment, 1 mild motor deficit, 2 oppositional defiant disorder, 2 specific language impairment, and 13 different syndromes (Crouzon, DiGeorge, Down, Goldenhar, Kabuki, lacrimo-auriculo-dento-digital [LADD], Moebius, Shprintzen, velocardiofacial). There were no intraoperative or perioperative permanent complications. One patient undergoing bilateral simultaneous ABI experienced pseudomeningocele formation, which was treated using revision surgery with fat grafting and resorbable cranioplasty plates, and is discussed later in this chapter.

The auditory performance is shown in Table 9.1, with CAP scores for the 74 children before implantation and at the last follow-up. All children showed improvement in auditory perception with implant experience. There was considerable variability in outcomes, and further analysis was undertaken to determine the causes. ABI outcome was analyzed as a function of the top score obtained, the age at implantation, the presence or absence of nonauditory disabilities, and etiology.

Fig. 9.1 shows the CAP scores of each child at the last follow-up. Among the nine children (12.1%) who ultimately were able to converse on the telephone (CAP level 7) the three postlingual children achieved this level by 3 years after ABI insertion, while the children with congenital deafness (CND without associated disabilities) achieved the same results later (at 6 years of follow-up). The 40% of the 10 children (13.5%) who achieved a CAP level of 6 achieved this level 3 years after ABI surgery. The 7 children (9.45%) who achieved the lowest open-set speech recognition CAP score of 5 took globally longer to achieve this level of performance—between 5 and 6 years. The 10 children (13.5%) who achieved closed-set discrimination of words (CAP level 4) took 4 to 6 years to achieve this score. A total of 26 children (35.1%) achieved some level of open-set speech recognition with the ABI (CAP levels 5, 6, and 7) and almost half of the children (36/74 = 48.6%) achieved CAP scores of 4 or better.

Age at implantation is well known to be a critical factor in the success of CIs. ^{1 , 2 , 3 , 4 , 14}

Fig. 9.2 shows the CAP score achieved as a function of the age at ABI surgery. Unsurprisingly, there is a trend for better performance in children implanted at a younger age (Kendall’s τ = –0.23, p < 0.01). This is particularly clear in children with no other disorders; many of these children implanted before 3 years of age were able to achieve a CAP score of 7 (p = 0.0088; Fig. 9.3).

Results in children with other congenital abnormalities (n = 38) and those without other complications (n = 29) were compared and the results are shown in Fig. 9.4. Children with no other disorders showed significantly higher CAP scores at the last follow-up (p < 0.001). Three years after ABI there was a three-category difference in the median CAP score between the two groups. The presence of additional disabilities was a significant predictor of the time to achieve CAP level 5 (p < 0.0001), regardless of whether the main deficit was of cochlear origin or nerve origin.

It is worth stressing that although children with other disabilities achieved low scores on the CAP, they still showed improved awareness of their environment and cognitive development. ^{7 , 8} In addition, while the median CAP score was only 2.5 for those children with additional disabilities, a few children in this category did obtain CAP scores of 4 or 5.

The effect of etiology of deafness was also reviewed. The 74 children were divided into five etiology groups: postlingual deafness due to trauma or severe ossification (n = 4), congenital deafness due to cochlear nerve aplasia (n = 25), cochlear malformations (n = 10), cochlear nerve aplasia with other nonauditory disabilities (n = 32), and NF2 and auditory neuropathy (n = 3). Children in the last category were considerably older at the time of ABI surgery (mean age 12.4 years) than children in other categories.

Fig. 9.5 shows the CAP score for each etiology group as a function of years of ABI use. While there appear to be clear differences in the median CAP scores between etiology groups, the top and bottom curves have too few subjects to achieve statistical significance, and the middle three curves did not achieve significant differences due to the high variability in performance within each etiology group. The four children who had prior hearing (three cochlear ossification, one trauma) clearly had the best outcomes, increasing in performance rapidly over the first 3 years and ultimately reaching the highest CAP level.

9.4 Discussion

In this study group, within 1 year of activation 83.8% of the children had obtained awareness of environmental sounds and 50% responded to speech sounds. Within 2 years of activation, 51.4% of children were able to identify environmental sounds and discriminate speech sounds (CAP levels 3 and 4). Of the 64 children with 3-year follow-up data, 28.1% were able to understand common phrases without the aid of lipreading and 12.5% of the children could use the telephone with a known speaker.

This study confirms previous findings that the ABI is an appropriate device for auditory rehabilitation in children with cochlear and cochlear nerve malfunctions that cannot benefit from CIs. A comparison of the ABI outcomes obtained from this series of children versus a large group of children fitted with CIs clearly shows better performance outcomes obtained in a shorter time period in the CI group. ²⁴ However, when CI results are compared with ABI children who have heard before, then performance is comparable and the developmental trajectory is comparable. In addition, when ABI performance is compared with congenitally deaf children who received a CI at the same age, then performance levels and trajectory over time are similar. ²⁵ This group of ABI children had previous hearing but lost their auditory nerve from head trauma or severe ossification following meningitis. This result suggests that the ABI could be considered as salvage option for patients with progressive ossification.

Considering the cohort of children with cochlear nerve aplasia or hypoplasia who had a CI first, none showed satisfactory auditory development with a CI. In this cohort of children, the time spent trying out the CI was not only time (and expense) wasted, but also it may have prolonged the period of auditory deprivation. It is today well known that in the absence of auditory stimulation, neural structures show a failure to mature and can degenerate, ^{26 , 27} and, in addition, auditory cortical areas can be reallocated to other modalities. ^{28 , 29}

In light of this, when should a trial with a CI be skipped and move directly to an ABI? And second, under what conditions can one be confident that a CI will not provide useful hearing? Recent outcomes in children with CI have clear indicated etiologies where CI results can be poor. ^{6 , 12} In cases where no auditory nerve is visible on high-resolution MRI of the internal auditory meatus and when the EABR evoked by the CI is distorted or absent, auditory results were very poor. In such cases, an ABI may provide better performance than a CI. We recommend high-resolution CT imaging of the internal auditory meatus ^{20 , 30} and EABRs, stimulated either through an existing CI or from a wick electrode on the round window.

Today, there is compelling evidence that outcomes are better when CIs are provided as young as possible ^{1 , 2 , 3 , 4 , 14} and that it is of critical importance to have auditory input during the period of greatest neural plasticity in order to develop speech perception. Children who receive CIs below the age of 1 have clearly better and more rapid auditory development than children who receive CIs between 1 and 2 years of age. ³¹ If a CI is tried initially, clinicians must remain vigilant for the early signs of CI efficacy. If no progress is being made on simple auditory tasks, it may be necessary to move to an ABI as soon as possible to make the best use of that early neural plasticity. It is necessary to explant the CI, re-evaluate the child with neuroimaging studies and perform ABI surgery as soon as possible after the lack of progress with a CI has become evident. Children previously fitted with CIs and subsequently with ABIs may demonstrate a slower development of auditory perception, possibly because of the major difference in the neural pattern of activation from the two devices and possibly because the time window of plasticity has partially closed.

A topic of further concern is why some children fitted with the ABI can detect and discriminate environmental sounds but do not develop speech perception and language. Several of the following conditions may responsible for the poor or low progression in speech perception abilities: incorrect positioning of the ABI array, incomplete development of the cochlear nuclei and auditory areas undetected by MRI, programming difficulties, or other negative psychological and cognitive factors. Most of the children with associated psychological and cognitive deficits could perceive the sounds and discriminate some speech patterns only a few months after ABI fitting. However, their overall auditory perceptual development has been very slow, and they continue to have trouble translating the electrical stimulation into speech and language development. Even if children are not able to achieve open-set speech recognition with the ABI, they may receive cognitive benefits. Access to auditory information from the ABI has been demonstrated ^{7 , 8} to influence the development of specific cognitive functions. Scores on two tests evaluating cognitive function (form completion and repeated patterns) increased significantly during the first 12 months of ABI use. These data demonstrated that the auditory stimulation of the ABI in preverbal children may facilitate the development of cognitive parameters related to selective visual-spatial attention and fluid (multisensory) reasoning.

The prevalence of surgical complications observed in the present group of 74 children fitted with ABI is comparable with what can be observed in children fitted with CI. ³² Clearly, the potential for complications is greater for an ABI than for a CI.