Bilateral cochlear implantation in children

1. Introduction 

For a deaf child a cochlear implant may give new chances for the speech and language development through audition. In general, in educational settings, the listening environments are fare from optimal. In many cases, room acoustics and the noise generated by the children in a classroom disturb speech perception for the hearing impaired children.

With cochlear implants, it is also known that the speech analyses in the cochlear implant processors are extremely sensitive for a bad signal to noise ratio such as in classrooms.

In general binaural fitting of hearing devices improve directional hearing and listening abilities in less optimal signal to noise ratio conditions (S/N). Two mechanisms in this process have to be mentioned: the head shadow effect and the binaural squelch effect. The head shadow effect is caused by the physical barrier of the head for noises presented to the contralateral side. This creates an attenuation in the high frequencies [1]. The binaural squelch effect sometimes can be hardly separated from the head shadow effect. With input from two sides, the brain might have a better representation of the noise and speech and separate them internally, improving the intelligibility [2].

It is expected that a binaural fitting with cochlear implants may facilitate the speech perception in bad signal to noise ratios too [3]. Bilateral cochlear implantation is currently still an uncommon management option for patients with profound deafness. Publications on bilateral cochlear implantation are rare, especially in children [4], [5]. The availability of the BTE type speech processors, although, make that also for children these potential benefits of a bilateral cochlear implantation are realistic.

This case describes the initial effects of bilateral cochlear implants in a young child. The profound hearing loss in this child is caused by hyperbilirubinemia. It is known that the auditory system is highly sensitive to bilirubin toxicity [6]. In literature damage to the auditory nervous system including auditory neuropathy and auditory processing problems are reported. Auditory dysfunction may occur in children with or without other signs of classical kernicterus [7]. The brain stem, including the auditory pathways, is particularly vulnerable to elevated bilirubin while the auditory periphery may be less so [8]. The characteristics of auditory neuropathy are observed on clinical audiologic tests as normal otoacoustic emissions (OAEs) in the presence of an absent or severely abnormal auditory brainstem response (ABR) [9]. Cochlear implantation in this population of patients is controversial. Reports can be found showing significant benefits [10].

Because of the hyperbilirubinemia and the profound deafness after exchange transfusion in this child, it is likely that the nucleus cochlearis is affected and auditory neuropathy results.

Ten months after the initial fitting of the second implant, a comparison has been made between the two implants separately as well as the binaural condition. Aided thresholds, speech identification and speech recognition are measured for the three conditions. The speech and language development and the observations about daily life situations made by the parents and teachers are presented.

2. Methods 

The subject of this initial report is a 5.2-year-old profoundly deaf girl with a bilateral cochlear implantation. The girl has a congenital bilateral hearing loss with a PTA (average of 0.5, 1, 2, 4 kHz) of more than 110 dB HL. Hearing aids were provided at an age of 1.2 years. Because no benefits of the hearing aids could be obtained, she received a Nucleus CI24 implant on the right side at an age of 2.5 years. At an age of 4.4 years she received an additional implant, a Nucleus CI 24-CONTOUR at her left side. Both implants make use of the SPEAK speech coding strategy.

The child goes to a mainstream school and receives speech and language therapy in a special speech and hearing rehabilitation centre.

Nine months after the second implantation, aided thresholds, speech identification and speech recognition are measured for three conditions. The conditions are: the first implant only, the second implant only and the binaural condition.

For measuring aided thresholds, warble tones generated by a conventional clinical audiometer were presented through a loudspeaker in a sound proof booth. The thresholds are measured in dBA.

For measuring the speech identification, the closed set sentence identification of the translated Tyler-Holstad Speech Perception Test for Hearing impaired Children [11] has been used. This test is developed for children aged 4–5 years and above. It's a closed set test. The test has four levels of difficulty. The highest level should be used when possible. In this case the highest level (4×4 matrix) was used. This means that the child must choose between four items. Each sentence is presented once and the child has to respond. The test is scored for each word identified correctly by the child. The chance level is 25%.

Speech recognition is measured with the Language-Specific Sentences (EARS, [12]) adapted for Flamish [13]. This test is developed for children of 5 years and older. Each sentence is presented once. The test is scored for each word and for each sentence repeated correctly by the child.

Language development was assessed by means of the Taaltest voor Kinderen (TvK), a standardised language development battery for Dutch speaking children [14].

3. Results 

The aided thresholds are presented in Fig. 1. In the first implant condition the child has an aided PTA of 28 dBA. In the second implant condition the aided PTA is 22 dBA and in the binaural condition the aided PTA is 23 dBA.

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Fig. 1. Aided thresholds in the monaural and binaural conditions.

Fig. 2 shows the results for the speech identification and recognition test. All the tests were performed in both the monaural and binaural conditions.

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Fig. 2. Speech identification (translated Tyler-Holstad Speech Perception Test for Hearing impaired Children) and recognition scores (Language Specific Sentences) for the monaural and binaural conditions.

Table 1 shows the results of speech and language development based on the TvK. Age at the time of testing was 4.8 years.

Table 1. TvK results assessed while wearing both implants

	Testitem	Age equivalent
	Morphology-active	4.5 years
	Lexicon-active	4.4 years
	Lexicon-passive	5.0 years
	Syntax-passive	4.10 years

During therapy, the speech and language therapist report a better speech production in the binaural condition. At school, the teacher as well as the mother report a better sense of rhythm and in daily listening situations lip-reading doesn't seem needed anymore. Since the bilateral implantation the child responds appropriately, even when the speaker stands behind the child.

4. Discussion and conclusions 

For frequencies above 1 kHz, the aided thresholds are well balanced. Differences between the individual implant thresholds are never more than 5 dBA. This is of importance for the potential directional hearing abilities as well as for the potential binaural interactions.

Due to ceiling effects the results of the closed set speech identification test are identical for the individual implants only and the bilateral listening situation. Differences can be found with the more demanding speech recognition tests. The speech recognition scores obtained with the second implant only are better than the results obtained with the first implant. Probably the slightly better aided thresholds for the second implant play a role in this better performance.The binaural listening situations are superior in the speech recognition tasks. It is clear that the binaural fitting facilitates the speech perception significantly.

These results are in line with the observations of the parents, the teacher and the speech and hearing therapist. Compared with the monaural listening situations the binaural listening results in a better perception of rhythm, a better speech production and makes speech perception and understanding possible without the need of additional visual information such as lip-reading.

At this time the speech and language development show no delays compared with normal hearing children. This is remarkable because of the congenital profound deafness of this child. The implantation of the first implant at an age of 2.5 years as well as the significant speech perception facilitating binaural fitting must have contributed to this result.

In addition, this report shows that a child with a profound hearing loss caused by hyperbilirubinemia can have significant benefits from a cochlear implant. Nevertheless, some authors have had reservations regarding cochlear implantation in cases of auditory neuropathy [15]. The outcomes for this child, are in contrast to the findings of these authors. Shallop et al. [10] also reports positive outcomes in five cases of auditory neuropathy who received cochlear implants. It is known that, in cases of auditory neuropathy, temporal processing abilities are disrupted. Auditory neuropathy disrupts the synchronous activity of the auditory nerve without affecting the function of the outer hair cells. They find that the preoperative dyschronous auditory brainstem neural potentials which characterize auditory neuropathy, are restored to some degree after cochlear implantation.