In 1990, the Food and Drug Administration (FDA) first gave approval for cochlear implantation in children aged 2 to 18 years. Initially, children who received a cochlear implant (CI) had total profound deafness, and most were older than 5 years of age. Early speech perception results demonstrated that congenitally or prelingually deafened children with a CI displayed substantial closed-set abilities (e.g., wherein children identify a word by selecting from a limited set of response alternatives), but only minimal open-set spoken word recognition abilities (i.e., in which no response alternatives are provided).1, 2 Since then, as cochlear implantation has been extended clinically to younger children, and with continued improvements in electrode design and signal processing,^3–6 pediatric CI recipients have achieved much higher levels of open-set word recognition.^7–13 For example, Eisenberg and colleagues¹⁴ reported mean Phonetically Balanced Kindergarten word lists (PB-K) scores of approximately 50% words correct for oral pediatric CI users. Open-set word recognition is an important diagnostic yardstick for determining cochlear implant success because it indicates that these children have established neural representations of words in their long-term lexical memory, a process that is fundamental to the development of spoken language.¹⁵ Although these average results are very encouraging and clearly establish the efficacy of CIs, individual patients vary greatly in outcome.^{1–3, 13, 16–21} Some children can communicate extremely well using the auditory/oral modality and acquire age-appropriate language skills, whereas other children display only minimal spoken word recognition skills or demonstrate severe language delays, or both.^22–27 Accounting for this enormous variability in the effectiveness of CIs on a wide range of outcome measures presents the most serious challenge facing cochlear implant clinicians and researchers today. Gaining an understanding of the nature of the individual differences and sources of variability in cochlear implant outcomes is crucial for predicting individual benefits before implantation and for selecting appropriate intervention strategies after implantation.

Despite the variability in individual outcomes, cochlear implantation is no longer questioned as a therapeutic option for children with prelingual deafness. However, in part because the outcomes are not guaranteed, controversy exists regarding the appropriate expansion of evolving technology into new patient populations. The current trend toward earlier implantation and the implantation of children with more residual hearing mandates careful documentation of performance limits with cochlear implants as well as with nonsurgical alternatives (e.g., hearing aids). Only through rigorous longitudinal studies will these issues be clarified. This chapter reviews current implant technology, patient selection criteria, and performance results for pediatric cochlear implant recipients and considers the challenges inherent in the broadening of cochlear implant candidacy.

Background

PEDIATRIC COCHLEAR IMPLANT SELECTION CRITERIA

Current selection criteria for pediatric cochlear implantation include the following:

≥12 months of age

Severe to profound bilateral sensorineural hearing loss (SNHL)

Minimal benefit from hearing aids

No medical contraindications

High motivation and appropriate expectations

Enrollment in a program that emphasizes development of auditory skills

COCHLEAR IMPLANT SYSTEMS

The cochlear implant devices available for implantation, as well as the speech processing strategies used, continue to undergo technologic improvements. Currently, three types of multichannel, multielectrode cochlear implant devices are commercially available for children in the United States. These devices have several characteristics in common. All have an electrode array that is surgically implanted into the cochlea and an external unit, consisting of a microphone that picks up sound energy and converts it to an electric signal, and a signal processor that modifies the signal, depending on the processing scheme in use. The processed signal is amplified and compressed to match the narrow electrical dynamic range of the ear. (The typical response range of the ear to electrical stimulation is on the order of only 10 to 20 dB, and even less in the high frequencies.) Transmission of the electrical signal across the skin from the external unit to the implanted electrode array is most commonly accomplished by the use of electromagnetic induction or radiofrequency transmission. The neural elements stimulated appear to be the spiral ganglion cells or axons. These devices use place coding to transfer frequency information in addition to providing temporal and amplitude information.

The Nucleus (Cochlear Corporation, Englewood, CA) family of cochlear implant systems (the 22-channel and 24-channel devices) are currently the most commonly used multichannel system. The Nucleus implantable electrode array consists of platinum-iridium band electrodes placed in a silastic carrier.28 Several generations of speech processors have been employed with the Nucleus multichannel cochlear implant. The initial Nucleus speech processors used a feature-extraction scheme in which selected key features of speech were presented through the implanted electrode array. An early speech processing strategy, the F0F1F2 strategy, primarily conveyed vowel information, including the first and second formant frequencies and their amplitudes, as well as voice pitch. A later coding scheme, the MULTIPEAK strategy, presented these acoustic features along with additional information from three high-frequency spectral bands to aid in consonant perception. One current Nucleus speech processing strategy is the Spectral Peak (SPEAK) strategy. This strategy uses a vocoder in which a filterbank consisting of 20 filters covering the center frequencies from 200 to 10,000 Hz is employed. Each filter is allocated to an active electrode in the array. The filter outputs are scanned and the electrodes that are stimulated represent filters that contain speech components with the highest amplitude. Depending on the acoustic input, the number of spectral maxima detected, and thus the number of electrodes stimulated, on each scan cycle can vary from one to ten, with an average of six per cycle. The rate at which the electrodes are stimulated varies adaptively at 180 to 300 pulses per second.

The Clarion multichannel cochlear implant (Advanced Bionics, Sylmar, CA) has an eight-channel electrode array that uses a radial bipolar configuration through electrode pairs positioned adjacent to the osseous spiral lamina in a 90-degree orientation.²⁹ The Clarion multichannel cochlear implant offers two types of speech-processing strategies: simultaneous analog stimulation (SAS) and continuous interleaved sampling (CIS). Both strategies represent the waveform or envelope of the speech signal.³⁰ The Clarion SAS strategy first compresses the analog signal into the restricted range for electrically evoked hearing and then filters the signal into a maximum of eight channels for presentation to the corresponding electrodes. Speech information is conveyed via the relative amplitudes and the temporal details contained in each channel. The CIS strategy filters the incoming speech into eight bands, obtains the speech envelope, and compresses the signal for each channel. Stimulation consists of interleaved digital pulses that sweep rapidly through the channels at a rate of 833 pulses per second when using all eight channels for a maximum pulse rate of 6664 pulses per second (8×833=6,664). With the CIS strategy, rapid changes in the speech signal are tracked by rapid variations in pulse amplitude. The pulses are delivered to consecutive channels in sequence to avoid channel interaction.

The MED-EL COMBI 40-Cochlear Implant system (Medical Electronics, Innsbruck, Austria) uses the CIS (continuous interleaved sampling) strategy, which provides both spectral and temporal resolution. Up to eight active electrodes can be used. The electrode array used has the capability of deep insertion into the apical regions of the cochlea.³¹ The MED-EL has the capacity to provide the most rapid stimulation rate of any of the currently available implants (maximum of 12,000 biphasic pulses per second).³²

SURGICAL IMPLANTATION

Cochlear implantation in children requires meticulous attention to the delicate tissues and small dimensions. Skin incisions are designed to provide access to the mastoid process and coverage of the external portion of the implant package while preserving the blood supply of the postauricular skin. The incision employed at the Indiana University Medical Center has eliminated the need to develop a large postauricular flap. The inferior extent of the incision is made well posterior to the mastoid tip to preserve the branches of the postauricular artery. From here the incision is directed posterosuperiorly and is then directed superiorly without an superior anterior limb. In children, the incision incorporates the temporalis muscle to give added thickness. A subperiosteal pocket is created for positioning the implant induction coil. A bone well tailored to the device being implanted is created, and the induction coil is fixed to the cortex with a fixation suture or periosteal flaps.

After the development of the skin incision, a mastoidectomy is performed. The horizontal semicircular canal is identified in the depths of the mastoid antrum, and the short process of the incus is identified in the fossa incudis. The facial recess is opened using the fossa incudis as an initial landmark. The facial recess is a triangular area bounded by (1) the fossa incudis superiorly, (2) the chorda tympani nerve laterally and anteriorly, and (3) the facial nerve medially and posteriorly. The facial nerve can usually be visualized through the bone without exposing it. The round window niche is visualized through the facial recess approximately 2 mm inferior to the stapes. Occasionally, the round window niche is posteriorly positioned and is not well visualized through the facial recess or is obscured by ossification. Particularly in these situations, it is important not to be misdirected by hypotympanic air cells. Entry into the scala tympani is best accomplished through a cochleostomy created anterior and inferior to the annulus of the round window membrane. A small fenestra slightly larger than the electrode to be implanted (usually 0.5 mm) is developed. A small diamond burr is used to “blue line” the endosteum of the scala tympani, and the endosteal membrane is removed with small picks. This approach bypasses the hook area of the scala tympani allowing direct insertion of the active electrode array. After insertion of the active electrode array, the round window area is sealed with small pieces of fascia.

SPECIAL SURGICAL CONSIDERATIONS

In cases of cochlear dysplasia, a cerebrospinal fluid (CSF) gusher may be encountered. The senior author prefers to enter the cochlea through a small fenestra and tightly pack the electrode at the cochleostomy with fascia. The flow of CSF has been successfully controlled in this way. In patients with severe malformations of the labyrinth, the facial nerve may follow an aberrant course. In these cases, the most direct access to a common cavity deformity may be by a transmastoid labyrinthotomy approach. The otic capsule is opened posterosuperior to the second genu of the facial nerve, and the common cavity is entered directly. Four patients have been treated in this way with no vestibular side effects.33

In cases of cochlear ossification, our preference is to drill open the basal turn and create a tunnel approximately 6 mm in length and partially insert a Nucleus electrode. This approach permits implantation of 10 to 12 active electrodes, yielding very satisfactory results. Gantz et al.³⁴ described an extensive drill-out procedure to gain access to the upper basal turn. The benefits of this extended procedure are under investigation. Steenerson et al.³⁵ described the insertion of the active electrode into the scala vestibuli in cases of cochlear ossification. This procedure has merit. However, the scala vestibuli is frequently ossified when the scala tympani is completely obliterated.

RESULTS OF COCHLEAR IMPLANTATION IN CHILDREN

Nucleus Cochlear Implant Systems

Pediatric clinical trials with the Nucleus 22-channel cochlear implant began in 1986, and in 1990 the FDA approved this device for use in children. The children originally implanted with the Nucleus 22-channel system used the F0F1F2 feature extraction speech-processing strategy. Children implanted after 1989 were provided with the Multipeak (MPEAK) strategy, and the Spectral Peak (SPEAK) strategy was approved in 1994. Pediatric clinical trials for the Nucleus 24-channel device with the SPEAK strategy were initiated in April 1997, and FDA approval was granted in June 1998.

One of the first large-scale reports of pediatric performance with the Nucleus cochlear implant was presented by Staller et al.²

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