Greg R. Licameli and Frank W. Virgin
Permanent sensorineural hearing loss (SNHL) in the moderate-to-severe range is reported to occur in approximately 1 to 3 of every 1000 children born in the United States and other developed countries.1Hearing loss that is presumed to be late in onset and at least moderate in severity is diagnosed in 1.2 to 3.3 per 10,000 school-aged children.2 Approximately 50% of congenital hearing loss is associated with environmental factors such as infection, prematurity, or other perinatal insults. Genetic factors account for the other 50%, of which 15% are associated with syndromes and 35% are nonsyndromic with autosomal inheritance being the most common form.
The cost to society of severe-to-profound hearing loss is significant. These costs include the negative effects in productivity as well as direct costs such as education and spending to provide equal access to services. It is estimated that approximately 42% of individuals with severe-to-profound hearing loss between the ages of 18 and 44 years are unemployed compared with 18% of the general population.3Despite substantial expenditures in the education of severe-to-profoundly deaf children, 44% fail to graduate from high school as compared with 19% of normal-hearing population. Only 5% of severe-to-profoundly deaf children graduate from college as compared with 13% of the normal-hearing population. Much work has been done to evaluate the cost-effectiveness of cochlear implantation. Overall, including indirect costs such as reduced educational spending, cochlear implants provide a savings exceeding US$ 50,000 per child.4,5
Cochlear implants were first approved for use in adults in the United States by the Food and Drug Administration in 1984 and for children 2 years of age or older in 1990. Subsequently, the age was lowered to 1 year in 2002. With the benefits of cochlear implantation becoming established and experience gained in patient selection, device development, and programming, cochlear implantation is become an increasingly common method of treating children with profound SNHL.
Technology of Cochlear Implants
These devices function primarily by stimulating surviving neural elements within the inner ear and provide auditory stimulation that aids in the development and/or maintenance of the spoken language. There are several manufacturers of cochlear implants and each device has subtle differences, the details of which are beyond the scope of this chapter. The most commonly implanted systems include the cochlear implants systems manufactured by Cochlear Corporation, Advanced Bionics, and Medical Electronic (Med-El). Each of the devices currently available has several essential components. An external microphone picks up pressure differences in a sound field and converts it to electrical signals. An externally worn processor then processes the signal according to a predefined strategy and then transmits stimuli to the electrode array which is surgically implanted in the scala tympani of the cochlea.
The speech-coding strategy of an implant defines the method by which pitch, loudness, and timing of sound are translated into a series of electrical pulses. Each implant in the market today is capable of using a variety of algorithms but in general there are two major coding strategies. The first is called “simultaneous strategy” in which a device is capable of simultaneous stimulation. The second coding strategy is “continuous interleaved sampling,” which stimulates each electrode serially, with no stimulation out of order. Signal processing strategies such as feature extraction, spectral peak, and above combination encoder are included in this category. Programming of cochlear implants is an area of ongoing research and is largely responsible for the gains made in obtaining successful outcomes in implant patients.
Identification of appropriate cochlear implant candidates is critical to the overall success of implantation. In countries with universal newborn screening, the early identification of hearing loss has produced a significant increase in the number of implant candidates, making selection criteria increasingly more important. Papsin and Gordon reported that a child's duration of deafness; age at receipt of cochlear implantation; educational setting; form of communication; cognitive, motor, and social development; speech language development; family structure and support; intelligence; and socioeconomic status all play a critical role in the outcome for each implant recipient.1 Use of a preimplant questionnaire can be useful in the evaluation of a patient's candidacy and in predicting communication outcomes.6
Children undergoing evaluation for cochlear implantation fall into several broad categories of patients. The most common are children with prelingual deafness, usually congenitally acquired. A second group is of children with hearing loss, who have developed spoken language with the use of hearing aids, but have undergone deterioration of hearing over time to a point where hearing aids are no longer beneficial. Children with auditory neuropathy spectrum disorders make up the third category. The fourth category includes children with significant medical or developmental disorders in whom a lack of hearing may not be their greatest challenge. The final group of children is of those who are deaf and primarily use other nonverbal methods to communicate. Each of these groups presents its own set of challenges and highlights the need for a comprehensive team approach for implant selection.
Audiologic evaluation will consist of auditory brainstem response (ABR), otoacoustic emissions, and verification of hearing aid fitting. It is of benefit for the audiologist who will perform preoperative behavioral audiologic examination after surgery to become familiar with a child's response style. In addition to standard audiologic testing, there are other testing criteria for children based on their individual language achievement. Children who have not developed word-recognition ability can meet the audiologic criteria by showing a lack of progress or developmental lag on a scale such as the Infant-Toddler Meaningful Auditory Integration Scale or the Meaningful Auditory Integration Scale.7 Young children may meet the criteria with best-aided, word-recognition ability no greater than 20% for the Advanced Bionics and Med-El devices and no greater than 30% for the Cochlear Corporation device using the Multisyllabic Lexical Neighborhood Test or the Lexical Neighborhood Test.8 Older children who are able to repeat sentences completely, but have had progressive hearing loss can meet the criteria by using the adult criteria, which is no more than 60% best-aided word score on a sentence recognition test such as the Hearing in Noise Test.9 In the future there will be continued evolution of the audiologic evaluation and criteria for cochlear implantation.
Age of Implantation
There is clear evidence that implanting children early in life is advantageous.10 Language development begins at birth and develops rapidly during the early childhood years. Speech, vocabulary, and language skills are enhanced by early auditory stimulation. In 2001, Kileny et al published data which demonstrated that children who were implanted between the ages of 12 and 36 months outperformed children implanted between the ages of 37 and 60 months.11With the exception of patients with postmeningitic hearing loss where there is concern for cochlear ossification if surgery is delayed, implantation in the less than 12-month population is not universally performed.
Auditory neuropathy is a form of hearing impairment that is characterized by moderate-to-profound SNHL in which the function of the outer hair cells is preserved, but afferent neural activity in the cochlear nerve and central auditory pathways is disordered.12 These patients warrant a thorough evaluation including electrocochleography, ABR, and magnetic resonance imaging (MRI) in an attempt to identify the underlying pathological condition.13
Developmental disability coexists with hearing loss in 30 to 40% of cases.14 Cochlear implant centers see these children for evaluation in increasing numbers. This population of patients epitomizes the need for a comprehensive evaluation with a multidisciplinary team approach. Additionally, it is important to give realistic expectations of success to family and caregivers.
The medical evaluation should include a determination of the patient's ability to undergo a surgical procedure as well as radiologic evaluation to determine anatomical abnormalities that may preclude implantation or result in variation of normal surgical technique. Traditionally, high resolution computed tomography (HRCT) has been used for radiologic examination of patients being evaluated for cochlear implantation. However, recent investigations have assessed the usefulness of MRI in conjunction with HRCT or alone. HRCT provides a superior evaluation of the boney anatomy whereas MRI can provide improved evaluation of soft tissues including evaluation of the cochlear nerve and membranous portion of the cochlea. Currently, HRCT is the imaging modality of choice in all age groups, including children, to identify cochlear dysplasia, labyrinthine ossification, and other temporal bone anomalies associated with congenital hearing loss, which might contribute to intraoperative complications (Fig. 4.1).15–17Although MRI has the advantage of providing improved resolution of soft tissue structures, it has the disadvantage of higher cost and longer image acquisition times requiring general anesthesia in the pediatric population. However, it has been clearly shown that HRCT is not capable of demonstrating early cochlear obliteration nor is it capable of detecting cochlear nerve aplasia (Fig. 4.2).16 In 2007, Trimble et al assessed the usefulness of HRCT and MRI in the evaluation of cochlear implant candidates. The results of their work proposed a model that would use either imaging modality based on a patient's history. These authors concluded that selective use of each imaging modality would not miss any findings relevant to implantation and would result in cost improvements.18 Currently, the majority of centers use HRCT on all patients and MRI in the setting of postmeningitic hearing loss and in cases where there is concern for cochlear nerve aplasia.19
Figure 4.1 Examples of abnormalities that can be identified on a computed tomography scan. (A) Bilateral dysmorphic cochlea with absent modiolus, small and narrow basal turn and complete absence of separation of the middle and apical turns as well as stenosis of the cochlear aperture. (B) Severe stenosis of the cochlear aperture raising the question of cochlear nerve aplasia.
Figure 4.2 (A) T1-, fat-suppressed, gadolinium-enhanced magnetic resonance image demonstrating cochlear enhancement (arrow) in an infant with bacterial meningitis. (B) Computed tomography scan was normal in appearance.
The patient is placed under general anesthesia and is positioned with the operative ear up. Preoperative antibiotics are given based on institutional guidelines. Skin incisions for cochlear implants have evolved and vary widely. In our institution, a lazy-S incision is used and infiltrated with 0.5% lidocaine with 1:200,000 epinephrine to aid in hemostasis. The incision is planned so that it is no less than 1 cm from the edge of the implant at any given site. An anteriorly based periosteal flap is elevated to expose the mastoid and skull. A subperiosteal pocket is made to house both the device and ground electrode depending on the device to be implanted. Mastoidectomy is performed and the facial recess is identified and opened. It is essential to obtain a view of the stapes to provide perspective used in accurately identifying the round window niche. The superior portion of the round window niche is frequently drilled away to provide a clear view of the round window membrane.
Once the round window is identified, a well is developed to house the device and provide a channel for the electrode to travel to the mastoid. The well is carefully developed and in young children it is often necessary to expose dura to facilitate a good implant fit. It is important to ensure that there are no sharp edges and that the electrode can pass freely from the stimulator/receiver into the mastoid. Cochleostomy is generally performed in the anterior inferior portion of the round window to provide direct exposure of the scala tympani for electrode insertion. The size of the cochleostomy ranges from 1 to greater than 2 mm depending on the type of implant being inserted. Care is taken to keep the cochlea clear of blood and to avoid trauma to the basilar membrane from aggressive suctioning.
The implant is then brought into the field and placed in the well. The device may or may not be secured with sutures or mesh to the skull, depending on the surgeon's preference. The electrode is inserted into the scala tympani to the full extent atraumatically and in accordance with the manufacturer's specifications. Once the electrode is inserted, the extra length is coiled into the attic and attempts are made to keep the electrode out of the mastoid tip to avoid the theoretical risk of electrode migration as the patient grows and the mastoid tip enlarges. The periosteum is then closed over the electrode and implant, and the skin is closed ideally with offset suture lines. In our institution the integrity of the implant is tested intraoperatively by a member of the audiological team. Additionally, an anterior–posterior film of the skull may be taken to confirm placement before awaking the patient from anesthesia.
Cochlear implantation has become an established intervention in children with severe-to-profound SNHL. Clear benefit has been established and great strides have been made in predicting which candidates will succeed with cochlear implantation. The primary goal in cochlear implantation is the development of spoken language, however, children vary greatly in their performance. In general, the most successful patients are those with congenital hearing loss that are cognitively normal, who are implanted early, and are in an environment with motivated families, educators, and an environment rich in oral communication.
Multiple factors seem to influence the outcomes in cochlear implant recipients. Shorter duration of deafness before cochlear implantation has been demonstrated to be a positive predictor of better outcome.20Additionally, an implant recipient's method of communication and educational environment also influence outcome. When compared with patients who use total communication (oral and sign language), those who use oral communication only achieved higher in speech perception, speech intelligibility, verbal rehearsal skills, and literacy.21 Children with cognitive delay have been shown to demonstrate some forms of measurable postimplant gains in speech perception and word or sentence recognition, but these gains are slower than in typically developing children.22 Although auditory gains in the medically complex patient may be slower, spoken language as a primary mode of communication may be attainable in some patients and even if spoken language does not develop, speech production is not the only indication of benefit from cochlear implantation.6
Many children with cochlear implants achieve open-set speech recognition within the first year of implantation.23 In one long-term study, 67% of patients developed intelligible speech, 78% attended mainstream school, and 79% were able to use the telephone. On average, recipients had 72% open-set word recognition in quiet and 45% in noise. Implanted children have also demonstrated increased difficulty with receptive language when compared with their normal hearing peers with 75% scoring below the mean. These differences have been shown to persist into adolescence.24,25
There is growing group of patients who have received bilateral cochlear implants, either simultaneously or sequentially placed. Bilateral implant users have the advantage of spatial separation between target and competing speech.26–28 Additionally, it has been shown that bilaterally implanted children also demonstrate a significant increase in speech discrimination when compared with the best-performing unilateral implant.29,30
There have been significant advancements in cochlear implantation since the first devices were designed. In a recent study that evaluated the Advanced Bionics cochlear implant systems, one group of patients used the Clarion 1.2 system and the other group used the newer CII/HiRes 90K system. When the two groups were compared it was clearly demonstrated that the children who used the newer system developed better speech perception skills.31 Over the past two decades there have been advances in cochlear implant devices as well as processing and rehabilitation strategies. Currently, the first groups of children implanted are entering adulthood. Although clear benefits of cochlear implantation have been established, it is still to be determined how these effects translate into adulthood. It is anticipated that as device processing strategies and rehabilitation improves, outcomes for children receiving cochlear implants will continue to improve.
Cochlear implant programs have significantly evolved over the past 20 years and great strides have been made in the identification and rehabilitation of this unique group of patients. Significant socioeconomic gains have been demonstrated in the implanted patient, which has led to greater utilization of this technology over time. As the implant field moves forward, advances in candidate selection, rehabilitation, device production, programming strategies, and surgical technique will continue to provide robust areas of research and innovation.
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