Hearing Loss and Cochlear Implantation in Children
Hearing loss is the most common congenital sensory impairment, with an incidence of 2 to 3 per 1000 live births. Approximately half of these newborns have severe-profound loss. Hearing loss can significantly hinder a child’s expressive and receptive communication and, as a result, be detrimental to cognitive, educational, and psychosocial development. Fortunately, with the advent of universal newborn hearing screening, which is now mandated in most states, hearing loss is usually discovered very early in life. This screening provides the opportunity to intervene in the nascent stages of childhood development. Early intervention has far-reaching impact not only in terms of furthering a child’s communication abilities, but also in its potential to change the direction of a child’s life.
Cochlear implantation has become a very successful treatment option for some children with severe-profound hearing loss. Over the past several decades, significant advances in implant technology, coupled with increased understanding of auditory development, have enabled many hearing impaired children to successfully become a part of the “hearing world.”
Types of hearing impairment
To understand the role of cochlear implantation, the different types of hearing impairment must first be understood. Hearing loss is primarily characterized as conductive or sensorineural in nature. Mixed hearing loss is a combination of the two. Another category of hearing impairment is auditory neuropathy spectrum disorder (ANSD).
Conductive hearing loss occurs when sound transmission is physically blocked between the opening of the external ear and the cochlear hair (receptor) cells. The most common cause of conductive hearing loss in children is otitis media and related conditions, such as middle ear effusion and Eustachian tube dysfunction. Conductive loss has numerous other causes, including cerumen impaction, tympanic membrane perforation, and ossicular fixation or disruption. In a conductive loss, no dysfunction of the cochlear hair cells or auditory nerve is present. Cochlear implantation is not used to treat conductive hearing losses.
A sensorineural hearing loss (SNHL) is caused by a neural defect in the transmission of sound at the level of the cochlea, which is where the receptor cells for sound, known as hair cells, are found. SNHL may be congenital or acquired, and both congenital and acquired losses may be hereditary (genetic) or nonhereditary in nature. Approximately 30% of hereditary hearing loss is considered syndromic. More than 400 genetic syndromes that include hearing loss have been described. The other 70% are nonsyndromic, or not associated with any visible abnormalities or medical problems. Well-recognized risk factors for SNHL in infants and children include family history of childhood SNHL, birth weight less than 1500 g, low Apgar scores, craniofacial anomalies, hypoxia, in utero infections (eg, TORCH), and history of mechanical ventilation for more than 5 days.
SNHL is also characterized as being prelingual or postlingual. Prelingual hearing loss is present before speech develops and is very often congenital; postlingual hearing loss occurs after the development of normal speech.
Auditory neuropathy was first recognized as an entity in the 1990s, and was renamed ANSD in 2008 [1]. It is diagnosed when there is a discrepancy between cochlear and neural function. In ANSD, the outer hair cells function normally, but sound is not properly transmitted from the outer hair cells to the auditory cortex. This dysfunction is thought to be caused by desynchronized action potentials in the auditory nerve. Presentation varies widely. Children with ANSD may have hearing thresholds that range from normal to profound levels on audiometric testing; however, they usually exhibit disproportionately poor speech perception abilities for their degree of hearing loss. Some patients with ANSD have only very mild impairment, whereas others present as functionally deaf. Some have impairment that fluctuates in severity. Children with ANSD typically have difficulty hearing in noise. ANSD children may hear words but have difficulty understanding what is being said.
ANSD may not be detected on newborn hearing screening if otoacoustic emissions (OAE) testing is used, because this modality only tests the function of the cochlear outer hair cells, and not the auditory nerve. Auditory brainstem response (ABR) is either absent or grossly abnormal on testing. ANSD accounts for a relatively small percentage of hearing impairment. Risk factors include prematurity, severe jaundice, low birth weight, and hypoxia. It is genetic in some cases. Some children with ANSD actually have deficient or absent cochlear nerves, whereas others have global neuropathies, such as seen in Charcot-Marie-Tooth disease or Friedreich ataxia.
Cochlear implantation is an option for some children with severe-profound sensorineural hearing loss or significant impairment caused by ANSD.
The cochlear implant
The U.S. Food and Drug Administration (FDA) approved the modern, multichannel cochlear implant in 1985 for use in adults. The FDA approved the device in 1990 for children down to the age of 2 years, and in 2002 down to the age of 12 months.
A cochlear implant is not a hearing aid. Hearing aids merely amplify sound. A cochlear implant bypasses the cochlear hair cells and directly electrically stimulates the auditory nerve. It does not restore normal hearing, but rather allows for the perception of sound sensation.
The device has internal and external parts (Figs. 1 and 2). The external parts include a microphone, speech processor, and transmitter. The microphone picks up sounds from the environment. The speech processor digitizes the sounds and then these signals are sent via the transmitter to the internal portion of the device, which is surgically implanted. The receiver, which is under the scalp, picks up the signals and delivers them to the electrode array that is inserted into the cochlea. The electrodes along the array directly stimulate the auditory nerve. The cochlea is tonotopically organized: high frequencies are picked up at the base of the cochlea, and low frequencies are detected at the apex of the snail-shaped organ.
Fig. 1 This diagram shows how the internal and external portions of a cochlear implant are positioned. The microphone, which is worn on the ear, picks up sounds which are then digitized by the speech processor. The transmitter sends these signals through the scalp to the receiver, and the impulses are sent into the electrode array which directly stimulates the auditory nerve.
(Reprinted from Papsin BC, Gordon KA. Cochlear implants for children with severe-to-profound hearing loss. N Engl J Med 2007;357:2380–7; with permission.)
Fig. 2 The internal portion of a cochlear implant. The receiver is surgically implanted under the scalp, and has a ground electrode and a curved electrode array which is placed in the cochlea.
(Photo provided Courtesy of Cochlear Americas, 2009 Cochlear Americas; with permission.)
Currently available cochlear implants have up to 24 electrodes along the array. The speech processor contains software that transforms sounds into electrical signals. The software contains speech-encoding algorithms designed to optimize the conveyance of speech information. Different manufacturers use different processing strategies and no one strategy has consistently produced better overall results than the others. As of this writing, there are three major manufacturers in the world.
Candidacy for implantation
Before any discussion regarding candidacy for implantation, it is important to note that not all people desire cochlear implants or wish to be a part of the “hearing world.” Some view deafness not as a disability, but rather a different human experience. For this population, Deafness is a cultural identity, much like an ethnic identity, and the capital “D” is used to signify use of the term as a cultural label. American Sign Language (ASL) forms the linguistic basis for their identity. The Deaf population prides itself on being a close-knit, supportive community, with its own language and rich heritage. Thus, cochlear implantation, and the view that hearing-impaired children must be brought into an oral-hearing world, are antithetical to the Deaf identity, and have been the topic of many contentious debates. Both sides of this debate share the common goal of promoting language development in and bettering the life of deaf children. Both sides have pros and cons.
The decision of whether to implant a child is pivotal, and one of lifelong consequence, because it can profoundly influence the future direction of a child’s life. It is a complex decision based on many factors in addition to hearing status, such as available educational options and support systems. The surgical placement of the device alone does not result in improved hearing and, in fact, is only the beginning of a long and involved learning process. Success depends on motivation and a long-term commitment to therapy and education. For this reason, in many centers a team approach is used to evaluate candidates, with the involvement of otolaryngologists, audiologists, speech–language pathologists, social workers, psychologists, and educational specialists.
From a medical standpoint, a candidate must be healthy enough to withstand general anesthesia and surgery, and must have a cochlea and an auditory nerve. Rarely, deafness is the result of an absent auditory nerve or cochlear agenesis. Other cochlear and inner ear malformations are not usually contraindications to surgery but must be evaluated on a case-by-case basis.
Obtaining an accurate assessment of an infant or child’s hearing status is necessary to determine candidacy. Previously, the belief was that only children with bilateral profound (pure-tone averages >90 dB) sensorineural hearing loss, receiving little to no benefit from hearing aids, should be considered for implantation. However, as understanding of auditory development and outcomes has evolved, the criteria have broadened to include patients with greater degrees of residual hearing. The trend has been toward a more functional assessment of a candidate’s hearing, with a focus on language acquisition abilities, instead of reliance on pure-tone averages alone.
All cochlear implant candidates should receive a trial of binaural amplification, fit appropriately, according to American Academy of Audiology pediatric hearing aid fitting guidelines. A 3- to 6-month trial of hearing aid use, in conjunction with intensive therapy, is typically necessary to evaluate benefit. To determine whether the hearing aids are providing enough useful auditory input to allow for communication and language acquisition, open-set aided speech recognition testing is performed in older children. In younger children, closed-set speech recognition tests may provide useful information. The FDA criteria vary slightly for different manufacturers’ devices, but speech discrimination scores of 50% to 60% or below with optimal amplification are currently required for candidacy. Receiving benefit from amplification no longer disqualifies one from receiving a cochlear implant, because most patients who are able to recognize approximately 50% of words using hearing aids have now been shown to generally have even higher speech recognition scores postimplantation, compared with their best preoperative scores [2,3]. For children with asymmetric hearing loss, the literature supports continued use of a hearing aid in the better-hearing, nonimplanted ear, with implantation of the worse-hearing ear. Greater residual hearing at implantation and shorter duration of deafness seem to be associated with improved language outcomes; conversely, poorer outcomes seem to be associated with prolonged use of hearing aids before implantation [4,5].
For infants and very young children who cannot undergo speech perception testing, the assessment relies heavily on a team approach. Audiometric testing and appropriately fit hearing aids are the first requirement, and then evidence of benefit, or lack thereof, is based on regular audiologic evaluations, results of outcome questionnaires, and reports from parents and early intervention providers. A speech-language pathologist who is familiar with this population works collaboratively with the implant audiologist to assess progress in auditory skills. Because the evaluation of infants is not straightforward, a hearing aid trial of at least 6 months, with appropriate early intervention and speech-language services, is usually desired before making a decision regarding implantation.
A situation in which the decision to implant is often expedited is when deafness occurs secondary to bacterial meningitis, particularly if Streptococcus pneumoniae is the pathogen. In these cases, because of the powerful inflammatory response triggered by the pneumococcal cell wall, rapid fibrosis and ossification of the cochlea may occur, often beginning within several weeks of the infection [6]. Ossification can lead to a more difficult surgery, incomplete electrode insertion, and poorer outcomes. For this reason, many advocate a more aggressive time line for surgery, including implantation under the age of 12 months if necessary. This recommendation is a matter of some debate, because hearing recovery in postmeningitic patients has been reported; for advocates of the watchful waiting approach, serial MRI scanning is performed, with urgent cochlear implantation for signs of ossification.
The age of implantation continues to decrease. Understanding of auditory development and brain plasticity has given physicians the impetus to implant children at a younger age, and earlier implantation has been correlated with improved communication outcomes [4,7–12]. A narrow window of time exists during development, known as a sensitive period, within which the central auditory cortex must be stimulated by sound if it is to develop normally. If the auditory cortex is deprived of stimulus during this period, it will reorganize to be used for other purposes. For example, studies using functional MRI have shown that in deaf individuals, areas of the auditory cortex were activated by visual stimuli; this was not seen in normal hearing controls [13]. Because of this reallocation of brain function, once the sensitive period has passed, auditory development becomes much more challenging.
Sharma and colleagues [13,15] looked to assess the effect of age at cochlear implantation on auditory development and found that the brain is maximally plastic for the first 3.5 years, and that the optimal time for implantation is within this period. Plasticity then declined, but was found to remain in varying degrees until approximately 7 years of age, which marked the end of the sensitive period. Some studies have shown that different language skills may have different sensitive periods. For example, one study concluded that there might be an earlier sensitive period for vocabulary-building skills than for speech perception skills [7].
Research has been directed toward further narrowing down the optimal age for implantation. No consensus has been reached, but most literature supports that younger is better [7,10,16]. The FDA currently approves cochlear implantation down to 12 months of age, but some centers have been implanting even younger infants.
As understanding of ANSD has grown, cochlear implants have also become recognized as a viable option for some children with this disorder. One might question the efficacy of a cochlear implant in patients with abnormal auditory nerve conduction, but it is postulated that the discrete electrical signals produced by a cochlear implant may be more effective at eliciting a synchronous neural response than acoustic stimuli. Several studies report that children with ANSD derive benefit from cochlear implantation, and some report outcomes comparable to those of implanted children with SNHL [17–20]. In 2008, a consensus panel recommended that cochlear implants should be considered in children with ANSD, regardless of audiometric threshold, if progress is not being made in language development, despite appropriate amplification.
However, children with ANSD are a heterogeneous group, and not all patients with ANSD should be implanted. Research has shown that for individuals with a cochlear nerve deficiency or hypoplastic cochlear nerve, outcomes are much poorer [21,22]. This does not mean that this group should automatically be excluded from consideration, but they should be carefully assessed for any auditory sensation, and expectations should be tempered.
There is also a chance that auditory function may improve in ANSD. This has been documented in several studies. In a 2002 study, 50% of children with ANSD and evidence of severe SNHL spontaneously improved, with a mean improvement time of 5.8 months. The children whose ANSD was attributed to hyperbilirubinemia had a greater tendency to improve. These children achieved a stable audiogram at a mean age of 18 months [23]. Thus, caution must be exercised before proceeding with implantation in infants and very young children who carry this diagnosis.
Cochlear implant programs in the past tended to exclude deaf children with other handicaps, but the number of children with other disabilities being implanted is increasing rapidly, and accounts for a large percentage of implant recipients at some centers. For most children, the goal of implantation is spoken language. For children with developmental delays or other disabilities, the goals of implantation may be different. Speech and language outcomes may be limited by cognitive deficits, but these children may still derive benefit from improved use of language, as well as sound awareness, and increased environmental and social connectedness. Communication by nonverbal modalities may still be supported by sound. Mere sound awareness can be useful in terms of social attachment, attention, and safety. Thus, these children should not automatically be excluded from implant candidacy [24,25].
