Introduction
The cochlear implant is an electrical device that directly stimulates the auditory nerve Cochlear implants differ from hearing aids whose aim is to amplify the remaining haircells caused by cochlear haircell damage. According to Zwolan (2000), people with moderate to extremely low frequency and those with profound high-frequency sensorineural hearing loss may not get much help from hearing aids.
The cochlear implant’s purpose, therefore, is to rehabilitate a hearing loss where a conventional hearing aid/s proves ineffective (Zwolan, & Kileny, 2005). Most sources show that cochlear implants show improved outcomes despite some implanted people still having hearing difficulty in noisy backgrounds. (Zwolan, & Kileny, 2005; Pedley, & Giles, 2005). To determine whether an individual is eligible for cochlear implantation, one first goes through a thorough evaluation undertaken by a team of highly trained professionals that can take several months to be completed. (Zwolan, & Kileny, 2005).
The selection criteria for cochlear implant candidates in adults and paediatrics have essentially similar criterion, but also have specific differences. Notably, because of the advances and improvements of the cochlear implantation technology, its candidacy criteria will continue to evolve and expand. Zwolan & Kileny (2005) state that candidacy criteria will certainly change over the years as advances are made in device technology, surgical technique and speech processing strategies.
In a later study, Zwolan (2006) mentioned candidacy changes in other centres that included implanting individuals with both increasing amounts of residual hearing and preoperative open-set speech perception skills, implanting children at a younger age, as well as implanting a larger number of individuals with abnormal cochleae. In his 2001 study, Dillon mentions that selection candidacy may vary between countries, different centres, or even between different implant companies. According to the law, all devices have to receive criterion and candidacy approval from the Food and Drug Administration (FDA) (Zwolan, 2000).
General Selection Criteria for All Age Groups
Generally, people considered for a cochlear implant have a severe to profound hearing loss that has gained little or no benefit from hearing aids (SCIC, 2010a). Dillon (2001) mentions that the range of hearing loss should be greater than 90dB HL at some frequencies and more than 60dB HL at all frequencies in both ears. In addition to this criteria, the Sydney Cochlear Implant Centre (SCIC, 2010) mentions that individuals who may benefit from cochlear implantation include:
- Young children lacking significant hearing to benefit from hearing aids.
- Older children, adolescents and adults who have successfully tried hearing aids, but whose hearing has changed so the hearing aids are no longer a useful aid for communication purposes.
- Older children who have used hearing aids and find that they are now unable to communicate effectively due to environmental changes in their learning situations.
- Individuals whose hearing has deteriorated suddenly.
- Individuals who have not obtained the expected benefit from hearing aids and are unable to communicate despite having good hearing threshold levels.
According to SCIC (2010), due to changes in devices and individual variation on performance with cochlear implants, potential implantees could also consist of:
- Children and adults with a congenital hearing loss.
- Sudden hearing loss from either meningitis or trauma.
- A progressive hearing loss such as Large Vestibular Aqueduct Syndrome (LVAS).
- Additional disabilities – previously children with cognitive delay or with additional disabilities were excluded from participating in early clinical trials. However, more children displaying additional disabilities are routinely receiving cochlear implants (Zwolan, 2006). Supporting studies indicate that cochlear implant can improve the quality of life of profoundly deaf individuals with associated disabilities thus increasing their listening and communication skills, their self-sufficiency, and ability to interact with others (Filipo, et al, 2004; Donaldson, et al, 2004).
- Auditory Dys-synchrony or Auditory Neuropathy – cochlear implant candidacy may result in complications with some of these individuals as they can demonstrate recovery or improvement of speech recognition/detection skills over time (Zwolan, 2006). It has also been noted that outcomes of individuals with auditory neuropathy tend to vary. Therefore, candidacy for cochlear implantation for individuals with auditory neuropathy should be determined on a case-to-case basis (Zwolan, 2006).
- Ski-slope losses (with good residual hearing).
According to SCIC (2010), the rationale for this criterion, is that the guidelines are set such that at least 75% of people would have improved speech perception.
Selection Criteria for Adults
Additional selection criteria to be included for potential cochlear implant adult candidates is for them to score 40% or less on open-set sentences when using hearing aids. They would also need motivation, be emotionally stable, have realistic expectations, and be willing to attend the pre- and post-implantation assessments, and the cochlear implant mapping and training sessions required. (Dillon, 2001).
Selection Criteria for Pediatrics
Initially, the accepted age for children undergoing implantation was 18 months but this is changing. (Dillon, 2001). Age is a big factor in the cochlear implant selection process for children. Today, there is ongoing controversy concerning whether to implant as early as possible or delay implantation until the child is older. Come people propose that the implant should be done early enough to allow children to have greater access to auditory information that is essential for speech and language developmental skills (Hoffman, 1997; Osberger, 1997). However, with no sound stimulation, the auditory neural pathways to the brain would not develop.
Owens, Espeso, Hayes, & Williams (2006) mention that when age increases, the ability to produce neural connections diminishes until at approximately the age of 10, this skill is inherently lost. The evaluation of speech and language skills, however, poses a challenge, as it will be difficult to assess, particularly for younger children during postoperative follow-ups. According to Zwolan (2006), younger children cannot provide a conditioned response that is routinely used for adjusting and setting the device. Notably, the presence of a severe language delay would demonstrate the need for intervention. Zwolan further asserts that speech, language, and communication skills are assessed in older children to:
- Determine if the child demonstrates developmental language and/or articulation disorders.
- Describe the child’s communicative status with respect to normative models of language development.
- Assist in defining appropriate expectations for speech and language skills following cochlear implant. Speech perceptual assessment is certainly important for the pre- and post-operative process for children, as they are more sensitive to differences in individual performance. Preoperatively, speech perception testing can determine if a child is receiving auditory information with his/her current hearing aids, and can assist in determining if speech perception skills will improve with a cochlear implant. Alternatively, postoperatively, speech perception testing can provide information regarding speech cues that are or are not being perceived by the child. (Zwolan, 2006).
According to Beiter & Estabrooks (1994), the recommended candidacy criterion for pediatrics entails:
- Bilateral profound sensorineural hearing loss.
- Ages 2 -17 years.
- No medical or radiological contraindications.
- Demonstrates little or no benefit from fitted amplification.
- Be enrolled in an educational program with a strong auditory and oral component.
- Be psychologically suitable.
- Have a family with appropriate expectations and support.
Other Selection Criteria Considerations
One essential factor for determining the candidacy of cochlear implantation is that there should be no medical contraindications such as cochlear ossification, an absent cochlea, chronic middle ear infection, or retrocochlear hearing loss. The etiology of a hearing loss can certainly affect the decision of whether to implant or not, and may create uncertainty. For instance, cochlear ossification may make it impossible to implant the electrodes or could reduce the effectiveness of the implant when inserted (Dillon, 2001).
Thus, it would be essential to carefully investigate these factors during the pre-operative process via a detailed case history and comprehensive assessments, to prevent any arising complications. It is also important to note that outcomes will vary significantly among individuals, and will depend on pre-existing conditions, onset of different conditions, and etiology of hearing loss (SCIC, 2010).
Other commonly applied factors towards the cochlear implant candidacy criteria include:
- Individuals who need to be using hearing or have the need to use hearing in the future
- Individuals who need to have certain auditory structures (such as their auditory nerve) intact
- There should be realistic goals/expectations with cochlear implantation. (SCIC, 2010)
In sum, current candidacy guidelines indicate that there is appropriateness in providing cochlear implants to individuals with increasing amounts of residual hearing, to individuals with increasing amounts of preoperative open-set speech perception skills, and to children as young as 12 months. (Zwolan, 2006).
Cochlear Implant Candidacy Assessment
To be eligible for cochlear implantation, a complete preoperative evaluation is first performed. This is an extremely vital process for cochlear implantation, as it determines whether the patient is medically and audiologically suitable for a cochlear implant (Zwolan, 2006).
Audiological Assessment
The aim of the preoperative audiological assessment is to determine the type and severity of hearing loss. This evaluation includes unaided air and bone conduction thresholds, unaided speech discrimination, speech reception threshold (SRT), speech detection threshold (SDT), otoacoustic emissions (OAEs), and immittance testing that is inclusive of tympanometry and acoustic reflexes. Previous clinical trials have indicated that individuals demonstrating a bilateral profound sensorineural hearing loss were considered for cochlear implant candidacy. However, present guidelines have been more lenient and have indicated that individuals who have slightly better hearing can now be considered for cochlear implantation.
One should note, however, that there would be variations in terms of availability and criteria of devices. For instance, the guidelines for the Nucleus Freedom with Contour Advance cochlear implant indicates that potential cochlear implant adult candidates who are postlingual have a moderate-to-profound hearing loss in the low frequencies and a profound hearing loss in the mid-to-high frequencies. (Zwolan, 2006).
Electrophysiologic Assessment
Several centres employ electrophysiologic tests in their preoperative test battery. Tests such as the Auditory Brainstem Response (ABR) can look at how the cochlear nerve and part of the auditory pathway to the brainstem responds to sound (SCIC, 2010). The ABR not only verifies audiometric results, but it also helps in identifying patients with auditory neuropathy. It’s also used to rule out the possibility of functional deafness, as well as verify electrical stimulation of the auditory system. This informs the surgeon about the presence or absence of an eighth nerve in the ear (Zwolan, 2006).
Hearing Aid Assessment
The purpose of the hearing aid assessment is to evaluate the individuals’ performance with the appropriate amplification. Aided detection thresholds are obtained, which can be compared to those expected with a cochlear implant, as well as determine whether the individual’s speech detection skills are likely to improve with the cochlear implant. It is also essential to consider the amount of time the potential candidate has used appropriate amplification (Zwolan, 2006).
Psychological Assessment
This evaluation is primarily focused on evaluating the cognitive ability of pediatric individuals, in order to determine whether factors other than hearing are hindering the child’s auditory development. However, adult patients presented by cognitive or mental status could also fall under this category of testing. The presence of cognitive impairment could greatly influence the ability to develop spoken language skills with cochlear implantation. In children, psychological deficits may not be identified until the child is two years (Zwolan, 2006).
Medical Assessment
In medical testing, CT/MRI scans are often conducted together. The conjoined usefulness of both the CT/MRI can assist the surgeon in locating the nerves during the surgical process, assist him/her in deciding which ear would be the most favourable to implant, and can provide helpful information to individuals and families regarding the outcome of the surgery if specific abnormalities are detected (SCIC, 2010). Moreover, these scanning techniques aid in evaluating candidacy and surgical planning (Cohen, & Roland, 2006). After cochlear implant surgery, it would also not be possible for the individual to have another MRI scan as the internal piece of the cochlear implant contains a magnet. (SCIC, 2010)
Cochlear Implant Rehabilitation
The fitting and tuning of the external part of the cochlear implant occurs 3 weeks after operation. Learning to listen with an implant requires practice, and one may require using various strategies for improving listening in various situations. However, individuals may take different amount of times to get used to listening with the cochlear implant. Unlike hearing aids where approximately three review appointments are arranged, the individual with a cochlear implant will require an ongoing rehabilitative process.
During this process, the cochlear implantee receives mapping services as well as thorough assessment services that aid in examining the function of the implant, its position within the cochlea, and the auditory response to the electrical stimulation over time. Tests conducted to monitor the device and electrical stimulation over time include the Electrocochleography (ECochG) which provides information about the integrity of part of the cochlear nerve, the Electrical ABR (EABR) to monitor the implant. (SCIC, 2010).
Another key factor that contributes to a successful rehabilitation program is one that is conducted holistically by a multidisciplinary team of professionals who are trained in the area of audition. Beiter & Estabrooks (1994) advises that candidates for a cochlear implant needs professionals from otolaryngology, audiology, auditory habituation, auditory verbal therapy, speech-language pathology, education, and psychology. Children need their families since it’s the support structure for their child and will be able to provide information to the professionals involved about how the child uses their hearing and other modalities for communication purposes.
Often, parents in conjunction with professionals make the final decision on whether to proceed with hearing aids or not. (Beiter & Estabrooks, 1994)With adults, the focus of a successful program is to be able to deal with the individual as a whole, which confirms the need for a multidisciplinary team. (Bosco, Ballantyne, & Argiro, 1996).
During the rehabilitative process, a mode of communication is often implemented. According to Estabrooks (1994), there is no communication method designed to meet all of the individual needs of a child who is hearing impaired. However, auditory-verbal therapy (AVT) as compared to all the other modes of communication is certainly one of the options that is widely used and most commonly employed by cochlear implant rehabilitative programs.
Despite its commonness with children, auditory training can also be used on adults (Bosco, Ballantyne, & Argiro, 1996). Estabrooks (1994) defines AVT as a diagnostic tool, with the application and management of technology, strategies, techniques, and procedures to enable children with hearing impairments to learn to listen and understand spoken language for the purposes of communicating through speech. Certainly, rehabilitation programs are tailored for children with cochlear implants. Rehabilitation or auditory training programs, as mentioned by Owens, Espeso, Hayes, & Williams (2006), are highly individualised depending on age, cognition and level of hearing gain.
Age at implantation and age at onset of deafness are greatly considered both for child and adult patients. According to Bosco, Ballantyne, & Argiro (1996), age at onset of deafness influences the rehabilitation process at all levels, as it facilitates the planning of suitable work units and the creation of adequate proposals for acoustic and phonatory assessment and training. It is essential in any rehabilitation process to be able to adhere to a flexible approach. This is achieved by adjusting to the various needs of the patient while considering his social and cultural environmental interactions (Bosco, Ballantyne, & Argiro, 1996).
Operative/Post-operative Complications
There are always associated risks or complications that could arise when implanting a foreign body. Hoffman & Cohen (1995) state that an implanted foreign body can increase the risk of postoperative infection. Nevertheless, risks can be avoided through evaluation of the patient, careful surgical planning, meticulous surgical procedures, and conscientious follow-up (Cohen, 1997). According to Owens, et al (2006) some risks in cochlear implantation surgery include:
- Wound and skin infection
- Inner ear infection and meningitis
- Cochlear implant infection
Wound and skin infection
Wound and skin infection is the most common risks involved in surgery and is usually treated with local wound care and oral antibiotics. If severe, the infection may spread and re-implantation may be warranted. Alternatively, skin infection can be caused by erosion due to the magnetic attachment of the device, particularly when the device is too powerful (Owens, Espeso, Hayes, & Williams, 2006).
Ear infection and Meningitis
With inner ear risk reports, many patients are known to experience temporary vestibular dysfunctions, although there are reports of permanent vestibular dysfunction. If there is no history of bilateral vestibular dysfunction with rehabilitation, often the symptoms should resolve. To avoid this, a history of balance problems should be elicited at the candidacy interview to prevent such risk from occurring (Owens, Espeso, Hayes, & Williams, 2006).
Following cochlear implantation, meningitis may also occur. Risks of meningitis should be highly considered in children below the age of 24 months and adults above 65 years, as well as those with a history of meningitis. Other individuals at risk include those who have a diminished immune capacity and the presence of a dysplastic cochlea in the ear to be implanted. (Cohen, & Roland, 2006).
Other surgical risks/complications
Other surgical complications that can arise from cochlear implants include the incision and flap erosion and infection, electrode insertion, device migration, and facial nerve injury (Cohen, & Roland, 2006). Although rare, other surgical complications may include middle ear infections, bleeding, tinnitus, dizziness, and Chorda Tympani damage. Surgery could also result in perforation of the tympanic membrane, which can usually be repaired during the surgical process (SCIC, 2010).
Device complications
There can also be device complications that are often related to malfunctions and failures of the device and its various components, as well as faulty design, failure of electronic circuits or components, or external trauma caused by a blow to the head, a fall or exposure to excessive current. When these complications occur, they would require replacement of the device and additional surgery (Cohen, & Roland, 2006). Additionally, not all the electrode array can be inserted into the cochlear. This can be due to technical difficulties, congenital abnormalities such as common cavities and CHARGE syndrome, scar tissue in the cochlea or bone inside the cochlea (SCIC, 2010).
References
Beiter, A., & Estabrooks, W. (1994). Chapter 7: The cochlear implant and auditory-verbal therapy. In W. Estabrooks. (Ed.). Auditory-Verbal Therapy for parents and professionals. Washington, DC: The Alexander Graham Bell Association for the Deaf.
Bosco, E., Ballantyne, D., & Argiro, M. T. (1996). Chapter 12: Rehabilitation procedures adapted to adults and children. In D. J. Allum. Cochlear implant rehabilitation in children and adults. San Diego: Singular Publishing Group, Inc.
Cohen, N. L. (1997). Surgical techniques to avoid complications of cochlear implants in children. Advances in oto-rhino-laryngology, 52, 161-163.
Cohen, N. L., & Roland, Jr. T. J. (2006). Chapter 10: Complications of cochlear implant surgery. In S. B. Waltzman, & J. T. Roland Jr. Cochlear implants. New York: Thieme Medical Publishers.
Dillon, H. (2001). Hearing aids. Sydney: Thieme Medical Publishers.
Donaldson, A. I., Heavner, K. S., Zwolan, T. A. (2004). Measuring progress in children with autism spectrum disorder who have cochlear implants. Archives of Otolaryngology – Head and Neck Surgery, 130(5), 666-671.
Filipo, R., Bosco, E., Macini, P., & Ballantyne, D. (2004). Cochlear implants in special cases: Deafness in the presence of disabilities and/or associated problems. Acta oto-laryngologica. Supplementum, 552, 74-80.
Hoffman, R. A. (1997). Cochlear implant in the child under two years of age: Skull growth, otitis media, and selection. Otolaryngol – Head and Neck Surgery, 117(3): 217-219.
Osberger, M. J. (1997). Cochlear implantation in children under the age of two years: Candidacy considerations. Otolaryngol – Head and Neck Surgery, 117(3): 145-149.
Owens, D., Espeso, A., Hayes, J., & Williams, R. G. (2006). Cochlear implants: Referral, selection and rehabilitation. Current Paediatrics, 16(5), 360-365.
SCIC. (2010). Who can benefit from a cochlear implant? Web.
SCIC. (2010a). Cochlear implant candidacy. [Handout]. Sydney: Author.
Zwolan, T. A., & Kileny, P. R. (2005). Chapter 87: Auditory rehabilitation with cochlear implants. In J. A. DeLisa, B. M. Gans, & N. E. Walsh. (Eds.). Physical medicine and rehabilitation: Principles and practice. Philadelphia: Lippincott Williams & Wilkins. (pp. 1905-1910).
Zwolan, T. A. (2006). Chapter 6: Selection of cochlear implant candidates. In S. B. Waltzman, & J. T. Roland Jr. Cochlear implants. New York: Thieme Medical Publishers.