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Surgery for Cochlear Implantation
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The advances seen in cochlear implantation have been nothing short of remarkable. Although the first attempt to stimulate the auditory system electrically occurred over two centuries ago, the development of a cochlear prosthesis to restore hearing to patients with sensorineural hearing loss (SNHL) has happened only over the past six decades. The early pioneering work of Simmons, Michaelson, and House provided the stimulus to encourage others, including Bonfai, Chouard, Clark, Eddington, and the Hochmairs. The initial acceptance of cochlear implants (CIs) was slow; safety and efficacy were the concerns of the early investigators, and the greatest champions of the implants were the patients themselves. Time and technology have increased the benefits gained by most patients from their CIs, and thus CIs have become the most successful prosthesis ever used to attempt to restore a sensory deficit. In the United States, the Food and Drug Administration (FDA) has approved CIs from Advanced Bionics, Cochlear, Med-EL, and Oticon for use in adults and children.
Over the years, incremental improvements in technology have resulted in the approval of multiple devices from each manufacturer. As performance with CIs has improved, the criteria for cochlear implantation have expanded ( Table 29.1 ). Bilateral cochlear implantation has provided additional benefit beyond a single CI in more challenging listening conditions, and is now considered an accepted medical practice outside of research protocols. Hybrid CIs have been successfully utilized in the treatment of severe to profound high-frequency SNHL with the opportunity to preserve low-frequency acoustic hearing. Finally, CIs have increasingly been used for hearing restoration in the setting of single-sided deafness (SSD).
Table 29.1
Criteria for Cochlear Implantation.
| 1985 | 1990 | 1998 | Current | |
|---|---|---|---|---|
| Age | Adults | Adults Children (>2 years) |
Adults Children (>18 months) |
Adults Children (>9 months) |
| Onset of hearing loss | Postlingual | Postlingual adults Prelingual and postlingual children |
Adults and children Prelingual and postlingual |
Adults and children Prelingual and postlingual |
| Degree of hearing loss | Profound | Profound | Adults: severe to profound Children: profound |
>2 years: moderate to profound <2 years: profound |
| Adult open-set sentences | 0% | 0% | <40% | <50% in implanted ear <60% in contralateral ear |
| Pediatric speech scores | NA | 0% open-set | <20% (MLNT or LNT) Lack of auditory progress |
<30% (MLNT or LNT) Lack of auditory progress |
LNT, Lexical neighborhood test; MLNT, multisyllabic lexical neighborhood test; NA, not applicable.
Patient Selection
Candidate selection for cochlear implantation has evolved as the devices and patient performance have improved. In general, adults and children with bilateral moderate to profound SNHL, who receive little to no benefit from conventional hearing aids, are in good physical and mental health, and possess the aptitude and motivation to participate meaningfully in the auditory rehabilitation program are potential candidates for cochlear implantation.
Patients with severe to profound high-frequency hearing loss with relative preservation of the low frequencies may be candidates for a hybrid CI. The concept behind hybrid cochlear implantation is to provide high-frequency electric stimulation to the base of the cochlea and preserve the low-frequency acoustic hearing. The high-frequency information is particularly important for speech discrimination, which is often lacking in this patient population.
Finally, CIs have been increasingly used to rehabilitate hearing for patients with SSD. In this population, CIs were first investigated for their benefit in tinnitus reduction. The additional benefits conferred for speech recognition and sound localization have led the FDA to approve the use of CIs for patients with SSD.
Medical Evaluation
A thorough medical evaluation is necessary to detect problems that may contraindicate surgery or interfere with the patient’s ability to complete postimplantation rehabilitation. Rarely, the cause of hearing loss may be a contraindication to surgery. Aplasia of the cochlea or cochlear nerve is a contraindication to implantation secondary to the lack of auditory nerve elements to receive stimulus from the implant, but anatomical studies have suggested that, in cases of cochlear nerve aplasia, cochlear nerve fibers may travel along the vestibular nerve. Cochlear nerve hypoplasia, on the other hand, defined as a small or barely perceptible cochlear nerve on preoperative magnetic resonance imaging (MRI), is not an absolute contraindication to cochlear implantation. With current imaging modalities, it may not be possible to differentiate between cochlear nerve aplasia and cochlear nerve hypoplasia, together termed cochlear nerve deficiency. Some patients with cochlear nerve deficiency will receive benefit from cochlear implantation, but the prognosis for obtaining open-set speech understanding and participating in mainstream education is more limited. A prior history of meningitis with cochlear ossification or fibrosis does not exclude the patient from implantation but may necessitate modification of the surgical technique.
Two crucial factors influencing auditory performance following cochlear implantation include the age of onset of deafness and duration of profound hearing loss. The ideal adult candidate has moderate to profound acquired SNHL. A period of auditory experience adequate for the development of normal speech, speech perception, and language offers a significant advantage in learning to use the implant. These so-called postlingually deafened patients represent the majority of adults undergoing cochlear implantation. In these patients, there is a significant correlation between duration of bilateral profound hearing loss and performance. Those with prolonged bilateral auditory deprivation receive similar auditory information to other implant patients but are not able to use the information as effectively. This is thought to be due to the loss of central auditory processing. Evidence has emerged that the duration of monaural auditory deprivation is correlated less strongly with performance than bilateral auditory deprivation. In other words, implanting a single long-deafened ear is not contraindicated, particularly if the contralateral ear has been aided, thereby stimulating the central auditory system. , Still, with the expansion of criteria for CI, a long duration of deafness is no longer considered to be an absolute contraindication to implantation.
A small number of adult implant recipients are congenitally or prelingually deafened, with prolonged auditory deprivation and little to no experience with sound. These patients typically have greater difficulty assimilating the new auditory information and, in general, have performed less well than have those with some degree of auditory memory. Again, these situations do not pose absolute contraindications to implantation.
Analogous to adults, delayed onset of deafness in children (rather than congenital profound hearing loss) is also correlated with better speech and language skills following cochlear implantation. On the other hand, children with congenital profound hearing loss receive the maximal auditory benefit from cochlear implantation by undergoing the procedure at the youngest age possible. The current FDA guidelines require a child to be at least 9 months old, to have profound SNHL, to demonstrate no benefit from conventional hearing aids, and to be free from medical contraindications. In addition to these criteria, the commitment of the family and the child’s educational setting to postimplantation rehabilitation is a crucial determinant of successful implant use.
Audiological Assessment
The basic testing battery includes aided and unaided pure-tone and speech-detection thresholds, environmental sound recognition, and speech perception tests. In general, a patient is considered an audiological candidate for cochlear implantation if the following criteria are met: bilateral moderate to profound SNHL, with an unaided three-frequency pure-tone average of 70 dB hearing loss or poorer in the better ear; a speech sentence discrimination score of less than 50% in the ear to be implanted; and a speech discrimination score of less than 60% in the best-aided binaural condition.
Hybrid CI criteria are based on hearing thresholds and word recognition scores. Preoperative hearing thresholds may range from normal to 60 dB at 125 to 500 Hz. This should be accompanied by a severe to profound mid- to high-frequency hearing loss, defined as a threshold average of greater than 75 dB at 2000, 3000, and 4000 Hz. Roughly speaking, the consonant-nucleus-consonant word recognition score criteria are 10% to 60% in the ear to be implanted and up to 80% in the contralateral ear.
Audiological screening in children requires auditory brainstem response and otoacoustic emissions testing in addition to conventional behavioral audiometry. An aggressive trial of appropriately fit hearing aids and intensive auditory and speech training is an integral component of candidacy assessment in children. The global evaluation of CI candidacy in children is considerably more challenging than in adults and is best approached by a dedicated team comprising speech and hearing professionals, social workers, psychologists, and educators. The ultimate candidacy of a child is determined by not only a demonstrated physiological need but also by the strength of the child’s social and educational background.
Physical Examination
Prior to implantation, it is important to identify any external or middle ear disease, such as tympanic membrane perforation or chronic otitis media. Furthermore, for young children, the size of the implant in relation to the size of the child’s skull should be considered. The distance between the cochlear promontory and the mastoid cortex increases by about 1.7 cm from birth to adulthood, with one-half of the increase occurring during the first 2 years of life. The electrode must be long enough to tolerate the increase in height and width of the skull that will occur with the child’s growth. The accommodation occurs through the gradual straightening of the excess electrode length within the air-containing mastoid cavity.
Vestibular evaluation
Vestibular evaluation prior to cochlear implantation remains somewhat controversial. In certain centers, vestibular evaluation is performed as a matter of routine for all implant candidates. In other centers, only patients with preoperative vestibular symptoms and patients at elevated risk of vestibular dysfunction (e.g., elderly patients) undergo preoperative vestibular testing. Testing modalities described include videonystagmography, vestibular evoked myogenic potentials, head impulse testing, and posturography. Advocates of preoperative vestibular evaluation argue that the early identification of peripheral vestibular abnormalities can lead to more aggressive and timely management of vestibular complications following implantation.
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