Cochlear Implant Surgery 

Cochlear implantation has become a routine procedure in the United States and worldwide for the management of severe-to-profound sensorineural hearing loss. The decision to embark upon cochlear implantation is made either by the patient (if adult) or by the parents or caregivers of a child. The 60- to 75–minute procedure is well tolerated and routinely performed on an outpatient basis in both adults and children.

The team concept in cochlear implant evaluation allows for an exchange of information between the surgeon and other members of the implant and rehabilitation process, including audiologists, speech and language therapists, social workers, and psychologists. Typically, the patient is referred to a cochlear implant center, and initial contact is made. The patient may first be seen and identified as an implant candidate by an audiologist. Hence, a patient can enter the evaluation process in a number of different ways. Nonetheless, various issues are taken into consideration, including medical aspects of the patient's history, the audiologic evaluation, and radiographic studies.

An image depicting cochlear implant surgery can be seen below.

Although the team evaluation concept is explained at greater length in the Indications section, it is notable because it allows for proper selection of patients, the continuous flow of pertinent dialogue, and the promotion of realistic expectations on the part of the patient and the patient's family.

The evaluation process used by the authors at the implant center at the Case Medical Center/University Hospitals of Cleveland and Rainbow Babies and Children's Hospital is summarized below. At the time of the medical evaluation, the patient's general medical history and issues regarding hearing loss are reviewed. A complete neuro-otologic and otolaryngologic examination is performed, and obvious conditions (eg, tympanic membrane perforation, chronic otitis media, congenital anomalies) are noted. The patient's history is reviewed to establish the potential etiology of the hearing loss. Audiologic tests are reviewed and repeated as necessary. Once the patient is deemed to be a potential cochlear implant candidate, the various cochlear implant options are discussed, and audiologic evaluation commences.

Typically, the audiologist measures the patient's hearing with and without hearing aids. Evaluation with pure-tone audiometry and auditory brainstem response (ABR) testing (in the case of children) is often performed. Otoacoustic emission (OAE) testing complements these studies; OAE results often indicate the need for a trial of newer and sometimes stronger hearing aids.

A CT scan is obtained to evaluate the status of the cochlea and to establish the presence of a patent (nonossified) cochlea or to identify a common cavity, Mondini dysplasia, enlarged vestibular aqueduct, or an ossified cochlea. In some cases, an MRI is used instead of the CT when questions exist regarding the presence of the eight nerve or severe ossification. In children and young adults, speech and language evaluation and educational placement discussions are performed next. Finally, a psychosocial evaluation is completed. Once a patient has been evaluated, a team meeting commences to recommend cochlear implantation advice. If the patient is cleared for cochlear implantation, the patient proceeds with preoperative medical clearance, chooses a cochlear implant device, and proceeds with surgery.

In 1957, Djourno and Eyries made the observation that activation of the auditory nerve with an electrified device provides auditory stimulation in a patient. This observation is considered the seminal observation that paved the way for modern cochlear implantation. In 1963, Doyle and Doyle's early experiments in scala tympani implantation preceded the first House/3M single-channel implant in 1972.^[1] Multichannel devices introduced in 1984 have replaced single-channel devices by virtue of improved speech recognition capabilities. As of 2009, nearly 150,000 cochlear implants are estimated to have been performed worldwide, and approximately 7,000 procedures take place annually in the United States. Three US Food and Drug Administration (FDA)–approved multichannel devices are routinely used in the United States currently, including the Nucleus 5 cochlear implant system (Cochlear Corporation), the Clarion 90K (Advanced Bionics Corporation), and the Combi 40+ (MED-ELCorporation).

Severe-to-profound hearing loss, as evidenced by the lack of useful benefit from hearing aids, often determines one's candidacy for cochlear implantation. In children, this is confirmed via auditory testing and failure to develop basic auditory skills. In adults, candidates should receive limited or no benefit from appropriate hearing aids (ie, a score of 50% or less on sentence recognition tests in the best-aided listening situation).

Frequency

The incidence of congenital hearing loss varies by study. Niparko reviewed studies from the 1980s and 1990s and noted that one of the most carefully performed epidemiologic studies was that of Van Naarden et al, which noted an overall prevalence rate of serious hearing impairment of 1.1 cases per 1000 children aged 3-10 years.^[2] By age 75 years, 360 of 1000 adults have a disabling hearing loss. According to the 1996 National Institute on Deafness and Other Communications Disorders survey, more than 28 million Americans are deaf or hearing impaired.^[3] This statistic may reach 40 million by the year 2020.

Common etiologies of deafness that lead to consideration of cochlear implantation in pediatric patients include idiopathic, genetic, and acquired causes that result in congenital and delayed-onset hearing loss. Genetic hearing loss can be dominant or recessive. Infectious etiologies, including bacterial and postviral meningitis, can lead to severe hearing loss. Meningitis-related deafness has decreased with the routine use of the Haemophilus influenzae vaccine in children. Adult patients presenting for implantation include those with progressive hearing loss that began in childhood, viral-induced sudden hearing loss, ototoxicity, otosclerosis, Ménière disease, trauma, autoimmune conditions, presbycusis, and bacterial infections.

Typically, patients presenting with severe-to-profound deafness have had a direct or indirect injury to the organ of Corti, leading to degeneration or dysfunction of the hair cell system. Therefore, success of cochlear implantation depends on stimulation of surviving spiral ganglion neurons. The number of surviving neuron populations needed for successful implantation remains unclear. In 1991, Linthicum et al reported successful speech understanding in a patient who demonstrated less than 10% of the normal complement of neurons via a temporal bone study.^[4] Therefore, despite the wide range of surviving neurons present in various pathologic causes of deafness (10-70% of the normal 35,000-40,000 cells), most patients are likely potential implant candidates.

In the past, children with hearing loss presented to the physician after their parents developed a concern about their child's lack of response to noise and voices. This may have brought the child to the attention of an otolaryngologist promptly (within a few weeks to months), or consultation may have been delayed up to a number of years. With the addition of universal infant screening, babies are identified at birth as having a hearing loss. The loss is confirmed and quantified with auditory brainstem testing, and, if profound, the patient is referred for cochlear implant evaluation. Children are fitted with hearing aids, and a decision to implant is based on progress or lack of language development and careful counseling of the family. If a child is clearly found to be an implant candidate, an earlier implantation results in superior hearing and speech outcomes.

Thus, implantation at age 12 months is now considered ideal, and, in some instances, implantation at an earlier age is performed. Adults with progressive loss that ultimately fails to be managed via amplification also may present for implant consideration. Patients are increasingly informed of the various options for cochlear implantation via the Internet and often have specific questions regarding different device options.

For excellent patient education resources, visit eMedicine's Ear, Nose, and Throat Center. Also, see eMedicine's patient education article Hearing Loss, as well as the eMedicine article.

The main indication for cochlear implantation is severe-to-profound hearing loss that is not adequately treated with standard hearing aids. The clinical conditions that lead to such an indication include various scenarios, as follows:

• Congenital hearing loss and prelingual deafness
• Acquired hearing loss and postlingual deafness
• Severe hearing loss that can be aided and that deteriorates to profound loss in childhood, adolescence, or adulthood (perilingual) and coexists with various degrees of language development

Generally, the candidacy for implantation is considered separately for adults and children. As outlined in the 1995 National Institutes of Health (NIH) consensus statement on cochlear implantation, adult candidacy is noted as being successful in postlingually deaf adults with severe-to-profound hearing loss with no speech perception benefit from hearing aids.^[5] In addition, the statement notes that "most marginally successful hearing aid users implanted with a cochlear implant will have improved speech perception performance." Medicare guidelines as of January 2005 allow for cochlear implantation in patients with 50% aided sentence discrimination scores and allow for 60% sentence scores in clinical trials. Clearly, the trend over time is that relaxed guidelines are better, and better cochlear implant performance and outcome have been demonstrated.

Prelingually deafened adults, although potentially suitable for cochlear implantation, must be counseled in regard to realistic expectations, as language and open-set speech discrimination outcomes are less predictable. A strong desire for oral communication is paramount for this group of patients

Children are considered candidates for cochlear implantation at age 12 months, and, because of meningitis-related deafness with progressive cochlear ossification, occasional earlier implantation is necessary. Investigations are ongoing into extending the age of early routine implantation to younger than 12 months. Audiologic criteria include severe-to-profound sensorineural hearing loss bilaterally and poor speech perception under best-aided conditions, with a failure to progress with hearing aids and an educational environment that stresses oral communication. The use of objective testing in this age group includes auditory brainstem response (ABR) testing and otoacoustic emission (OAE) testing in addition to trials of various auditory training programs, which are essential before cochlear implantation. For further discussion, see the eMedicine article Cochlear Implants, Indications.

The surgeon performing cochlear implant surgery must be experienced in otologic surgery and, ideally, some aspects of neurotologic surgery. Intimate knowledge of the relevant surgical anatomy of the mastoid cortex, retromastoid region, and posterior/middle cranial fossa dura is important in properly performing the approach to the facial recess and in properly creating an implant receiver well that provides low-profile placement of the internal device.

In addition, the relationship of the facial nerve, incus, chorda tympani, and the facial recess needs to be properly understood to safely perform the posterior tympanotomy to gain access to the middle ear. Once the facial recess has been opened, knowledge of the round window anatomy as it relates to normal or abnormal middle ear topography is vital. The ability to visualize the round window membrane by removing the bony round window niche is important for creating a proper cochleostomy. Variations in anatomy, ossification of the scala tympani, and various strategies of dealing with cerebrospinal fluid oozers and gushers should be anticipated.

For more information about the relevant anatomy, see Auditory System Anatomy, Skull Base Anatomy, and Facial Nerve Anatomy.

Contraindications to cochlear implantation may include deafness due to lesions of the eighth cranial nerve or brain stem. In addition, chronic infections of the middle ear and mastoid cavity or tympanic membrane perforation can be contraindications. The absence of cochlear development as demonstrated on CT scans remains an absolute contraindication. Certain medical conditions that preclude cochlear implant surgery (eg, specific hematologic, pulmonary, and cardiac conditions) also may be contraindications. The lack of realistic expectations regarding the benefits of cochlear implantation and/or a lack of strong desire to develop enhanced oral communication skills poses a strong contraindication for implant surgery.