A cochlear implant (CI) is a surgically implanted device used for hearing rehabilitation of adults and children with advanced sensorineural hearing loss (SNHL) and/or poor speech discrimination who gain limited benefit from conventional hearing aids.
At a fundamental level, a CI is a transducer that transforms acoustic energy into an electrical signal, which is used to stimulate surviving spiral ganglion cells of the auditory nerve.
There are currently three FDA-approved CI manufacturers: Advanced Bionics Corporation (Valencia, CA, USA), Cochlear Corporation (Lane Cove, Australia), and Med-El GmbH (Innsbruck, Austria).
Over the last three decades, advancements in surgical technique, electrode design, and improved speech processing strategies have led to increasingly better outcomes in CI recipients. As a result, FDA candidacy criteria have gradually expanded from initially only implanting postlingual deafened adults with profound bilateral SNHL to now implanting adults and children with greater degrees of residual hearing (Table 20-1). Furthermore, a growing proportion of patients are undergoing CI for off-label or nontraditional indications including single-sided deafness, retrocochlear hearing loss, asymmetrical SNHL in adults and children with at least one ear that is better than performance cutoff for age, and children less than 12 months of age.
|Cochlear Implant Candidacy Guidelines|
(≥ 18 y)
|Hearing threshold||Moderate to profound SNHL in both ears (> 40 dB)||Severe to profound SNHL (> 70 dB)||Profound SNHL (> 90 dB)|
Normal to moderate hearing loss (thresholds better than 60-dB HL) in the low frequencies (≤ 500 Hz) and severe to profound mid- to high-frequency hearing loss, PTA (2,3,4 kHz) ≥ 75-dB HL in the ear to be implanted and moderately-severe to profound mid- to high-frequency hearing loss, PTA (2,3,4 kHz) ≥ 60-dB HL in the contralateral ear
|Word recognition||Limited benefit from binaural amplification defined by ≤ 50% sentence recognition in the ear to be implanted (or ≤ 40% by CMS criteria) and ≤ 60% in the contralateral ear or binaurally.||Limited benefit from binaural amplification defined by ≤ 20%-30% word recognition scores||Limited benefit from binaural amplification trial based on MAIS|
Aided CNC word recognition score between 10% and 60% (inclusively) in the ear to be implanted.
Contralateral ear equal to or better than that of the ear to be implanted but not more than 80% correct
Cochlear Implant Design
Components of the CI device are illustrated in Figure 20-1.
Microphone: placed near earhook and receives natural acoustic information and converts it to an analog electrical signal, which is sent to the behind-the-ear (BTE) or single unit off-the-ear sound processor.
Sound processor: alters the signal through the processes of amplification, compression, and filtering. The analog signal is digitized via Fourier analysis making it possible to convey timing, frequency, and intensity information in a way that is meaningful to the central nervous system.
Radiofrequency (RF) transmitter antenna: sends the processed signal transcutaneously to the implanted receiver-stimulator.
External magnet: aligns with the internal magnet to facilitate transcutaneous RF signal transmission.
Internal RF antenna.
Receiver/Stimulator: translates the electrical signal into rapid electrical impulses that is distributed to the individual wires within the electrode array.
Continuous-interleaved-sampling (CIS) is the most common signal processing strategy used. The CIS strategy minimizes problems associated with channel interaction by presenting nonsimultaneous (or interleaved) brief, rapid impulses across the electrode array. As a result, the tonotopic organization of the cochlea is better represented.
Electrode array: designed to be placed within the scala tympani. Contains individual wires that transmit data for each respective channel along the implanted multichannel electrode. The electrodes stimulate the spiral ganglion cells located in Rosenthal canal, which in turn propagate signal along the auditory nerve to the brain for further processing.
Standard electrode arrays contain 12 (MED-EL), 16 (Advanced Bionics), or 22 (Cochlear) electrodes, though some studies have found that the majority of users have less than 10 perceptually unique channels active at any one time.
Multiple electrode designs are available from each company to enhance the ability of the surgeon to preserve residual hearing and minimize insertion trauma. Electrodes have been made shorter, thinner, and more flexible. Surgeons can choose the electrode array best suited for the patient.
A thorough otologic medical history should attempt to determine the etiology of the hearing loss, though not always possible. Factors that may lead to cochlear ossification or scarring should be elicited, such as history of meningitis, temporal bone fracture, or otosclerosis.
Duration of deafness is valuable to understand as very long-standing severe or profound hearing loss may predict a poorer outcome.
Patients with chronic otitis media may require staged surgery, where infection, perforation, or cholesteatoma is initially treated and the implant is inserted during a staged second operation.
Recurrent acute otitis media (RAOM) should not be allowed to unduly delay implantation, particularly in children.
CIs can safely coexist with pressure equalizing (PE) tubes.
Children who are still susceptible to RAOM may keep PE tubes in place prior to undergoing surgery without placing the success of the CI or the patient at undue risk.
Replacement of PE tubes after CI is also acceptable for treatment of RAOM.
Pertinent non-otological medical history that may result in poor wound healing include previous radiation to the area, immunodeficiency, poorly controlled diabetes mellitus, tobacco use, malnutrition, a prominent history of allergic hypersensitivity reactions, or widespread dermatological disease.
Neurocognitive decline in the elderly (eg, dementia) or cognitive developmental delay in children and adults may reduce implant performance.
The goal of imaging is to ensure no contraindications to cochlear implantation exist and to facilitate safe surgical planning.
Superior modality for confirming the presence of the cochlear nerve since it allows direct visualization of the nerve. The cochlear nerve is best imaged on heavily T2-weighted MRI (eg, CISS, FIESTA) in the sagittal plane.
Gold standard for determining cochlear patency because it is able to distinguish between fluid versus cochlear fibrosis versus ossification.
Useful in identifying retrocochlear pathology, such as vestibular schwannoma, that may interfere with signaling to the brain stem and auditory cortex.
Spares the patient ionizing radiation but is more time consuming and costly compared to CT.
Provides the patient with their last opportunity to obtain high-quality brain imaging without artifact.
Useful in diagnosing bony cochlear malformations.
Useful in confirming presence of a cochlear nerve in cases of a narrow internal auditory canal (IAC) (< 3 mm) where MRI resolution is limited in its ability to adequately identify and separate nerves. In this setting HRCT is used to evaluate the bony cochlear nerve canal (BCNC), whereby a closed BCNC supports the diagnosis of cochlear nerve deficiency/aplasia.
Provides details on the surgical anatomy, which may help execute safe surgery. Potential surgical obstacles include:
Poorly pneumatized mastoid and/or facial recess
Extremely anterior sigmoid sinus
High-riding jugular bulb
Aberrant facial nerve course, particularly either anterior or medial displacement of the proximal mastoid segment
Inner ear malformation with risk of CSF gusher (increased in the setting of a thin cribriform area between the modiolus [eg, IP3 deformity], a widened IAC, or enlarged vestibular aqueduct).
In children, the method of candidacy testing is dependent upon age and level of cognitive development. In very young children, candidacy testing relies on otoacoustic emissions, auditory brain stem responses, behavioral testing, and parental questionnaires. In older children, behavioral audiometry and age-appropriate speech perception testing are performed. In most children, a minimum of a 3-month hearing aid trial should be completed before implantation. Evidence-based pediatric minimum speech test battery guidelines have been developed and published.
In adults, candidacy evaluation should follow published adult minimum speech test battery guidelines that rely on speech perception testing using Arizona Biomedical Sentences (AzBio), Consonant-Nucleus-Consonant (CNC) monosyllabic words, and/or Bamford-Kowal-Bench Speech-in-Noise test (BKB-SIN).
Sentence testing is performed in the unilateral and binaural best-aided condition with properly fitted hearing aids.
Performed for exclusionary reasons to identify subjects who have severe cognitive impairment, undetected psychosis, or unrealistic expectations.
Valuable information related to the family dynamics and other factors in the patient’s milieu that may affect implant acceptance and performance are assessed.
Identification of mild dementia can help with planning and structuring an effective rehabilitation plan for after implantation. The relationship between cognition and hearing loss is complicated and not yet well understood.
Current Audiometric Selection Criteria
FDA criteria exist for each device but are not consistent across devices or companies. Insurance companies also have varied criteria but are generally similar to FDA guidelines (see Table 20-1).
Audiometric testing protocols are not standardized (eg, some centers use AzBio in quiet and others test in noise), and thus patients may be considered a candidate in one center but not another. The goal of testing is to identify candidates who will benefit from implantation. FDA and insurance company guidelines are generally felt to be restrictive and understandably lag current practice and evidence. Most large centers in the United States find AzBio testing in noise and CNC word scores to be most helpful for counseling patients. In general, candidates have the following characteristics:
Bilateral moderate-to-profound SNHL
Aided sentence scores less than or equal to 50% in the ear to be implanted and less than or equal to 60% in the best-aided condition (usually binaural hearing aids)
Centers for Medicare and Medicaid Services (CMS) has stricter criteria: binaural-aided sentences less than or equal to 40%
Criteria for children is even less standardized than that for adults due to young age and evolving cognitive and language abilities.
More than or equal to 12 months of age based on the FDA, but earlier implantation has been demonstrated to be safe and may result in improved language outcomes.
Little to no benefit from hearing aids over a 3- to 6-month trial period
Trial period waived for children who had meningitis with radiographic evidence of ossification
Bilateral profound SNHL (for patients 12-24 months old); severe-to-profound SNHL (for patients > 24 months old)
Aided open-set word recognition scores less than or equal to 30% in children capable of testing
Multisyllabic Lexical Neighborhood Test (MLNT) or Lexical Neighborhood Test (LNT) are commonly used tests.
Patients with ANSD have normal otoacoustic emissions but an absent auditory brain stem response. Pure tone thresholds range from normal to severe.
Reliable open-set speech recognition can be achieved with CI in ANSD.
CI for SSD has been shown to reduce or alleviate tinnitus and improve sound localization in many patients. This indication is currently not FDA approved.
Neurofibromatosis Type II With Bilateral Vestibular Schwannomas or Sporadic Vestibular Schwannomas in an Only Hearing Ear
Historically this patient population was not considered for cochlear implantation; however, more recent experience demonstrates that many CI recipients achieve open-set speech recognition. When an intact cochlear nerve exists, outcomes are generally superior to auditory brain stem implants (ABIs).
Patients with superficial siderosis, pachymeningitis, sarcoidosis, history of CNS radiation, brain stem lesions, or similar present as unique situations. The decision to proceed or not is difficult, particularly because the results are unpredictable. However, successful restoration of hearing is possible, thus making the attempt reasonable.
Complete labyrinthine aplasia
Complete cochlear ossification
Absent cochlear nerve
Retrocochlear pathology along auditory pathway (relative contraindication; may be considered in some scenarios such as neurofibromatosis type II as mentioned earlier)
Though many of these serve as a contraindication to implantation, some surgeons would attempt CI if surgically feasible prior to performing an ABI because CI surgery is generally safer and if it is successful, the outcomes are typically better than what is achieved with an ABI. Furthermore, if unsuccessful, an ABI is still an option.