A cochlear implant is a small, complex electronic device that can help to provide a sense of sound to a person who is profoundly deaf or severely hard of hearing. The implant consists of an external portion that sits behind the ear and a second portion that is surgically placed under the skin (see figure). An implant has the following parts:
- A microphone picks up sound from the environment.
- A speech processor selects and arranges sounds picked up by the microphone.
- A transmitter and receiver/stimulator, receive signals from the speech processor and convert them into electric impulses.
- An electrode array is a group of electrodes that collects the impulses from the stimulator and sends them to different regions of the auditory nerve.
An implant does not restore normal hearing. Instead, it can give a deaf person a useful representation of sounds in the environment and help him or her to understand speech.
How does a cochlear implant work?
A cochlear implant is very different from a hearing aid. Hearing aids amplify sounds so they may be detected by damaged ears. Cochlear implants bypass damaged portions of the ear and directly stimulate the auditory nerve. Signals generated by the implant are sent by way of the auditory nerve to the brain, which recognizes the signals as sound. Hearing through a cochlear implant is different from normal hearing and takes time to learn or relearn. However, it allows many people to recognize warning signals, understand other sounds in the environment, and understand speech in person or over the telephone.
Who gets cochlear implants?
Children and adults who are deaf or severely hard of hearing can be fitted for cochlear implants. As of December 2019, approximately 736,900 registered devices have been implanted worldwide. In the United States, roughly 118,100 devices have been implanted in adults and 65,000 in children. (Estimates provided by the U.S. Food and Drug Administration [FDA], as reported by cochlear implant manufacturers approved for the U.S. market.)
The FDA first approved cochlear implants in the mid-1980s to treat hearing loss in adults. Since 2000, cochlear implants have been FDA-approved for use in eligible children beginning at 12 months of age. For young children who are deaf or severely hard of hearing, using a cochlear implant while they are young exposes them to sounds during an optimal period to develop speech and language skills. Research has shown that when these children receive a cochlear implant followed by intensive therapy before they are 18 months old, they are better able to hear, comprehend sound and music, and speak than their peers who receive implants when they are older. Studies have also shown that eligible children who receive a cochlear implant before 18 months of age develop language skills at a rate comparable to children with normal hearing, and many succeed in mainstream classrooms.
Some adults who have lost all or most of their hearing later in life can also benefit from cochlear implants. They learn to associate the signals from the implant with sounds they remember, including speech, without requiring any visual cues such as those provided by lipreading or sign language.
How does someone receive a cochlear implant?
The use of a cochlear implant requires both a surgical procedure and significant therapy to learn or relearn the sense of hearing. Not everyone performs at the same level with this device. The decision to receive an implant should involve discussions with medical specialists, including an experienced cochlear-implant surgeon. The process can be expensive. For example, a person’s health insurance may cover the expense, but not always. Some individuals may choose not to have a cochlear implant for a variety of personal reasons. Surgical implantations are almost always safe, although complications are a risk factor, just as with any kind of surgery. An additional consideration is learning to interpret the sounds created by an implant. This process takes time and practice. Speech-language pathologists and audiologists are frequently involved in this learning process. Before implantation, all of these factors need to be considered.
Indications
Candidates for cochlear implants are determined medically, by the FDA, and by insurance qualifications.
General indications[rx]:
- Sensorineural hearing loss (prelingual or postlingual)
- Greater than 6 months of age
- Cochlear and cranial nerve VIII must be present with relatively preserved anatomy
- Bilateral or single-sided deafness
- Auditory neuropathy
- Reliability to follow up with audiology and otolaryngology team
- Able to undergo general anesthesia and surgical procedure
Insurance qualifications:
- Bilateral sensorineural hearing loss moderate to profound over 18 years of age (failed benefit from hearing aids)
- Bilateral sensorineural hearing loss severe to profound in patients from 2 to 18 years of age (failed benefit from alternative hearing amplification)
- Bilateral sensorineural hearing loss profound in children under 2 years of age (failed benefit from alternative hearing amplification)
- Minimum age of 12 months (younger if there is a concern for cochlear ossification which is a complication after meningitis)
- Adult open sentence scores with speech recognition less than 50% ear to be implanted, below 60 % in the contralateral ear (with amplification used)
- Pediatric Multisyllabic Lexical Neighborhood Test below 30% with a lack of auditory progress
- HINT Audiometric battery less than 50% in the implanted ear, less than 60% in the best-aided ear (scores depending on the manufacturer)
- MLNT audiometric battery less than 20 to 30 % (scores depending on the manufacturer)
- Hybrid system – (this device aids patients with high-frequency sensorineural hearing loss)
- Patients with profound high-frequency hearing loss and preserved low-frequency hearing thresholds range from 0 to 60 dB at 125 to 500 Hz
- A severe to profound middle- to high-frequency hearing loss as defined by a threshold average of greater than 75 dB at 2000, 3000, and 4000 Hz
- 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
- Reliability to follow up with otolaryngology and audiology to perform auditory rehabilitation
Contraindications
Cochlear implants are contraindicated in several patient populations. It merits mentioning that patients who meet criteria but wish to avoid surgery are not candidates. There are alternative forms of communication that require explanation, such as sign language. Patients born without a cochlea (cochlear aplasia) or cranial nerve VIII would not be candidates. Conversely, cochlear hypoplasia (a form of altered cochlear anatomy) is not a contraindication to cochlear implantation; such as a Mondini malformation (incomplete turns or partitions of the cochlea, a congenital defect). Patients who cannot tolerate general anesthesia are not candidates. Patients who have conductive hearing loss, unilateral sensorineural hearing loss, or hearing loss serviceable by hearing aids would all be better served by an alternative management plan that an otolaryngologist would be able to recommend. Cochlear implants do not fix all types of hearing loss. Thus it is imperative to have a thorough evaluation by an otolaryngologist in conjunction with an audiologist to ascertain the type and severity of hearing loss and then provide measures to correct the hearing loss.
Equipment
All cochlear implant systems have both external and internal hardware. The external equipment includes a microphone, a sound processor, and a transmission system. The internal device includes a receiver/stimulator and an electrode array.
In general, an external microphone picks up sound and/or speech in the environment and sends the information to a sound processor. The speech processor converts the mechanical vibration (sound) into an electric signal, which gets sent through the skin via radio frequency transmission to the internal receiver/stimulator. For successful transmission through the skin, the external magnet on the transmitter must align with the internal magnet on the receiver (stimulator). The receiver/stimulator takes the electrical signal and moves it to the electrode positioned within the cochlea. The electrodes provide stimulation to the auditory nerve, and the signal is sent along the auditory pathway to the auditory cortex in the brain. There are several manufacturers of cochlear implants in the United States, each with different specifications to customize the device to individual patient needs. A full review of all the different companies and models is beyond the scope of this article.
Companies: Each company has device specifications required for implantation and patient selection; please refer to these companies for device-specific details.
Personnel
An audiologist is a person who provides the audiometry test to determine the type and severity of the hearing loss. They also work closely with patients after cochlear implant insertion to provide therapy, which allows patients to receive maximum benefit from the device. They may also adjust the device to the patient’s needs. Otolaryngologists are the medical physicians who provide the diagnosis of sensorineural hearing loss and determine which patients meet the criteria for cochlear implants. Otolaryngologists also perform the surgery of cochlear implantation. There are general otolaryngology physicians, otologists, and neurotologists who may perform this surgery. Otology and neurotology require an additional 1 to 2 years of training after completing an otolaryngology residency.[rx]
Regularly scheduled follow-up is necessary for successful outcomes after cochlear implantation. The adult cochlear implant patient should have his or her external devices evaluated and programmed annually. Pediatric patients require evaluation at least twice yearly. Many children are seen more frequently than twice a year if any questions arise on how their listening skills are developing and to ensure that their devices are programmed appropriately.
Preparation
Preparation for cochlear implantation begins with a proper diagnosis of sensorineural hearing loss. History and physical exam will initiate the patient encounter. Special note to rule out secondary causes of hearing loss including tympanic membrane perforation, middle ear effusion/infection, or canal atresia. These would have to be corrected before the placement of a cochlear implant as they can affect the audiological findings that determine if patients are candidates for hearing aids. The next step is to obtain a basic audiogram with tympanometry. In children who are unable to respond appropriately to sound, an auditory brainstem response (ABR) will be necessary. The ABR is a test that transmits sound through the ear and determines if it reaches the cochlear nerve and secondary structures of the hearing apparatus by measuring the electrical potential that occurs from being stimulated. It is a useful test for both pediatric patients and to rule out hearing loss in patients who may be malingering. After there is confirmation that bilateral sensorineural hearing loss is present and meets the criteria for cochlear implantation, imaging is the next step. A solid history and physical exam may reveal findings that warrant further investigations, and referrals to rule out coexisting abnormalities. In pediatric patients deemed to have profound sensorineural hearing loss evaluation by geneticists should be a consideration.[rx]
Typically a CT of the temporal bones without contrast and an MRI of the internal auditory canals with and without contrast is obtained, however one or the other would suffice.[rx] These images determine the presence of the cochlear nerve; also, it gives important anatomic detail regarding surgical anatomy if inserting a cochlear implant.[rx] Prior to placing a cochlear implant, a trial of hearing amplification should take place. In newborns, it is generally recommended to have hearing aids by the age of 6 months with a trial of 6 months before pursuing a cochlear implant. In adults, the period is shorter (1 to 3 months), and the benefit of hearing aids can be analyzed with a repeat audiometric battery. If patients have failed amplification measures and are candidates for hearing aids, then a discussion of the risks and benefits, as well as alternatives to the surgery should take place. After appropriate informed consent is obtained, a patient may schedule for surgery.[rx] The CDC recommends vaccinations against Streptococcus pneumoniae including PCV13 and PPSV23 (for those over the age of two). PCV 13 is safe for children under the age of two years old; this has been shown to reduce the risk of meningitis associated with cochlear implants.[rx]
In the United States, insurance companies do not always cover bilateral cochlear implants, causing difficulty for the cochlear implant team in determining which ear to place a cochlear implant. Generally, the ear with the shortest duration of deafness, the worse hearing ear, and the handedness of a patient for device manipulation may be factors to help determine appropriate implantation. If there are no audiometric differences between ears, then choose the better surgical ear. Some studies suggest that it may not matter which ear receives the implant (better or worse hearing ear), but the intervention of a cochlear implant will be of great benefit when the patients meet criteria.[rx] Factors to consider are an ideal mastoid to drill, the side with the least ossification or fibrosis in the cochlea, and the side with normal anatomy.[rx][rx]
Technique
A cochlear implant takes place in an operating room at a hospital or surgical center. A team of nurses, anesthesiologists, and the otolaryngology team will see the patient on the day of surgery and discuss their role in the surgery. After addressing questions and concerns and obtaining consent, the patient will proceed to the operating theatre. The room will be sterile. A timeout will be performed to assure the proper patient and surgery. Anesthesia will then be initiated, typically a general anesthetic by endotracheal tube. Nerve monitoring is often used to monitor the facial nerve during surgery. The patient will undergo prepping in a sterile fashion, and sterile gowning and gloving takes place.
At this time, the surgery may commence. A standard mastoidectomy is performed with exposure of the facial recess using a mastoid drill per surgeon preference. The facial recess is an area bounded by the incudal bar, facial nerve (mastoid segment) and chorda tympani.[rx] This area should provide adequate visualization of the round window of the cochlea. The operating surgeon may perform a cochleostomy if necessary. The processor is then slipped underneath the temporal fascia, or a bony well may be drilled to secure the device. The electrode is inserted after the opening of the round window or cochleostomy and inserted per manufacturer recommendations. At the time of implant, an audiologist or device representative should be present to test for proper device alignment within the cochlea. An X-ray is taken to confirm placement before the closure of the skin. Layered closure follows in a cosmetically pleasing fashion.[rx]
Complications
Complications include [rx][rx]:
- Bleeding, including life-threatening bleeding
- Stroke
- Infection
- Increased risk for meningitis
- Pain
- Skin breakdown overlying the area of the magnet
- Device failure including broken portions of the device failed device or improper placement in the cochlea
- Skull base damage
- Trauma to the brain
- Cerebrospinal fluid leaks
- Facial nerve paralysis/paresis
- Loss of taste on the ipsilateral side of the tongue
- Death
- Dizziness/vertigo
- Loss of residual hearing in select populations
- Complete deafness
What does the future hold for cochlear implants?
The NIDCD supports research to enhance the benefits of cochlear implants. Scientists are exploring whether using a shortened electrode array, inserted into a portion of the cochlea, for example, can help individuals whose hearing loss is limited to the higher frequencies while preserving their hearing of lower frequencies. Researchers also are looking at the potential benefits of pairing a cochlear implant in one ear with either another cochlear implant or a hearing aid in the other ear.
