The cochlear implant is a surgically placed device that converts sound to an electrical signal. This electrical signal is transmitted via electrodes to the spiral ganglion cells in the cochlear modiolus.
Although individual responses to cochlear implants are highly variable and depend on a number of physical and psychosocial factors, the trend toward improved performance with increasingly sophisticated electrodes and programming strategies has dramatically expanded indications for cochlear implantation. Although cochlear implants originally were touted as an aid to speech reading for individuals with profound hearing impairment, a growing population of implanted patients are exceeding their preoperative hearing performance (which was aided with conventional hearing aids). Due to the overall success of cochlear implants and ongoing advances in performance, an article addressing indications for cochlear implantation attempts to describe a target that moves almost yearly.
Hearing loss is one of the most common sensory impairments and affects 28 million Americans. Approximately 1-3 out of 1000 newborns has hearing impairment. The elderly are more commonly affected with 40-50% of people over age 75 having hearing loss. Depending on the degree of hearing loss, many affected individuals can be successfully fitted with hearing aids. For patients with hearing loss that is not mitigated with hearing aids, a cochlear implant may provide an opportunity for hearing.
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, which picks up sound from the environment.
A speech processor, which selects and arranges sounds picked up by the microphone.
A transmitter and receiver/stimulator, which receive signals from the speech processor and convert them into electric impulses
An electrode array, which 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. An estimated 85,000 patients worldwide have received cochlear implants. However, this number represents only a small number of individuals with hearing impairment who may potentially benefit from implantation (an estimated 250,000 people). Many candidates for cochlear implants often do of financial reasons
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 and to get used to it. However, it allows many people to recognize warning signals, understand other sounds in the environment, and enjoy a conversation in person or by telephone.
Throughout the 1970s, the Food and Drug Administration (FDA) recommended that devices be implanted only in adults with profound hearing loss. In 1980, the FDA allowed children at least 2 years of age to be implanted. The age limit has recently been lowered to 12 months for all 3 devices available in the United States (Advanced Bionics, Med-El, and Cochlear Corporation devices). Over time, indications have been broadened to include adults with severe hearing loss who may achieve some benefit from conventional amplification.
Children and adults who are deaf or severely hard-of-hearing can be fitted with a cochlear implants. According to the Food and Drug Administration (FDA), as of April 2009, approximately 188,000 people worldwide have received implants. In the United States, roughly 41,500 adults and 25,500 children have received them. In India more than 1000 people have received implants.
Adults who have lost all or most of their hearing later in life often can benefit from cochlear implants. They learn to associate the signal provided by an implant with sounds they remember. This often provides recipients with the ability to understand speech solely by listening through the implant, without requiring any visual cues such as those provided by lipreading or sign language.
Cochlear implants, coupled with intensive postimplantation therapy, can help young children to acquire speech, language, and social skills. Most children who receive implants are between two and six years old. Early implantation provides exposure to sounds that can be helpful during the critical period when children learn speech and language skills. In 2000, the FDA lowered the age of eligibility to 12 months for one type of cochlear implant.
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. Prior to implantation, all of these factors need to be considered.
The best age for implantation is still being debated but research has clearly indicated that children who receive hearing early have the best results. The FDA, in 2002, lowered the age for inclusion in pediatric clinical trials to 12 months. However, many centers will implant children as early as six months if there is certainty as to the audiologic indications. In addition, the child must weigh about 8 kg, which is important from an anesthesia viewpoint. Another indication for early implantation is bacterial meningitis causing hearing loss because the cochlea (the organ of hearing) will sometimes scar after meningitis, preventing implantation.
The evaluation process is a team approach. Obtaining accurate audiological information is at the core of making appropriate recommendations related to cochlear implant candidacy. It is important that the testing be done by a specially trained pediatric audiologist who is experienced in fitting and facilitating preimplant hearing aid use. The team also includes speech and language specialists, a social worker, an educational consultant and, of course, the operating surgeon.
While assessing the patients following points need to be noted:
Audiometric:is the hearing loss of appropriate degree and are hearing aids not sufficient?
Speech and Language: what is the level of the child’s present speech and language and what postoperative interventions are necessary?
Social Work: to assure there is a stable family support mechanism and realistic expectations as to outcome
Educational: to plan for appropriate school placement to maximize learning
Medical: to be sure there is no contraindication to surgery such as chronic ear infections or abnormal inner ear anatomy
Cochlear implantation is a collaborative effort involving patients, families, schools, audiologists, speech/hearing therapists, and surgeons. A patient with hearing impairment does not have a surgical problem that responds to the simple intervention of an implant surgeon. Because preoperative expectation affects the patient’s postoperative satisfaction and use of the implant, all patients and families require attention and counseling from an implant team before they embark on the life-changing journey of cochlear implantation.
The human ear is capable of hearing frequencies from 20-20,000 Hz. Pure-tone audiometry is used to assess a subject’s response to a frequency at a specific intensity measured in decibels. In most cases, frequencies from 250 Hz to 8000 Hz are assessed, as these are most important for speech perception.
Although a number of speech-recognition tests are currently used for different reasons, one of the most common speech-recognition tests is the hearing in noise test (HINT), which tests speech recognition in the context of sentences. In determining cochlear implant candidacy, HINT is performed without background noise, despite its name. HINT measures word-recognition abilities to assess the patient’s candidacy for cochlear implantation, in conjunction with conventional pure-tone and speech audiometry. The HINT consists of 25 equivalent 10-sentence lists that may be presented in quiet or noise to assess the patient’s understanding of sentences.
As noted earlier, when used to assist in the determination of cochlear implant candidacy, the HINT is currently performed in quiet. (HINT sentences are found in the Minimum Speech Perception Test Battery for Adult Cochlear Implant Users CD.) Administer the first test in quiet by using 2 lists of 10 sentences, which are scored for the number of words correctly identified.
For adults and children who can respond reliably, standard pure-tone and speech audiometry tests are used to screen likely candidates. For children aged 12-23 months, the pure-tone average (PTA) for both ears should equal or exceed 90 dB. For individuals older than 24 months, the PTA for both ears should equal or exceed 70 dB. If the patient can detect speech with best-fit hearing aids in place, a speech-recognition test in a sound field of 55-dB HL sound pressure level (SPL) is performed. A number of speech recognition tests are currently in use.
Current US Food and Drug Administration (FDA) guidelines permit implantation in patients whose open-set sentence recognition (eg, HINT) is 60% or less in the best-aided condition. However, for patients receiving Medicare benefits, the current cutoff for cochlear implant candidacy is a HINT score of 40% or less. For Medicare patients enrolled in an acceptable clinical trial or study, the cutoff is 60% or less. Guidelines for other third-party payers may vary and should be consulted.
In children with prelingual deafness, cochlear implant candidacy is established when auditory skills fail to develop after amplification and aural rehab over a 3-month time period. Progress is typically monitored with the Meaningful Auditory Integration Scale or the Early Speech Perception test. Other scales of auditory benefit may be used in older children.
Imaging with CT or MRI is performed prior to implantation to evaluate the inner ear, facial nerve, cochleovestibular nerve, brain, and brainstem. MRI may reveal hypoplasia or aplasia of the cochleovestibular nerve, whereas CT may show a narrow internal auditory canal or absence of the bony cochlear nerve canal at the modiolus. Inner ear malformations ranging from the rare cases of cochlear aplasia to the more common enlarged vestibular aqueduct are easily visualized on CT or MRI. Such results may alter the choice of side of implantation or raise other issues such as electrode selection.
Patients with cochlear malformations are still candidates for cochlear implantation, but they may require a different type of electrode, a different surgical approach (ie, drill out), and may be more at risk for meningitis or cerebrospinal fluid gusher. Surgeons should be prepared for a cerebrospinal fluid leak caused by incomplete partition between the cochlea and the internal auditory canal. Absence of the cochlea or the cochlear nerve can usually be confirmed with imaging and are absolute contraindications for cochlear implantation.
Clinicians must take extra caution to identify the cochleovestibular nerve complex; auditory stimulation with a cochlea implant in patients with supposed cochlear nerve aplasia has been reported. These cases may be more appropriately labeled cochlear nerve hypoplasia. Families and patients must be adequately counseled and fully informed about the variable performance of patients with dysplastic cochleae and the potential risk of cerebrospinal fluid leakage and meningitis.
In pediatric or young adult patients with progressive hearing loss, exclude neurofibromatosis II by performing MRI before proceeding with implantation. MRI has more recently become the imaging study of choice at some institutions because the inner ear, cochleovestibular nerve, brain, and brainstem can all be visualized. MRI, unlike CT scanning, is also very useful in identifying early labyrinthitis ossificans, which typically begins with endoluminal fibrosis of the scala tympani at the basal turn.
In addition to the purely audiologic criteria discussed above, pediatric candidates must be enrolled in an educational program that supports listening and speaking with aided hearing. For patients of all ages, no medical contraindications (eg, cochlear or auditory nerve aplasia, active middle-ear infection) may be present. Communication among patients, families, schools, audiologists, therapists, and surgeons is required.
An age-appropriate pneumococcal vaccination series should be completed at least 2 weeks before surgery.
Adults who have lost all or most of their hearing later in life can often benefit from cochlear implants. These older candidates can often associate the sounds made through an implant with sounds they remember. This may help them to understand speech without visual cues or systems such as lip reading or sign language.
Many young children can also be candidates for implants. Cochlear implants, coupled with intensive post-implantation therapy, can help young children to acquire speech, language, developmental and social skills. The best age for implantation is still being debated, but it appears that earlier implantation performs better. Most children who receive implants are between two and six years old. Because of research conducted at Beth Israel/New York Eye and Ear Cochlear Implant Center and other centers, infants as young as seven months old can now be implanted with cochlear devices.
The surgery for the insertion of a cochlear implant requires an overnight hospital stay. After the operative wound has healed the implanted individual returns approximately one month later for the initial fitting of the speech processor and “turning on” of the device. Subsequent visits allow finer tuning of the processor.
Individuals who receive a cochlear implant require continual follow up. Children, in particular, require a long period of rehabilitation to teach them to listen to the new sounds and to optimally tune the device. All children who receive cochlear implants can learn to hear the everyday sounds we take for granted. Most children are able to hear conversation without lip-reading and use spoken language for everyday communication. More than 75% of the adults are able to use the telephone for conversation.
Our approach is particularly unique with children in that we believe that follow-up by our educational consultants from the implant team is crucial to a child’s long-term success. The Cochlear Implant Center serves children throughout a wide geographical area.
With advancements in technology and continued follow-up studies with people who already have received implants, researchers are evaluating how cochlear implants might be used for other types of hearing loss.