COCHLEAR IMPLANTS

A cochlear implant (CI) is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing.

Cochlear implants may help provide hearing in patients who are deaf because of damage to sensory hair cells in their cochleas. In those patients, the implants often can enable sufficient hearing for better understanding of speech. The quality of sound is different from natural hearing, with less sound information being received and processed by the brain. However, many patients are able to hear and understand speech and environmental sounds. Newer devices and processing-strategies allow recipients to hear better in noise, enjoy music, and even use their implant processors while swimming.

As of December 2012, approximately 324,000 people worldwide have received cochlear implants; in the U.S., roughly 58,000 adults and 38,000 children are recipients. The vast majority are in developed countries due to the high cost of the device, surgery and post-implantation therapy. A small but growing segment of recipients have bilateral implants for hearing stereo sound (one implant in each cochlea).

Candidates

There are a number of factors that determine the degree of success to expect from the operation and the device itself. Cochlear implant centers determine implant candidacy on an individual basis and take into account a person's hearing history, cause of hearing loss, amount of residual hearing, speech recognition ability, health status, and family commitment to aural habilitation/rehabilitation.

A prime candidate is described as:

• having severe to profound sensorineural hearing loss in both ears.
• having a functioning auditory nerve
• having lived at least a short amount of time without hearing (approximately 70+ decibel hearing loss, on average)
• having good speech, language, and communication skills, or in the case of infants and young children, having a family willing to work toward speech and language skills with therapy
• not benefitting enough from other kinds of hearing aids, including latest models of high power hearing instruments and FM systems
• having no medical reason to avoid surgery
• living in or desiring to live in the "hearing world"
• having realistic expectations about results
• having the support of family and friends
• having appropriate services set up for post-cochlear implant aural rehabilitation (through a speech language pathologist, deaf educator, or auditory verbal therapist).

Type of hearing loss

People with mild or moderate sensorineural hearing loss are generally not candidates for cochlear implantation. Their needs can often be met with hearing aids alone or hearing aids with an FM system. After the implant is put into place, sound no longer travels via the ear canal and middle ear but will be picked up by a microphone and sent through the device's speech processor to the implant's electrodes inside the cochlea. Thus, most candidates have been diagnosed with a severe or profound sensorineural hearing loss.

The presence of auditory nerve fibers is essential to the functioning of the device: if these are damaged to such an extent that they cannot receive electrical stimuli, the implant will not work. Some individuals with severe auditory neuropathy may also benefit from cochlear implants.

Age of recipient

Post-lingually deaf adults, pre-lingually deaf children and post-lingually hard of hearing people (usually children) who have lost hearing due to diseases such as CMV and meningitis, form three distinct groups of potential users of cochlear implants with different needs and outcomes. Those who have lost their hearing as adults were the first group to find cochlear implants useful, in regaining some comprehension of speech and other sounds. The outcomes of individuals that have been deaf for a long period of time before implantation are sometimes astonishing, although more variable.

The risk of surgery in the older patient must be weighed against the improvement in quality of life. As the devices improve, particularly the sound processor hardware and software, the benefit is often judged to be worth the surgical risk, particularly for the newly deaf elderly patient.

Another group of customers are parents of children born deaf who want to ensure that their children grow up with good spoken language skills. The brain develops after birth and adapts its function to the sensory input; absence of this has functional consequences for the brain, and consequently congenitally deaf children who receive cochlear implants at a young age (less than 2 years) have better success with them than congenitally deaf children who first receive the implants at a later age, though the critical period for utilizing auditory information does not close completely until adolescence. One doctor has said "There is a time window during which they can get an implant and learn to speak. From the ages of two to four, that ability diminishes a little bit. And by age nine, there is zero chance that they will learn to speak properly. So it’s really important that they get recognized and evaluated early." Additionally, there is another sensitive period for strong asymmetry in hearing that closes earlier in life. In cases of early single-sided deafness or unilateral cochlear implantation the brain reorganizes towards the hearing ear and that puts the deaf ear into a disadvantage. Consequently, the benefit of a sequential second implantation (on the deaf ear) is critically dependent on the delay between implantations. Periods of asymmetric hearing during early childhood should be avoided and therapy of deafness should be binaural.

The third group who will benefit substantially from cochlear implantation are post-lingual subjects who have lost hearing: a common cause is childhood meningitis. Young children (under five years) in these cases often make excellent progress after implantation because they have learned how to form sounds, and only need to learn how to interpret the new information in their brains.

The operation, post-implantation therapy and ongoing effects

The device is surgically implanted under a general anesthetic or local anesthetic without or with sedation, and the operation usually takes from 1½ to 5 hours. First a small area of the scalp directly behind the ear may be shaved and cleaned. Then an incision is made in the skin behind the ear and the surgeon drills into the mastoid bone, creating a pocket for the receiver/stimulator, and then into the inner ear where the electrode array is inserted into the cochlea. The patient normally goes home the same day or the day after the surgery, although some cochlear implant recipients stay in the hospital for 1 to 2 days. As with every medical procedure, the surgery involves a certain amount of risk; in this case, the risks include skin infection, onset of (or change in) tinnitus, damage to the vestibular system, and damage to facial nerves that can cause muscle weakness, impaired facial sensation, or, in the worst cases, facial paralysis. There is also the risk of device failure, usually where the incision does not heal properly. This occurs in 2% of cases and the device must be removed. The operation also destroys some or all of the residual hearing the patient may have in the implanted ear because of the shaving of hair cells in the cochlea; as a result, some doctors advise single-ear implantation, saving the other ear in case a biological treatment becomes available in the future.

After 1–4 weeks of healing (the wait is usually longer for children than adults), the implant is "activated" by connecting an external sound processor to the internal device via a magnet. Initial results vary widely, and post-implantation therapy is required as well as time for the brain to adapt to hearing new sounds. In the case of congenitally deaf children, audiological training and speech therapy typically continue for years, though infants can become age appropriate—able to speak and understand at the same level as a hearing child of the same age. The participation of the child's family in working on spoken language development is considered to be even more important than therapy, because the family can aid development by participating actively—and continually—in the child's therapy, making hearing and listening interesting, talking about objects and actions, and encouraging the child to make sounds and form words. Professionals trained to work with children who have received cochlear implants are a major part of the parent-professional team when addressing the task of teaching children to use their hearing to develop speech and spoken language. These professionals include, but are not limited to:

• Speech-Language Pathologists (SLP)
• Certified Auditory-Verbal Therapists (LSLS Cert. AVT)
• Pediatric Audiologist (AuD)
• Teacher of the Deaf (ToD) with a specialization in Oral Deaf Education

Some users, audiologists, and surgeons also report that when there is an ear infection causing fluid in the middle ear, it can affect the cochlear implant, leading to temporarily reduced hearing.

The implant has a few effects unrelated to hearing. Manufacturers have cautioned against scuba diving due to the pressures involved, but the depths found in normal recreational diving appear to be safe. The external components must be turned off and removed prior to swimming or showering, except for users of the Advanced Bionics Neptune processor, which is waterproof. In most cases, certain diagnostic tests such as magnetic resonance imaging (MRI) cannot be used on patients with cochlear implants without first removing the small internal magnet (an outpatient procedure usually performed with a local anesthetic), but some implants are now FDA approved for use with certain strengths of MRI machine. Large amounts of static electricity can cause the device's memory to reset. For this reason, children with cochlear implants are also advised to avoid plastic playground slides. The electronic stimulation the implant creates appears to have a positive effect on the nerve tissue that surrounds it.

Cost

In the India, medical costs run from INR 2,50,000.00 to 5,00,000; this includes evaluation, the surgery itself, hardware (device), hospitalization and rehabilitation. Some or all of this may be covered by health insurance. In the United Kingdom, the NHS covers cochlear implants in full, as does Medicare in Australia, and the Department of Health in Ireland, Seguridad Social in Spain and Israel, and the Ministry of Health or ACC (depending on the cause of deafness) in New Zealand. According to the US National Institute on Deafness and Other Communication Disorders, the estimated total cost is $60,000 per person implanted.

A study by Johns Hopkins University determined that for a three-year-old child who receives cochlear implants can save $30,000 to $50,000 in special-education costs for elementary and secondary schools as they are more likely to be mainstreamed in school and thus use fewer support services than similarly deaf children.

Efficacy

A cochlear implant will not cure deafness, but is a prosthetic substitute for hearing. Some recipients find them very effective, others somewhat effective and some feel worse overall with the implant than without. For people already functional in spoken language who lose their hearing, cochlear implants can be a great help in restoring functional comprehension of speech, especially if they have only lost their hearing for a short time.

Individuals who have acquired deafblindness (loss of hearing and vision combined) may find cochlear implants a radical improvement in their daily lives. It may provide them with more information for safety, communication, balance, orientation and mobility and promote interaction within their environment and with other people, reducing isolation. Having more auditory information than they may be familiar with may provide them with sensory information that will help them become more independent.

British Member of Parliament Jack Ashley received a cochlear implant in 1994 at age 70 after 25 years of deafness, and reported that he has no trouble speaking to people he knows; whether one on one or even on the telephone, although he might have difficulty with a new voice or with a busy conversation, and still had to rely to some extent on lip reading. He described the robotic sound of human voices perceived through the cochlear implant as "a croaking Dalek with laryngitis". Another recipient described the initial sounds as similar to radio static and voices as being cartoonish, though after a year with the implant she said everything sounded right. Even modern cochlear implants have at most 22 electrodes to replace the 16,000 delicate hair cells that are used for normal hearing. However, the sound quality delivered by a cochlear implant is often good enough that many users do not have to rely on lip reading in quiet conditions. In noisy conditions however, speech understanding often remains poor.

Adults who have grown up deaf can find the implants ineffective or irritating. This relates to the specific pathology of deafness and the time frame. Adults who are born with normal hearing and who have had normal hearing for their early years and who have then progressively lost their hearing tend to have better outcomes than adults who were born deaf. This is due to the neural patterns laid down in the early years of life, which are crucially important to speech perception. Cochlear implants cannot overcome such a problem. Some who were orally educated and used amplifying hearing aids have been more successful with cochlear implants, as the perception of sound was maintained through use of the hearing aid. Daily exercises such as side by side tracking, listening to audio books while reading a print book, synthetic training, and analytic training can improve the efficacy of an implant through practice.

Children without a working auditory nerve may be helped with a cochlear implant, although the results may not be optimal. Patients without a viable auditory nerve are usually identified during the candidacy process. Fewer than 1% of deaf individuals have a missing or damaged auditory nerve, which today can be treated with an auditory brainstem implant. Research published in 2005 has suggested that children and adults can benefit from bilateral cochlear implants in order to aid in sound localization and speech understanding, and a 2011 study indicated that the language skills of children with two implants were within the normal range for the age.

Efficacy of cochlear implantation in the setting of otosclerosis has been demonstrated

Risks and disadvantages

Some effects of implantation are irreversible; while the device promises to provide new sound information for a recipient, the implantation process inevitably results in shaving of the hair cells within the cochlea, which can result in a permanent loss of some or all residual natural hearing. While recent improvements in implant technology, and implantation techniques, promise to minimize such damage, the risk and extent of damage still varies. The goal of new implantation techniques is to reduce the risk of infection, operating time, and complications while improving the patient's ability to hear. Such improvements of implantation techniques include preserving low frequency hearing and using minimal invasive surgery to better secure the device. Still, the cause of deafness is not always identified before the surgery. It is possible but rare that the surgery does not restore hearing at all.

The United States Food and Drug Administration reports that cochlear implant recipients may be at higher risk for meningitis. A study of 4,265 American children who received implants between 1997 and 2002 concluded that recipient children had a risk of pneumococcal meningitis more than 30 times greater than that for children in the general population.^[49] A later, UK-based, study found that while the incidence of meningitis in implanted adults was significantly higher than the general population, the incidence in children was no different from the general population. As a result, the Centers for Disease Control and Prevention and the Food and Drug Administration both recommend that would-be implant recipients be vaccinated against meningitis prior to surgery.

Rarely, necrosis has been observed in the skin flaps surrounding cochlear implants. Hyperbaric oxygen has been shown to be a useful adjunctive therapy in the management of cochlear implant flap necrosis.

As the location of the cochlea is close to the facial nerve, there is a risk that the nerve may be damaged during the operation. The incidence of the damage is infrequent.

There are strict protocols in choosing candidates to avoid risks and disadvantages. A battery of tests is performed to make the decision of candidacy easier. For example, some patients suffer from deafness medial to the cochlea - typically vestibular schwannomas. Implantation into the cochlea has a low success rate with these people, as the artificial signal does not have a healthy nerve to travel along. Historically, patients with severe congenital anatomic anomalies of the cochlea were considered poor candidates for cochlear implantation. Many studies since the 1980s have demonstrated successful hearing outcomes after CI in this group. Blake Papsin et al. in 2005 published the largest series of patients with cochleovestibular anomalies undergoing implantation and found no significant difference in outcomes versus patients with normal anatomy. Michael Pakdaman et al. in 2012 presented a systematic review of studies reviewing cochlear implantation in anomalous inner ears and found increased surgical difficulty and lower speech perception among patients with more severe inner ear dysplasia. With careful selection of candidates, the risks of implantation are minimized.

Functionality

The implant works by using the tonotopic organization of the basilar membrane of the inner ear. "Tonotopic organization", also referred to as a "frequency-to-place" mapping, is the way the ear sorts out different frequencies so that our brain can process that information. In a normal ear, sound vibrations in the air lead to resonant vibrations of the basilar membrane inside the cochlea. High-frequency sounds (i.e. high pitched sounds) do not pass very far along the membrane, but low frequency sounds pass farther in. The movement of hair cells, located all along the basilar membrane, creates an electrical disturbance that can be picked up by the surrounding nerve cells. The brain is able to interpret the nerve activity to determine which area of the basilar membrane is resonating, and therefore what sound frequency is being heard.

In individuals with sensorineural hearing loss, hair cells are often fewer in number and/or damaged. Hair cell loss or absence may be caused by a genetic mutation or an illness such as meningitis. Hair cells may also be destroyed chemically by an ototoxic medication, or simply damaged over time by excessively loud noises. The cochlear implant bypasses the hair cells and stimulates the cochlear nerves directly using electrical impulses. This allows the brain to interpret the frequency of sound as it would if the hair cells of the basilar membrane were functioning properly (see above).

Transmitter

This is used to transmit the processed sound information over a radio frequency link to the internal portion of the device. Radio frequency is used so that no physical connection is needed, which reduces the chance of infection and pain. The transmitter attaches to the receiver using a magnet that holds through the skin.

Receiver

This component receives directions from the speech processor by way of magnetic induction sent from the transmitter. (The receiver also receives its power through the transmission.) The receiver is also a sophisticated computer that translates the processed sound information and controls the electrical current sent to the electrodes in the cochlea. It is embedded in the skull behind the ear.

Electrode array

The electrode array is made from a type of silicone rubber, while the electrodes are platinum or a similar highly conductive material. It is connected to the internal receiver on one end and inserted into the cochlea deeper in the skull. (The cochlea winds its way around the auditory nerve, which is tonotopically organized as is the basilar membrane). When an electrical current is routed to an intracochlear electrode, an electrical field is generated and auditory nerve fibers are stimulated.

In the devices manufactured by Cochlear Ltd, two electrodes sit outside the cochlea and act as grounds—one is a ball electrode that sits beneath the skin, while the other is a plate on the device. This equates to 24 electrodes in the Cochlear-brand 'nucleus' device, 22 array electrodes within the cochlea and 2 extra-cochlear electrodes.

Insertion depth is another important factor. The mean length of human being cochlea is 33–36 millimetres (1.3–1.4 in), due to some physical limitation, the implants don't reach to the apical tip when inserted but it may reach up to 25 millimetres (0.98 in) which corresponds to a tonotopical frequency of 400–6000 Hz. MED-EL produces deep insertion implants that can be inserted up to a tonotopical frequency of 100 Hz (according to Greenwood frequency to position formula in normal hearing), but the distance between the electrodes is about 2.5 millimetres (0.098 in), while in the Nucleus Freedom from Cochlear Ltd is about 0.7 millimetres (0.028 in). There is a strong research in this direction and the best sounding implant can be subjective from patient to patient.

Speech processors

Speech processors are the components of the cochlear implant that transforms the sounds picked up by the microphone into electronic signals capable of being transmitted to the internal receiver. The coding strategies programmed by the user's audiologist are stored in the processor, where it codes the sound accordingly. The signal produced by the speech processor is sent through the coil to the internal receiver, where it is picked up by radio signal and sent along the electrode array in the cochlea.

There are primarily two forms of speech processors available. The most common kind is called the "behind-the-ear" processor, or BTE. It is a small processor that is worn on the ear, typically together with the microphone. This is the kind of processor used by most adults and older children. Babies and small children wear either a "baby" BTE (pinned or clipped to the collar) or the body-worn processor, which was more common in previous years. Today's tiny processors can often take the place of bulky body-worn processors. MED-EL and Cochlear brands both carry a "baby BTE" configurations.