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Cochlear Implants

Description

Damage to the inner ear resulting in faulty transmission of auditory information to the brain is referred to as sensorineural hearing loss; damage which is often permanent. One of the more promising treatments is the cochlear implant, an electronic device that provides a sense of sound to people who would have little or no benefit from hearing aids.

A cochlear implant is surgically placed in the inner ear and activated by a device worn outside the ear. The implant functions similar to that of an artificial inner ear, taking over the job of the cochlea. The cochlea then translates sounds into electrical signals and sends them to the brain for interpretation. The implant directly stimulates the auditory nerve to send information to the brain.

The first research on cochlear implants began in the late 1950s when scientists began to experiment with ways to compensate for the damaged hair cells. Since then, cochlear implant technology has continually improved with approximately 70,000 people worldwide receiving implants, many of them children.

Although a cochlear implant does not restore normal hearing, it can dramatically improve the ability to hear and to understand speech. Benefits from the surgery vary from one individual to another, but most find that the implant allows them to handle such tasks as talking on the telephone. After a few months of wear, the user usually finds that other voices begin to sound more natural. For children, implants can help them acquire speech, language, and other essential developmental skills.

A cochlear implant is very different from a hearing aid, which amplifies sounds and delivers them to the ear canal. An implant does not make sounds louder. Instead, it compensates for damaged or nonworking parts of the inner ear, identifying useful sound information and translating this information into a form that the brain can understand.

Normally, the inner ear converts incoming vibrations from the middle ear into electrical impulses. The delicate hair cells stimulate the auditory nerve to send the electrical impulses to the brain. The brain recognizes the impulses as sound, unless the hair cells are damaged. In this case, they are unable to stimulate the auditory nerve. Although many nerve fibers may remain intact and can still transmit electrical impulses, these fibers are unresponsive because of the hair cell damage.

In people with mild or moderate hearing loss, sounds that are amplified by a hearing aid are converted into electrical impulses by the hair cells that are not damaged, in the same way that sounds are transmitted in a normal-hearing ear. But if there is profound sensorineural hearing loss, extensive hair cell damage prevents the ears from processing the auditory information, no matter how loud a hearing aid might amplify the sound.

Cochlear implants bypass the hair cells and stimulate the surviving nerve fibers in the cochlea. These fibers send electrical signals through the auditory nerve to the brain, allowing the perception of sound.

How Implants Function

Several different cochlear implant systems are available, but all work by identifying sounds in the environment electronically and sending the impulses to the brain. The implant is not a single unit but has both internal and external components. The external parts consist of a microphone, speech processor, transmitter, and connecting cords. The internal components are a receiver and electrodes.

These parts work together as follows:

• The microphone picks up sounds. It is located in a headset or case worn behind the ear, similar to a behind-the-ear hearing aid.
• A thin connecting cord carries sounds from the microphone to the speech processor, a small but powerful computer that digitally converts the sounds into coded electrical impulses. The coded impulses contain information about the frequency and loudness of sounds. Speech processors generally come in two styles. One is about the size of a pager and can be worn on a belt or in a pocket. The other is small enough to fit behind the ear and may be part of the same headset or case that contains the microphone.
• Coded impulses are sent to a transmitter, sometimes called a transmitting coil. A magnet holds the transmitter in place behind the ear, directly over the receiver that is implanted beneath the scalp.
• The transmitter relays the coded impulses as radio waves through the skin to the receiver. The receiver relays the signals to an array of electrodes threaded directly into the cochlea on a bundle of tiny wires.
• The electrodes stimulate nerve fibers in the cochlea that trigger the creation of electrical impulses. This information is sent to the auditory nerve and on to the brain for interpretation.

The process may sound complicated, but it all happens very quickly. The length of time between when the microphone picks up a sound to when the brain receives the information is just a few thousandths of a second. .

Criteria

Cochlear implants are not alternatives to hearing aids and help only those who cannot receive any benefit from hearing aids. Adults and children who are candidates for cochlear implants typically have severe to profound sensorineural hearing loss in both ears or have great difficulty understanding speech.

The best age for children is still being debated, but most who receive implants are between the ages of one and six. The younger the child at the time of implantation, the less delay there will be in the speech and language development – as long as there is appropriate therapy and education after the implantation.

Among adults, there is no upper age limit. Several studies have shown that people over the age of 65 can experience excellent results. The duration of hearing loss is the foremost predictor of any success with the implants: the shorter the duration, the better the results. The decision to receive an implant should be made only after talking to a cochlear implant audiologist and an experienced cochlear implant surgeon.

As well as having some degree of severe hearing loss, the best candidates for implants must have the following:

• Realistic expectations, that is, a clear understanding of the benefits and limitations of a cochlear implant;
• Willingness and ability to make a time commitment for the pre-implant evaluations and postsurgical follow-up services;
• Motivation, along with the support of family and friends to be a part of the hearing world.

Opposition

It may come as a surprise to know that many people in the deaf community strongly object to cochlear implants since many are often content in their unique culture. The deaf community usually includes a shared sign language, social customs and lifestyle, literature, art, and political, economic, and recreational organizations. However, not all people who are deaf participate in this culture and, for them, implants are a viable option.

For many in the deaf community, deafness is not regarded as a disorder to be altered. They have an especially negative reaction to implantation in children who are born deaf. Some parents have reported dealing with unfavourable comments and adverse reactions if they choose an implant for their child. However, some headway is being made in reconciling the two perspectives. Many are now recognizing the value of being fluent in both worlds – that is, continuing to use sign language and remaining part of the deaf culture while also participating in the larger hearing world.

Testing

The condition of the auditory nerve fibers will play a factor in the success of a cochlear implant. People with a greater number of functioning nerve fibers in the cochlea may benefit more from an implant. Although no test can determine the number or location of surviving fibers, such tests as magnetic resonance imaging (MRI) can indicate whether the cochlea can accommodate implant electrodes.

An otolaryngologist (ENT doctor), performs cochlear implant surgeries, although not all perform the procedure. Before proceeding with the implantation, several tests will be performed by an implant team, which includes an otolaryngologist and an audiologist. Tests will include:

• Otologic examination: An ENT will perform a medical exam involving the outer, middle, and inner ear to ensure that no active infection or any type of abnormality exists that would void the use of a cochlear implant. This exam will also determine if the patient can safely undergo general anesthsia.
• Imagery examinations: includes X-rays, CT scans (computerized tomography), and/or MRIs (magnetic resonance imagery) to see if the cochlea is suitable for inserting implant electrodes.
• Audiologic evaluation: includes extensive hearing tests to determine how much can be heard without a hearing aid. At the same time, hearing, speech, and language tests are conducted to establish a baseline of information for comparison with tests following implantation.
• Psychological examinations: will determine if the patient can cope with the implant. They will also examine issues that could affect adjustment to and satisfaction with an implant.

Once all the tests prove satisfactory, the surgery will be scheduled.

Surgical Procedure

Implant surgery is performed under general aneshetic and lasts from one to three hours. The procedure may also be done on an outpatient basis.

After anesthesia is administered, the surgeon will proceed as follows:

• An incision will be made behind the ear and a small depression in the skull behind the mastoid bone will be made. This will be where the receiver is placed.
• A second incision in the mastoid bone opens up the middle ear.
• A tiny hole is made in the cochlea and the electrodes are inserted.
• A few electronic tests are performed to make sure the device is functioning properly before the incisions are closed.

Bandages are usually removed a day or so after surgery. Complete healing takes about 4-6 weeks and, during this time, the implant will not be activated. Activation and programming will be done only after the surgical site heals completely.

Activating the Implant

When the healing process is complete, the patient returns to the cochlear specialist for the fitting of the external components and mapping the speech processor.

During the initial session, a headset or case containing the microphone is placed on the patient’s head and a transmitter is positioned on the side of the head. It is held in place by a magnet that couples with a magnet in the implanted receiver. The speech processor is connected to the microphone and to the audiologist’s computer.

One by one, implanted electrodes embedded in the cochlea are turned on. Each one carries a slightly different frequency and the patient will be asked to respond to the sound, indicating how loud it is. The audiologist uses these measurements to program the speech processor with special computer software. This processor is set to the appropriate levels of stimulation for each electrode.

After programming is complete, the speech processor is disconnected from the audiologist’s computer. Rechargeable or disposable batteries are inserted into the processor and the patient leaves with the whole system. It does take time to adjust, and each person has a different experience using the system.

Cost

Total costs for a cochlear implant, including evaluations, surgery, hospital fees, and all other fees and hardware, can range from $30,000 to $50,000. However, unlike hearing aids, cochlear implants are covered by most private insurance plans. In the U.S., Medicare, some state Medicaid programs, and Veterans Affairs provide partial coverage for cochlear implants. The implant specialist will be able to assist in determining the extent of a patient’s coverage.

http://www.innvista.com/health/ailments/earail/cochimpl.htm

Cochlear Implants, Surgical Technique

Introduction

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.

Postauricular incision for cochlear implant.

[ CLOSE WINDOW ]

Postauricular incision for cochlear implant.

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.

History of the Procedure

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-EL Corporation).

Problem

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.

Etiology

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.

Pathophysiology

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.

Presentation

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.

Indications

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.

Relevant Anatomy

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.

Contraindications

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.

http://emedicine.medscape.com/article/857242-overview

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