A cochlear implant (CI) is a surgically implanted neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for improved speech understanding in both quiet and noisy environments. A CI bypasses acoustic hearing by direct electrical stimulation of the auditory nerve. Through everyday listening and auditory training, cochlear implants allow both children and adults to learn to interpret those signals as speech and sound.
The implant has two main components. The outside component is the sound processor, which contains microphones, electronics that include digital signal processor (DSP) chips, battery, and a coil that transmits a signal to the implant across the skin. It is generally worn behind the ear, though some self-contained sound processors take the form of a pebble-shaped unit worn directly on the side of the head over the implant site, held in place by magnetic attraction to the internal implant. The sound processor could also be attached to clothing, for example, in the case of young children. The inside component is the actual implant, which contains a coil to receive signals, electronics, and an array of electrodes that stimulate the cochlear nerve, which is placed into the cochlea.
The surgical procedure is performed under general anesthesia. Surgical risks are minimal, and most individuals will undergo outpatient surgery and go home the same day. However, some individuals will experience dizziness, and on rare occasions, tinnitus or facial nerve bruising.
From the early days of implants in the 1970s and 1980s, speech perception via an implant improved greatly, though gains have plateaued since the introduction of modern sound coding strategies in the 1990s. More than 200,000 people in the United States had received a CI through 2019. Many users of modern implants gain reasonable to good hearing and speech perception skills post-implantation, especially when combined with lipreading. One of the challenges that remain with these implants is that hearing and speech understanding skills after implantation show a wide range of variation across individual implant users. Factors such as age of implantation, parental involvement and education level, duration and cause of hearing loss, how the implant is situated in the cochlea, the overall health of the cochlear nerve, and individual capabilities of relearning are considered to contribute to this variation.
History
André Djourno and Charles Eyriès invented the original cochlear implant in 1957. Their design distributed stimulation using a single channel.
William House also invented a cochlear implant in 1961. In 1964, Blair Simmons and Robert L. White implanted a single-channel electrode in a patient's cochlea at Stanford University. However, research indicated that these single-channel cochlear implants were of limited usefulness because they cannot stimulate different areas of the cochlea at different times to allow differentiation between low and mid-to-high frequencies as required for detecting speech.
The next step in the development of the CI was its clinical trial on a cohort of patients. In the late 1960s, Robin Michelson and colleague Melvin Bartz constructed a cochlear device with biocompatible materials that can be implanted in human patients. This system was implanted in 4 patients, and the report of the hearing results was a watershed for clinically applicable cochlear implants. Robin Michelson, Robert Schindler, and Michael Merzenich at the University of California, San Francisco (UCSF), conducted these experiments in 1970 and 1971. Michelson, a clinical pioneer, and Merzenich, a talented basic scientist with a solid foundation in neurophysiology, were integral in the development of the UCSF cochlear implant team. Michelson was recognized for implanting a single-channel device into a congenitally deaf woman. She demonstrated auditory sensations from stimulation, as well as pitch perception for stimulus frequencies less than 600 Hz. Unfortunately, this patient had no word recognition. His pioneering work was presented, but not well-received at the 1971 annual meeting of the American Otological Society (Michelson, 1971 and Merzenich et al., 1973). In 1973, the first international conference on the "electrical stimulation of the acoustic nerve as a treatment for profound sensorineural deafness in man" was organized in San Francisco.
NASA engineer Adam Kissiah started working in the mid-1970s on the design for a cochlear implant. Kissiah used his knowledge learned while working as an electronics instrumentation engineer for NASA. This work took place over three years, when Kissiah would spend his lunch breaks and evenings in Kennedy Space Center's technical library, studying the impact of engineering principles on the inner ear. In 1977, NASA helped Kissiah obtain a patent for his cochlear implant; Kissiah later sold the patent rights. A commercial product based on Kissiah's design, developed by BIOSTIM Inc., was tested but failed to reach the marketplace.
The modern multi-channel cochlear implant was independently developed and commercialized by two separate teams—one led by Graeme Clark in Australia and another by Ingeborg Hochmair and her future husband, Erwin Hochmair in Austria, with the Hochmairs' device first implanted in a person in December 1977 and Clark's in August 1978. The Hochmairs' 1977 implant, though an early milestone, did not in its original 8-channel form lead directly to the modern device. Testing of that first system failed to produce open speech understanding and the Hochmairs changed their approach. It was Clark's multi-channel design, commercialized as the Nucleus device by Cochlear Limited, that became the first multi-channel cochlear implant to receive FDA approval in 1985, and successive iterations of the Nucleus device went on to become the most widely used cochlear implants in the world.
Parts Cochlear implants bypass most of the peripheral auditory system, which receives sound and converts that sound into movements of hair cells in the cochlea; the deflection of stereocilia causes an influx of potassium ions into the hair cells, and the depolarization in turn stimulates calcium influx, which increases release of the neurotransmitter glutamate. Excitation of the cochlear nerve by the neurotransmitter sends signals to the brain, which creates the experience of sound. With an implant, instead, the devices pick up sound and digitize it, convert that digitized sound into electrical signals, and transmit those signals to electrodes embedded in the cochlea. The electrodes electrically stimulate the cochlear nerve, causing it to send signals to the brain.
There are several systems available, but generally they have the following components:
External:
one or more microphones that pick up sound from the environment a speech processor that selectively filters sound to prioritize audible speech a transmitter that sends power and the processed sound signals across the skin to the internal device by radio frequency transmission
Internal:
a receiver/stimulator, which receives signals from the speech processor and converts them into electric impulses an electrode array embedded in the cochlea Several manufacturers currently have a totally implantable cochlear implant (TICI) in development or undergoing clinical testing. This new type of cochlear implant incorporates all the current external components of an audio processor into the internal implant. The lack of external components makes the implant invisible from the outside and also means it is less likely to be damaged or broken. On the other side, the development of a TICI faces especially two technical hurdles: the energy supply and the design of the implantable microphone. Initial clinical trials of TICIs have demonstrated safety and comparable performance to conventional cochlear implants. A study involving a research TICI indicated that users experienced high levels of hearing performance, although some challenges with microphone sensitivity were noted.
Assistive listening devices Most modern cochlear implants can be used with a range of assistive listening devices (ALDs), which help people to hear better in challenging listening situations. These situations could include talking on the phone, watching TV, or listening to a speaker or teacher. With an ALD, the sound from devices, including mobile phones, or from an external microphone is sent to the audio processor directly, rather than being picked up by the audio processor's microphone. This direct transmission improves the sound quality for the user, making it easier to talk on the phone or stream music.
ALDs come in many forms, such as , pens, and specialist battery pack covers. Modern ALDs are usually able to receive sound from any Bluetooth device, including phones and computers, before transmitting it wirelessly to the audio processor. Most cochlear implants are also compatible with older ALD technology, such as a telecoil.
Surgical procedure
Surgical techniques Implantation in children and adults can be done safely with few surgical complications, and most individuals will undergo outpatient surgery and go home the same day.
Occasionally, the very young, the very old, or patients with a significant number of medical diseases at once may remain for overnight observation in the hospital. The procedure can be performed in an ambulatory surgery center in healthy individuals.
The surgical procedure most often used to implant the device is called mastoidectomy with facial recess approach (MFRA).
The procedure is usually done under general anesthesia. Complications of the procedure are rare, but include mastoiditis, otitis media (acute or with effusion), shifting of the implanted device requiring a second procedure, damage to the facial nerve, damage to the chorda tympani, and wound infections.
Cochlear implantation surgery is considered a clean procedure with an infection rate of less than 3%. Guidelines suggest that routine prophylactic antibiotics are not required. However, the potential cost of a postoperative infection is high (including the possibility of implant loss); therefore, a single preoperative intravenous injection of antibiotics is recommended.
The rate of complications is about 12% for minor complications and 3% for major complications; major complications include infections, facial paralysis, and device failure.
Although up to 20 new cases of post-CI bacterial meningitis occur annually worldwide, data demonstrates a reducing incidence. To avoid the risk of bacterial meningitis, the CDC recommends that adults and children undergoing CI receive age-appropriate vaccines that generate antibodies to Streptococcus pneumoniae.
The rate of transient facial nerve palsy is estimated to be approximately 1%. Device failure requiring reimplantation is estimated to occur 2.5–6% of the time. Up to one-third of people experience disequilibrium, vertigo, or vestibular weakness lasting more than one week after the procedure; in people under 70, these symptoms generally resolve over weeks to months, but in people over 70, the problems tend to persist.
In the past, cochlear implants were only approved for people who were deaf in both ears; , a cochlear implant had been used experimentally in some people who had acquired deafness in one ear after they had learned how to speak, and none who were deaf in one ear from birth; clinical studies had been too small to draw generalizations.
Alternative surgical technique