Cochlear implant
A cochlear implant 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 generally worn behind the ear, but could also be attached to clothing, for example, in young children. This component, the sound processor, contains microphones, electronics that include digital signal processor chips, battery, and a coil that transmits a signal to the implant across the skin. The inside component, the actual implant, has a coil to receive signals, electronics, and an array of electrodes which is placed into the cochlea, which stimulate the cochlear nerve.
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 the 1980s, speech perception via an implant has steadily increased. 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 re-learning 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 1960's Robin Michelson and colleague Melvin Bartz construct a cochlear device with biocompatible materials that can be implanted in human patients. This system is implanted in 4 patients, and the report of the hearing results represent a watershed for clinically applicable cochlear implants. Robin Michelson, Robert Schindler, and Michael Merzenich at the University of California, San Francisco, conducted these experiments in 1970 and 1971. Michelson, a clinical pioneer, and Merzenich, a talented basic scientist with a solid foundation in neurophysiology, was an integral element 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. 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 what would become the modern 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 the cochlear implant; Kissiah later sold the patent rights.
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.
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 depolarisation 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 which 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
- a receiver/stimulator, which receives signals from the speech processor and converts them into electric impulses
- an electrode array embedded in the cochlea
Assistive listening devices
Most modern cochlear implants can be used with a range of assistive listening devices, 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 of 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.
The procedure is usually done under general anesthesia. Complications of the procedure are rare, but include mastoiditis, otitis media, 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 ; 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; as of 2014 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 as of 2014 had been too small to draw generalizations.