What is a cochlear implant?
The Cochlear Implant is an electronic device that replaces the physiological cochlear function, converting sound into coded electrical impulses by an external sound processor and restoring or reestablishing the hearing function of a deaf person by directly stimulating and exciting the auditory nerve through a system of electrodes implanted in the body. In recent years, with the development of electronics, computer technology, phonetics, electrophysiology, materials science, and ear microsurgery, cochlear implants have been widely used in clinical practice and are currently the most successfully used biomedical engineering device. Cochlear implants are now routinely used worldwide as a treatment for severe to total deafness, with over 360,000 implants worldwide to date.
History and Current Status
The history of cochlear implants can be traced back to the discovery by Volta in Italy in 1800 that electrical stimulation of the normal ear could produce hearing, and the first implantation of electrodes into the cochlea of a totally deaf patient by Djourno and Eyries in France in 1957, which enabled the patient to perceive ambient sounds and gain a sense of sound, and the success of scientists in Europe and the United States in restoring hearing through electrical stimulation in the 1960s and 1970s. In 1972, the American House-3M single-channel cochlear implant became the first commercialized device, and in 1977, the world’s first multichannel cochlear implant was successfully implanted in Vienna, Austria. 1991 saw the introduction of the CIS, a high stimulation rate coding strategy, and the cochlear implant entered the era of multichannel high resolution. Today, the world’s major cochlear implant manufacturers are MED-EL in Austria, AB in the United States and Cochlear in Australia. To date, more than 360,000 deaf people worldwide have used cochlear implants, more than half of whom are children.
Multi-channel cochlear implantation began in China in 1995, and the technology has matured. With the increase in the number of cochlear implant cases and the expansion of the range of indications, the efficacy and safety of cochlear implantation in some special indications of deafness have been confirmed, further expanding the indications for cochlear implantation. For example, cochlear implantation in patients with no residual preoperative hearing, cochlear implantation in cases of inner ear malformation and cochlear ossification, cochlear implantation in patients with combined chronic otitis media, cochlear implantation in younger deaf patients, and cochlear implantation in older deaf patients.
The acquisition of normal speech in humans requires not only normal hearing, but also normal development of the auditory speech center. Studies have shown that the human auditory language center is fully developed by the age of about 5 years old, and the prime period of language development is from 0 to 3 years old. Therefore, congenital deafness does not reach its optimal level of language recovery and development until the age of 3 years old, especially before the age of 2 years old.
In adult patients with postlingual deafness, the cause of deafness may be sudden deafness, pharmacological deafness or hereditary delayed deafness based on congenital inner ear malformation (large vestibular conduction syndrome). These adult deaf patients are called adult postlingual deaf patients because they had normal hearing and acquired normal speech with adequate development of their auditory speech centers prior to their deafness. Adult postlingually deaf patients are one of the best indications for cochlear implants. These deaf patients have normal development of their auditory language centers prior to their deafness, and after receiving a cochlear implant, they regain their hearing and are able to recall their past memory of language, so they are able to regain their language skills in a relatively short period of time. An important issue for adults with postlingual deafness is that early cochlear implantation after deafness will quickly recall their past memory of language and obtain better language results. If the deafness is prolonged, the patient’s memory of past language will fade, leading to a decrease in the effectiveness of the cochlear implant. There is a growing concern about cochlear implants for elderly deaf patients. The majority of elderly deaf patients are postlingually deaf, and the cause of their deafness, in addition to the reasons mentioned above, is more often due to progressive hearing loss in old age until the use of hearing aids is ineffective. With the development of social economy and the increase of life expectancy of the population, the quality of life of the elderly has been more concerned by the society and families. Restoring the auditory speech ability of the elderly can enhance their speech communication ability, improve their psychological state, make them gain self-confidence and greatly improve their quality of life. Elderly deaf patients who receive cochlear implants are able to achieve excellent hearing and speech results.
Cochlear implant sound processing solutions
In the late 1970s, the University of Utah developed the first commercial multichannel cochlear implant device with a speech processor that divides the sound into four different channels and then compresses the analog signal output from each channel to accommodate the narrow dynamic range of the electrical stimulation. This speech processing scheme is known as compressedanalog (CA).
In the early 1980s, the Nucleus cochlear implant device with 22 intracochlear ring electrodes was developed at the University of Melbourne, Australia. The speech processing scheme has evolved from the initial extraction of only the fundamental frequency and the second resonance peak (F0F2) information, to the WSP processor with the first resonance peak (F0F1F2), to the multipeak processor with F0F1F2 plus 3 high frequency peaks, to the current one that extracts only any 6 of the 22 analyzed frequency bands. The current spectral peak (speatralpeak) processor that extracts information from any 6 of the 22 highest energy frequencies.
The continuous interlevedsampling (CIS) speech processor studied by Wilson et al. in the USA. In contrast to the feature extraction design of Nucleus, the CIS processor tries to preserve the original information in the speech, only divides the speech into 4-8 frequency bands and extracts the waveform envelope information in each band, then compresses the dynamic range with a logarithmic function, and continuously samples the compressed envelope with high-frequency biphasic pulses, and finally sends the pulse string with the speech envelope information to the corresponding electrode at intervals. From the information content point of view, CIS and CA processors are essentially the same, but CIS has the advantage of avoiding the problem of electric field interferences due to the simultaneous stimulation of multiple electrodes. Although both CIS and Nucleus use biphasic pulse-spaced stimulation, they differ in two ways: first, each electrode of CIS is stimulated with a high-frequency (800 to 2000 Hz) pulse train at a constant rate and continuously, even when silent, except that its pulse amplitude is reduced to a threshold level; second, the analysis band of CIS is consistent with the number of stimulated electrodes, and currently CIS speech processing solutions have been widely adopted by most of the world’s cochlear implant companies, and new improvements have been made on this basis. For example, ABC in the United States has introduced the S series processing solution, Nucleus in Australia has introduced the ACE solution for the CI24M type 24-channel device and MED-EL in Austria has introduced the fast CIS solution.
In recent years, the cochlear implant field has focused on research and development of the fine structure of sound (Fine Structure), mainly in the time domain and frequency domain.
In the time domain, two processes are involved: the analysis of the acoustic signal and the release of the electrical stimulation signal. Fine structure processing is added to the process of obtaining time-varying information based on envelope extraction. For example, MED-EL’s Fine Structure Processing (FSP) strategy improves the cochlear implant sound to a high-definition level of detail close to normal.
In the frequency domain, current steering, or “virtual channel,” breaks through the limit of the number of physical electrodes and provides more channels for the cochlear system, enriching the frequency domain information. Another challenge in cochlear implant technology is the ability to discriminate low frequency information (e.g., F0), which is one of the main reasons for the difficulties in listening in noisy environments, multi-person conversations, voice recognition, tonal language recognition (e.g., the four tones of Mandarin Chinese), and music appreciation. In addition to current directional techniques that provide more resolution of low-frequency information, the fine structure strategy that MED-El has started to adopt in cochlear implants such as PULSAR, in which variable stimulation rates are used in the low-frequency band to improve discrimination in the low-frequency region, and the FAME (Frequency Amplitude Modulation Encoding) strategy proposed by F, G, Zeng et al. The FAME (Frequency Amplitude Modulation Encoding) strategy proposed by F, G, Zeng et al. is also based on the principle of rate encoding in the frequency domain to achieve the same purpose. As a more important advance in improving low-frequency discrimination, Combined Electro-Acoustic Stimulation (EAS) has been the focus of research and development in recent years, providing natural low-frequency information to deaf patients where applicable, and its effectiveness in listening and music appreciation in noisy environments is gradually being validated in clinical trials.
Nowadays, new cochlear implants such as MED-EL’s SONATA and CONCERTO have adopted recent high-definition fine structure FSP coding and parallel stimulation coding technologies, which have further improved the effectiveness of cochlear implants and met the needs of learning and communicating in tonal languages such as the four tones of Chinese, while providing more than 250 tones recognition to meet the majority of users’ music appreciation The cochlear implant can also be used to improve speech recognition in the presence of noise.
Indications for cochlear implantation
1. Patients with prelingual deafness
(1) Children with severe or profound sensorineural deafness in both ears with hearing loss ranging from 1 kHz and higher with hearing thresholds of 90 dB or more. For those without preoperative residual hearing, hearing aid sound field audiometry is required to help determine residual hearing, and electrical stimulation of auditory brainstem evoked potentials (EABR) is performed if necessary.
(2) Hearing loss of unknown etiology, congenital, hereditary, pharmacological, or post meningitis, with lesions localized in the cochlea; patients with auditory neuropathy with lesions localized in the cochlea require preoperative EABR to estimate the site of the lesion, and given the current limitations in understanding auditory neuropathy from a medical perspective, the parents of the child need to be informed of the special risks. For most inner ear malformations, including Mondini malformation, common cavity malformation, and large vestibular aqueduct malformation are still indications for cochlear implantation, and the parents need to be informed of the special risks and that the parents have reasonable expectations.
(3) Time of onset of deafness For recent hearing loss, stable hearing changes need to be observed for at least 3 months.
(4) The optimal age is 12 months to 5 years; cochlear implantation should be done as early as possible due to the limitations of cerebral hearing and speech plasticity, with a minimum age of 4 months in Europe and 6 months in mainland China.
Children or adolescents older than 5 years of age need to have a certain hearing and speech foundation, a history of hearing aid wear and a history of hearing or speech training since childhood. Ineffective or very poor hearing aid is defined as open phrase recognition ≤ 30% or two-word word recognition ≤ 70% in the best hearing aid listening environment.
(5) No significant improvement in hearing ability after hearing aid fitting with appropriate hearing aids and no significant improvement in auditory language ability after hearing rehabilitation training.
(6) Have normal psycho-intellectual development.
(7) The family and/or the implant recipient has a correct understanding of the cochlear implant and appropriate expectations.
(8) There are conditions for hearing and language rehabilitation education.
(9) No contraindication to surgery.
2.Patients with postlingual deafness
(1) Hearing loss in adults with severe or very severe sensorineural deafness in both ears ranges from 1 kHz and higher with a hearing threshold of 70 dB or more. For those without preoperative residual hearing, hearing aid sound field audiometry is required to help determine residual hearing and, if necessary, EABR testing or psychophysical testing with electrical stimulation of the drum head.
(2) Patients of all ages with postlingual deafness who are advanced cochlear implant candidates need to have a proper understanding of cochlear implants and appropriate expectations.
(3) Time of onset of deafness For new onset of hearing loss, stable hearing changes need to be observed for at least 3 months.
(4) No significant improvement in speech recognition ability after hearing aid fitting.
(5) The patient has a normal psychological and mental condition and a correct understanding of the cochlear implant and appropriate expectations.
(6) No contraindications to surgery.
Contraindications to cochlear implantation
1. Absolute contraindications
(1) Severe malformation of the inner ear, such as Michel’s malformation or cochlear agenesis.
(2) Hearing nerve deficiency.
(3) Severe mental illness.
(4) Purging inflammation of the middle ear mastoid has not been controlled.
2. Relative contraindications
(1) Poor general condition due to concomitant diseases.
(2) Uncontrollable epilepsy.
(3) Patients with cerebral white matter lesions are not contraindicated for cochlear implantation, but parents should be informed of the special risks and have reasonable expectations.
(4) Secretory otitis media and glue ear are not contraindications to surgery. In chronic otitis media with tympanic membrane perforation, if the inflammation is under control, surgery in one stage or in stages may be an option.
Pre-operative evaluation
1. Medical history taking
Medical history taking and examination are used to understand the cause of the disease. The focus of otologic history taking should be on the etiology and pathogenesis of deafness. The patient’s hearing history, history of tinnitus and vertigo, history of ototoxic drug exposure, history of noise exposure, history of systemic acute and chronic infections, past history of otologic diseases, developmental factors (systemic or local developmental abnormalities, intellectual development, etc.), family history of deafness, history of hearing aid wear and other causes, such as epilepsy, mental conditions, etc. should be understood. Children with deafness should also include: maternal pregnancy history, pediatric birth history, pediatric growth history, and speech development history.
The patient’s language ability (e.g., articulation characteristics, clarity of constructions) and language comprehension and communication ability (e.g., oral, lip reading, sign language, written, guessing, etc.) should also be understood.
2. Otological examination
Including earwax, external auditory canal, tympanic membrane and eustachian tube, etc.
(1) Audiological examination (1) subjective hearing threshold determination of children under 6 years old can be measured by pediatric behavioral audiometry, including behavioral observation audiometry, visual reinforcement audiometry and play audiometry; (2) acoustic conductance measurement including tympanic chamber pressure curve and stapedius muscle reflex; (3) auditory brainstem response (ABR) 40Hz correlation potential (or multi-frequency steady-state evoked potentials); (4) otoacoustic emission (transient evoked otoacoustic emission or aberration product otoacoustic emission); (5) speech audiometry (5) speech audiometry speech hearing threshold test for language perception threshold and language recognition threshold; speech recognition test including speech test word list and pediatric speech test word list; (6) hearing aid matching requires professional audiologist for hearing aid matching, generally need to wear both ears, after matching, hearing threshold test and speech recognition test should be done, and then auditory language training for 3-6 months; (7) vestibular function examination (for those with a history of vertigo); (8) The electrical stimulation test of the drum head includes psychophysical examination of threshold, dynamic range, frequency discrimination, interval discrimination and temporal discrimination.
(2) Audiological assessment criteria ① Binaural pure tone air conduction hearing threshold measurement > 80 dBHL (average of 0, 5, 1, 2, 4 kHz, WHO standard) in patients with postlingual deafness. Cochlear implants may also be considered if the good ear does not achieve 30% helpful open phrase recognition and the hearing loss is greater than or equal to 75 dB [see FDA supplemental criteria]; ②Patients with prespeech deafness require a comprehensive assessment after multiple objective audiometric examinations and behavioral audiometry for infants and children, including: no auditory response at sound output on ABR examination ( 120dBSPL); no response at the loudest sound output at frequencies above 2kHz for 40Hz correlation potential detection and >100dB at frequencies below 1kHz; no response at 105dBHL at frequencies above 2kHz for multi-frequency steady-state audiometry; no response at all frequencies in both ears for aberration product otoacoustic emissions; hearing threshold not entering the auditory language area (banana chart) at frequencies above 2kHz for helpful sound field audiometry, speech (3) For patients without any residual hearing, cochlear implantation can still be considered if there is a clear auditory response to electrical stimulation of the tympanic capsule. If there is no auditory response to electrical stimulation of the tympanic capsule, the patient or parents should be informed of the situation, and the patient and family should consider the risks of surgery.
3. Imaging evaluation
Imaging is a crucial test for patient selection. A thin layer CT scan of the temporal bone, three-dimensional reconstruction of the cochlea and magnetic resonance examination of the inner ear canal should be routinely done, and cranial magnetic resonance examination should be done if necessary.
4. Language ability assessment
Patients with certain language experience or ability should be evaluated for speech ability (language structure and function), including speech intelligibility, vocabulary, comprehension, grammar, expression and communication ability. For children younger than 3 years old who were uncooperative, video observation of “parent-child games” was used to evaluate the patient’s current language ability.
5. Psychological, intellectual and learning ability assessment
For children over 3 years of age who lack language ability, the Schneider Learning Ability Test can be used, and for those under 3 years of age, the Greifers Mental Developmental Behavior Inventory can be used. For children with suspected mental retardation (IQ <68< span=""> on the Hine Learning Ability Assessment and <70 on the Greifers Mental Developmental Quotient) or abnormal psychological behavior, patients should be advised to go to an authoritative institution for further observation, diagnosis, and identification. Patients with socio-cultural mental retardation may be considered for cochlear implantation; while patients with non-socio-cultural mental retardation, or ADHD, autism and other mental retardation should be explained to their parents the great difficulties such disorders may bring to the patient's post-operative rehabilitation, and parents should be helped to establish objective psychological expectations.
6. Pediatric or internal medicine evaluation
Do whole body physical examination and related auxiliary examinations.
7.Family conditions and rehabilitation conditions assessment
Families who have received professional training or have regular guidance from a language training teacher can conduct auditory language training for the child at home; otherwise, the child should be sent to a rehabilitation school or institution for deaf children.
Cochlear implant surgery
The surgery is performed under general anesthesia and intravenous antibiotics are given before the surgical incision. Electrode impedance testing and neurological response telemetry (ART) are performed after implantation of the electrodes. EBAR monitoring and facial nerve monitoring are used in special cases such as inner ear malformations. The majority of surgical approaches are made using a facial saphenous approach. A retroauricular incision is usually used. The incision is divided into two layers, the superficial skin and subcutaneous tissue, and the deeper temporalis fascia and myo-periosteal flap. The entire flap is turned backward to expose the bony cortex of the mastoid area. A recipient/stimulator bone bed is created on the skull surface above the posterior mastoid process with an electric drill. A simple mastoidectomy is performed to expose the short pedicle of the anvil, which is used as a marker to open the facial fossa, and the cochlear tympanic step is opened below the round window niche. The receptive stimulator is inserted into the bone bed, the stimulating electrode is inserted into the cochlear tympanic step, and the reference electrode is placed on the skull surface under the temporalis muscle. The surgical approach is modified accordingly for cases of cochlear malformation (e.g. Mondini malformation, common cavity malformation) and cochlear ossification. The main surgical complications include wound infection, flap necrosis, facial palsy, meningitis, and electrode prolapse. A small number of patients with implanted electrodes in the cochlea experience mild vertigo after surgery, which mostly disappears on its own within a few days.
Cochlear implant post-operative adjustment
Switch-on is performed one month after the cochlear implant surgery. Different cochlear implant devices have different design principles and use different hardware and software, as well as different methods, processes and parameters for tuning. Cochlear implant devices consist of an internal implant and an external speech processor. Mapping is the process of adjusting the parameters of each cochlear implant device to provide the most comfortable and effective stimulation for the patient and to allow the patient to hear various sounds comfortably, using a computer and specialized equipment. The speech processor does not work unless the professional sets the appropriate values for a series of parameters through tuning. The parameters that need to be adjusted for post-cochlear implant tuning include: speech coding scheme such as HDCIS, FSP, Fs4-P scheme; electrical stimulation mode, either monopolar stimulation, bipolar stimulation, or co-local mode; the maximum number of channels used is set to a maximum of, say, 24 channels, the frequency assignment of the channel filter output, assigning a central frequency range of 70 Hz to 10,000 Hz to each channel; the threshold value for each channel (the minimum stimulation level at which the THR value can produce auditory stimulation); and the maximum comfort stimulation for each channel (the maximum comfort stimulation at which the MCL value can be felt by the patient).
Start-up is scheduled 3 to 5 weeks after surgery, when the in vivo portion of the cochlear implant, especially the electrode portion, is more stable. After start-up, most patients will have a gradual process of adaptation to outside sounds, with a period of psychological and physiological change and development before stabilization. The electrode parameters change the most and the fastest in the first 3 months after switching on, debugging once a month, trying to set the program for more than 3 months each time, patients change 1 set of program every week in the parents, and later they can debug once every 3 months, 6 months, or 1 year.
Auditory speech rehabilitation after cochlear implantation
Patients, parents of deaf children, and teachers should be made aware of the importance of auditory speech rehabilitation after cochlear implantation, especially to prepare children with prelingual deafness for how they should be rehabilitated after surgery and the choice of rehabilitation sites. Pre-operative rehabilitation training should be implemented according to the characteristics of different children’s ages and hearing and language levels. The content of the rehabilitation training should focus on the establishment of the patient’s auditory awareness and the understanding of the definition of the concept of things, so as to prepare him/her for the behavioral experience and learning psychology for post-operative start-up and rehabilitation training.
The “Auditory Oral Training Method” is a logical and strict guideline. For children with cochlear implants, it refers to the use of the cochlear implant signal to maximize the development of hearing, followed by the development of oral language, creating the best possible environment for the child. The auditory speech training for deaf children should be in accordance with the rules of language development in children, and should be carried out gradually in stages according to the “hearing age” of the deaf child, from shallow to deep. There are three stages: auditory training stage, vocabulary accumulation stage, and language training stage.
1. Auditory training stage
The main purpose of the auditory training stage is to use the deaf child’s residual hearing to listen to various sounds, to awaken the “sleeping state”, and to give frequent stimulation, repeated training, and repeated reinforcement, so that the deaf child can gradually adapt to various daily sounds and enter the audible society.
2. Vocabulary accumulation stage
The vocabulary accumulation stage is based on auditory training, supplemented by visual and other senses to make them know more social things, combine what they see and touch with sound signals in their brain to form signals, so that they gradually understand the meaning of speech.
3.Language training stage
The language training stage is based on vocabulary accumulation, training deaf children to speak more, from single words to short sentences, from simple to complex, from less to more, gradually to be able to understand other people’s language, so that others can understand their own language.
Cochlear implant rehabilitation should be implemented under the guidance of professionals, and services that undertake professional rehabilitation guidance provide appropriate rehabilitation training models for hearing impaired children and families.