What is a cochlear implant?
The Cochlear Implant is an electronic device that replaces the physiological cochlear sound perception function. The external sound processor converts sound into coded electrical impulses and restores or reestablishes 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 House-3M single-channel cochlear implant in the United States became the first commercialized device.
In 1977 the world’s first multi-channel cochlear implant was successfully implanted in Vienna, Austria, and in 1991 the high stimulation rate coding strategy CIS was introduced and the cochlear implant entered the era of multi-channel high resolution from then on. 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 hearing before surgery; cochlear implantation in cases of inner ear malformation and cochlear ossification; cochlear implantation in patients with combined chronic otitis media; cochlear implantation in patients with younger deafness; cochlear implantation in patients with advanced deafness.
Normal human speech requires not only normal hearing, but also normal development of the auditory language center. Studies have shown that the human auditory language center is fully developed by the age of 5, and the prime period of language development is between the ages of 0 and 3. Therefore, the optimal level of language recovery and development can be achieved by the age of 3 for congenital deafness, especially by implantation before the age of 2.
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 postlingually 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. The majority of elderly deaf patients are postlingually deaf. 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 Nucleus processor is characterized by bipolar stimulation, where different electrodes are stimulated at different times and the stimulation frequency does not exceed 500 Hz.
Speech processing schemes have 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 three high frequency peaks, to the current spectral peak processor that extracts only the highest energy frequency information from any six of the 22 analyzed frequency bands ( speatralpeak) processor.
The continuous interval sampling (continuousinterlevedsampling, CIS) speech processor studied by Wilson et al. in the United States. In contrast to the feature extraction design of Nucleus, the CIS processor tries to preserve the original information in speech, only divides the speech into 4-8 frequency bands and extracts the waveform envelope information on 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 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 spacing stimulation, they differ in two ways: First, each electrode in CIS is stimulated with a high frequency (800-2000 Hz) pulse train at a constant rate and continuously, even when silent, except that the pulse amplitude is reduced to a threshold level;
Secondly, the CIS analysis frequency band is consistent with the number of stimulation electrodes. The CIS speech processing solution has been widely adopted by most cochlear implant companies in the world 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 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 started to use in cochlear implants such as PULSAR improves the resolution of low-frequency regions with variable stimulation rates in the low-frequency band; the FAME (Frequency Amplitude Modulation Encoding) strategy proposed by F. G. Zeng et al. 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 frequencies 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 to the cochlea; patients with auditory neuropathy with lesions localized to 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 particular risks. For most inner ear malformations, including Mondini malformation, common cavity malformation, and large vestibular conduit malformation are still indications for cochlear implantation, and the parents of the child need to be informed of the special risks and that the parents have reasonable expectations.
(3) Duration 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; limited by brain hearing and speech plasticity, cochlear implantation should be done as early as possible, with a minimum age of 4 months for European implantation and 6 months for 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 aids are 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 speech 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 frequencies 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 tympanic capsule.
(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 deafness onset For new onset hearing loss, stable hearing changes need to be observed for at least 3 months.
(4) No significant improvement in speech recognition after hearing aid fitting.
(5)Have normal psychological and mental condition and the patient has correct understanding and appropriate expectation of cochlear implant.
(6) No contraindications to surgery.
Contraindications to cochlear implantation
1. Absolute contraindications
(1) Severe deformities of the inner ear, such as Michel’s deformity or cochlear agenesis;
(2) Hearing nerve deficiency;
(3) Severe mental illness;
(4) The purulent 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 a contraindication to cochlear implantation, but parents should be informed of the special risks and that they 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 controlled, a stage or staged surgery may be an option.