The cochlear implant is a hearing prosthesis for the inner ear, the cochlea, which gave the implant its name. This surgically implanted hearing device offers patients with profound hearing loss, the chance to hear again. Something that was previously not possible with either analog or digital hearing aids. However, the prerequisite for this is a still functioning auditory nerve.
What is the cochlear implant?
The cochlear implant is a hearing prosthesis for the inner ear. This surgically implanted hearing device offers patients with profound hearing loss the chance to hear again. A cochlear implant, or CI for short, can help children and adults who suffer from extreme hearing loss or deafness. Unlike a conventional hearing aid, the CI stimulates the auditory nerve fibers directly. The cochlear implant consists of two parts: An external one, which consists of a microphone, speech processor, coil, battery or accumulator. And an internal part, the actual implant, which is composed of coil, signal processor with associated stimulator and electrodes. The implant is surgically inserted behind the ear. The external part is worn by the patient like a hearing aid behind the ear. There have been some attempts to implant both parts, but they failed. The external microphone receives sound vibrations and converts them into electrical signals that are transmitted to the implanted coil. The internal coil now relays these signals to a stimulation circuit that generates currents for the electrodes in the cochlea. These currents are responsible for stimulating the auditory nerve. This is why it is so important that it is still intact, otherwise the implant will not work. The stimulation generates so-called action potentials, which are electrical excitations, and transmits them to the brain, where they are identified as acoustic signals such as noises, sounds and speech.
Function, effect and goals
When speech understanding is no longer possible via hearing and nothing is achieved with conventional hearing aids, the cochlear implant still offers a chance. This is especially the case with destroyed hair cells. An important prerequisite for the functioning of a CI is, however, that the patients are deafened only during or after the acquisition of speech, otherwise a spoken language understanding is not possible. However, this also makes it possible to treat children who suffer hearing loss at an early age, but who can already speak or are just learning to speak. Whether a CI can be considered for children is decided on the basis of the hearing threshold. This is the sound pressure level at which the human ear can just perceive tones and sounds. As a guideline, 90 decibels is used for the hearing level in children. Prior to implantation, a number of preliminary examinations are performed to investigate the cause of the deafness. Computer tomography and magnetic resonance imaging provide information about the auditory nerve and the auditory pathway. In order to correctly assess speech understanding, various tests are used in adults, such as the Freiburg one-syllable test. This tests how many monosyllables the patients understand. If the rate is less than 40 percent, a cochlear implant is advised. The chances of success with this method depend on several factors: the duration of the hearing loss, the patient’s linguistic competence, the condition of the auditory nerve, and ultimately the motivation of the patient, who must learn to hear completely anew. The surgery is performed under general anesthesia. An incision about eight centimeters long is made in the skin behind the ear. For the implant, the surgeon mills a recess in the skull bone. A hole is drilled in the cochlea through which the electrodes are inserted. During the operation, which lasts about two hours, the function of the implant is tested again and again. After about five days, the patient is discharged from the hospital. The healing process takes about four weeks. This is followed by outpatient fitting appointments. The speech processor is repeatedly readjusted on five consecutive days. After this, a long rehabilitation phase begins, which lasts about two years for adults and three years for children. The duration varies from patient to patient. Adults who have just become deaf and receive their implant quickly usually need only one year. Nevertheless, hearing must be completely relearned during this time.Sounds and voices have a completely different effect via the implant, so the hearing system requires a corresponding familiarization phase. Various adaptation periods, as well as hearing and speech therapies, complement the rehabilitation period. Subsequent annual check-ups are important to check the technical function of the implant and to perform hearing tests.
Risks, side effects and dangers
There are always risks associated with surgery. However, when inserting the cochlear implant, there are still a few special risks to consider, about which the operating physician must inform the patient intensively. Facial and taste nerves can be injured during the procedure, as the channel for the electrodes is milled immediately nearby. There can also be confusion when inserting the electrodes, which are inserted into one of the three arch canals instead of the cochlea. However, intensive monitoring during the procedure makes this error nearly impossible. There is also a risk of meningitis infection if germs enter the cochlea through the entry point of the electrodes. And the patient may develop an intolerance to the implant materials (silicone). The cost of a CI, including the entire rehabilitation phase, is around 40,000 euros. As a rule, the statutory health insurance funds cover the costs. Negotiations with private insurers must be conducted separately. Follow-up costs for batteries are usually not reimbursed.
