|| A cochlear implant is a medical prosthesis that restores hearing to those with severe nerve deafness. The cochlear implant was developed in the 1970s and has undergone many refinements since then. Essentially, a cochlear implant involves two parts, an external component and an internal component that is surgically implanted inside the body. The external component consists of a microphone that is worn at the ear level and functions to receive sound. This sound is then taken to a processor, which encodes the sound into an electrical signal. The electrical signal is then sent to a transmitter that transmits a tiny radio signal. The radio signal is then received by the internal component called a receiver stimulator unit. This unit is placed under the scalp in a small depression in the skull. This receiver unit takes the radio signal and encodes it into small electrical signals again which are then delivered through an electrode array that is carefully placed in the cochlea, or hearing portion of the inner ear. These tiny electrical currents stimulate the cochlear nerve, or hearing nerve, directly. This is necessary in cases in which the tiny nerve endings in the inner ear that usually respond to vibrations of sound are so diseased that they cannot be stimulated through vibrations and require a direct electrical stimulation. A cochlear implant is not a hearing aid. It is a medical prosthetic device that requires a surgery. A cochlear implant is not suitable for those individuals for whom a hearing aid is helpful. A good rule of thumb is the telephone test. If one can carry on a conversation over the telephone with
hearing aids on, then their hearing is probably too good to benefit from the cochlear implant. (Diagram
G - different components of a cochlear implant, Diagram H
- electrode array inserted into the cochlea)
Requirements for Cochlear Implantation
Currently it is necessary for a person to be at least one year old prior to cochlear implantation. It is also necessary that the
hearing loss be severe enough such that the patient gets little to no benefit from hearing aids. The patient must also be medically fit to undergo a general anesthetic lasting about two hours. The cochlea, or hearing portion of the inner ear, has to be formed and suitable for implantation of an electrode array. This is determined by obtaining a CT scan of the inner ear prior to surgery. Other considerations that are important in the cochlear implant evaluation include a motivation and commitment to learn to use the implant and appropriate educational opportunities for children that receive the implant.
Because all natural hearing is lost in the ear with cochlear implantation, great care is made to ensure a patient is a good candidate for the implant. That means we want to make sure that the implant is going to really help. This is done through a series of standardized testing. These hearing tests done prior to implantation may seem laborious and annoying at first, but they are essential to the evaluation process. An assessment about expectations is also done as well as questions about one's commitment to use the implant. The CT scan is always done to ensure that the anatomy is suitable for cochlear implantation.
The surgery for cochlear implantation lasts about two hours. It is done either as an outpatient or with an overnight hospitalization. The patient undergoes a general anesthetic and some hair is shaved off the scalp behind the ear. The incision is made in the crease behind the ear and then extended into the scalp behind the ear. A hole is drilled in the mastoid, or ear sinus, which is just behind the ear canal, and another depression is drilled in the skull behind this. The stimulator receiver unit is placed in the depression in the skull and, then a trough is drilled from there to the mastoid cavity. A small opening is made from the mastoid, or ear sinus, into the middle ear between the nerve that moves the face and the taste nerve so that the cochlea can be seen and entered. A small hole is drilled in the cochlea, and the electrode array is inserted into the cochlea. The stimulator receiver unit is then secured in the depression in the skull. The scalp wound is then closed and a fluffy dressing placed around the head. The next day the small drain that was placed in the wound is removed, and later that evening the fluffy wrapping can also be removed. The patient is then seen about one week later. It is important not to get the wound wet during that one week period. The wound is allowed to heal for about four weeks after surgery before the external component is placed in contact with the skin. This is to ensure that a full healing takes place. Magnets in each component hold the internal and the external components together. The surgery usually does not cause much pain, and it seems to be tolerated well. There is occasionally some dizziness following the surgery in about half the patients. This usually goes away in a few days. It is rare for any
dizziness to persist. Occasionally there is taste disturbance, such as a metallic taste, that usually goes away in a few weeks. The surgery takes away any natural hearing that remains in the ear. For this reason, several steps are taken to ensure that the patient is a good candidate for the cochlear implant. Once the implant is placed, it cannot be removed and a hearing aid used again.
Risk and Complications Associated With Surgery
Complications associated with cochlear implantation are rare but can be serious. The most common complication with the surgery is a metallic taste in the mouth. This usually goes away after two or three weeks but sometimes can persist. There is a small area in which the implant must be placed. This area is between the facial nerve and the taste nerve.
This is an extremely rare complication that is usually present only when there are irregularities in the anatomy associated with the nerve, or other complicated medical conditions. Sometimes this can be caused by a small swelling in the nerve that goes away with time, but can be permanent. The second most common complication of cochlear implant surgery is dizziness. About half of the patients experience transient dizziness that goes away within a few days. However, on rare occasions people can experience persistent imbalance. There are occasions in which the implant fails. Sometimes the internal component has a mechanical failure and needs to be replaced. Other less common complications can include infection and bleeding. Severe infections require removal of the implant.
Rarely there can be injury to the facial nerve, which can result in paralysis of the face.
Another drawback to cochlear implantation which would not be a complication but is a consideration, is the ability to perceive with MRI scans with cochlear implant in place. Usually other tests such as CT scans can be done in lieu of MRIs.
The benefit provided by the cochlear implant varies a lot based on the circumstances leading up to implantation. The best performance with the cochlear implant is in adults who have heard well all their lives and then lose their hearing suddenly. They do very well with a cochlear implant, and although they describe the hearing as different from previous hearing, they learn that hearing very well and very quickly. In a year most of these patients when asked would say that it is hard to determine whether it is normal hearing because it is their hearing now. The patients that do worst with the cochlear implant are children that have never heard before and are older before they are implanted. However, even these children do well with the proper education and habilitation. The Jackson Ear Clinic did a survey investigating how our first thirty-three patients implanted benefited from the cochlear implant. All reported that they were glad that they had gotten the implant and would recommend it to other patients. They all responded that if they had it to do over knowing what they know now, they would still choose to have the implant. The benefits received from the implant varied a lot depending on the circumstances of each patient. However, all were pleased with the hearing that they got. These surveys included responses from mothers of children that had the implant. The implant is not a cure all. Rather, it is a tool that allows the deaf to hear again and learn oral communication. In cases of children, it is important that they receive appropriate educational training in an oral school to learn to use the implant. In both adults and children, it is important to get the proper habilitation to adjust the implant. In the course of time immediately after implantation, several sessions are necessary to determine the appropriate computer program for the implant that best helps that individual. Many adjustments to the strategies of stimulation can be done that very much change the response each patient gets.
The cochlear implant is a medical prosthesis that allows deaf persons to hear by direct electrical stimulation of the nerve endings in the inner ear. Some commitment ot time spent before and after the surgical procedure is done is necessary to take full advantage of the implant. This is indeed a great technological advance in our day that allows deaf persons to hear again. If you have any questions about the cochlear implant please let us know.
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