Approximately one in a thousand persons is born deaf. Almost an equal number of persons born with hearing will develop deafness during their lifetime. This information is concerned with the cochlear implant for restoration of some hearing to the totally deaf.
The cochlear implant is an electronic instrument. Part of the device is implanted in the temporal (ear) bone and part is worn like a pocket-type hearing aid on the body. The cochlear implant, however, is not a hearing aid. It allows limited speech discrimination (understanding), makes communication easier and enables the user to hear many environmental sounds.
The cochlear implant is regulated by the Food and Drug Administration for use in both adults and children, but is available for children only from a few institutions. The names of those institutions are available on request.
Function of the ear
The ear is divided into three parts: external ear, middle ear and inner ear. Each part performs an important function in the process of hearing.
The external ear consists of an auricle and the ear canal. These structures gather sound and direct the sound toward the eardrum.
The middle ear chamber lies between the external ear and the inner ear and consists of the eardrum and three small bones (ossicles): malleus (hammer), incus (anvil), stapes (stirrup). These structures transmit sound vibrations to the inner ear. By so doing they act as a transformer, converting sound vibrations in the external ear into fluid waves in the inner ear.
The inner ear chamber (labyrinth) contains both the auditory (hearing) and vestibular (balance) mechanisms and is filled with fluid. The auditory chamber is called the cochlea. This term comes from Latin and means snail shell, which the cochlea resembles.
Fluid waves initiated by movement of the three small ear bones are transmitted to the cochlea where they in turn stimulate the delicate hearing cells (hair cells) of which there are over sixteen thousand. Movement of these hair cells generates an electrical current in the auditory cortex which recognizes these electrical stimulations as sound.
Types of hearing impairment
The external ear and middle ear conduct sound vibrations. The inner ear receives these vibrations and transforms them into electrical impulses.
When there is some disease or obstruction in the external ear or the middle ear, a conductive hearing impairment results. This impairment may be due to a variety of problems and is correctable by medical or surgical treatment.
When the hearing impairment is due to some problem in the inner ear, a sensorineural (nerve) impairment results. A sensorineural hearing impairment is not correctable by medical or surgical treatment but can be helped by the cochlear implant in selected cases.
A third type of hearing disorder, not commonly encountered, is the central hearing loss, so called because the problem is not in the ear but in the complicated interconnections in the brainstem or in the auditory cortex (hearing center of the brain).
Hearing impairments are measured in decibels (dB). Normal hearing individuals have thresholds between 0 and 25 dB. A threshold of 30 to 35 dB is considered to be mild impairment, a 60 to 85 dB impairment is called severe and a threshold of 90 dB or more is considered to be a profound impairment. Candidates for a cochlear implant have a threshold of 90 dB or greater in both ears, i.e. a profound sensorineural hearing loss.
In most cases of hearing loss it is not difficult to determine the type of impairment. Carefully administered hearing tests (of pure tone and speech) and tuning fork tests allow the otologist (ear specialist) to decide whether the problem is conductive or sensorineural.
Fortunately few hearing impairments progress to deafness (profound or total loss of hearing). Deafness may result though from a variety of causes such as an infection, lifesaving drugs, or a head injury. Congenital deafness (present at birth) usually results from unknown causes but may result from hereditary factors or a viral infection.
Most cases of sensorineural deafness are due to damage of the hair cells in the cochlea, the cells that initiate electrical current in the auditory nerve. Unfortunately, these cells, once destroyed, do not regenerate, in the same way that a finger lost through an accident does not regrow.
If fluid waves in the cochlea have no hair cells to stimulate, the nerve fibers, regardless of how normal they may be, do not transmit an electrical impulse. It is exactly as if there were a telephone wire but not a receiver; no amount of shouting at the wire would result in transmission of sound.
The cochlear implant
The cochlear implant consists of an internal coil, embedded under the skin behind the ear, and a wire (active electrode) introduced into the fluid filled convoluted turns of the cochlea. Through this system it is possible to supply electrical current to stimulate the auditory nerve, current that cannot be provided by the damaged hair cells.
Some implanted patients are able to understand a portion of speech without the aid of speech reading, with most obtaining only limited speech discrimination (understanding). They can detect their own voices and are therefore better able to monitor the loudness of their own speech. Most have improvement in the quality of their voice. They are aware of the conversation of others and may understand some words; they avoid interrupting conversations. Most implant users can tell the difference between a man’s and a woman’s voice, but they describe speech as sounding artificial. Speech reading (lipreading) is improved in implant users. They are able to use clues from the sounds and rhythms of speech to combine these with what they see.
Some implanted patients can carry out limited conversations over the telephone but more are not able to understand words, and for this reason they are still denied easy access to the telephone for full communication. They are able to determine if there is a dial tone, a busy signal, a ringing tone or if someone has answered at the other end. Communication codes may be devised with family and friends to help in the use of the telephone. Simple messages about coming home for dinner, that the car has broken down, and about urgent problems, may be transmitted.
One of the cochlear implants principal advantages at this stage of development has been to enable the user to hear more environmental sounds.
Many implant patients report a greater sense of security since they can now hear approaching footsteps, a fireball at their place of employment, doors open and close, etc. They feel less isolated.
Severe tinnitus (head noise) often has been decreased by the implant.
Becoming a choclear implant patient
Becoming a cochlear implant patient involves an evaluation which includes otologic, audiologic, radiographic and psychological tests.
Cochlear implant surgery is performed under general anesthesia. This requires one or two days in the hospital.
Rehabilitation and implant training begin about two months following surgery. Rehabilitation training begins at home over the next nine to twelve months.
Otologic (ear) Evaluation
An examination must be performed to determine that there is no active infection or other problem within the middle or inner ear that would preclude the surgical placement of the implant
Audiologic (hearing) Evaluation
Extensive hearing tests are performed to determine the degree of hearing with and without a hearing aid.
Radiographic (x-ray) Evaluation
Special x-rays (CT scan) are taken to evaluate the condition of the inner ear bone.
Electric Promontory Testing
This test involves stimulation of the inner ear through an electrode inserted through the eardrum. Small electrical currents are passed through the electrode to determine if a sensation of sound is produced. This gives us information as to the suitability for cochlear implant surgery in totally deaf patients.
A general physical examination may be performed by an internist or pediatrician to determine if there are any contraindications to general anesthesia or surgical implantation of the electrode assembly.
Cochlear implant surgery
The poorer hearing ear of he two ears is usually selected for surgery.
Cochlear implant surgery is performed under general anesthesia through an incision behind the ear and involves opening the mastoid and middle ear. Surgery lasts about two and a half hours. Hair is shaved several inches above and behind the ear.
A coil (internal receiver) is embedded under the skin behind the ear and a wire (active electrode) is placed into the fluid which fills the cochlea (inner ear).
The patient is usually discharged from the hospital the day following surgery and can return home by air or ground transportation.
Rehabilitation and training
One to two months following surgery the patient returns and is fitted with the external speech processor, similar to a body hearing aid with a coil (external transmitter) which sits over the internal coil (internal receiver). This device receives incoming environmental and speech sounds through a microphone and transforms them into electrical currents. By means of magnetic coupling between the implanted and external coils, the current is able to stimulate the inner ear fluids and the nerve fibers which results in a sensation of sound.
The external device differs from a hearing aid in that a hearing aid amplified the environmental and speech sounds and delivers this amplified sound to the ear canal. The cochlear implant, on the other hand, is not a hearing aid because it does not stimulate the inner ear by amplified sound vibrations, but rather through direct electrical stimulation of the remaining hearing nerve fibers.
The first few sessions of rehabilitation involve using a computer to program the speech processor to the patient’s own responses. It often takes several sessions to fine tune the device, making it suitable for the patient.
After fitting to the external device, the next month is devoted to weekly rehabilitation sessions. This is to acquaint the patient fully with the implant equipment and its most efficient and effective use.
This program involves both the patient and the family; it is directed toward utilization of the new auditory clues available, as well as improvement in communication ability. Included is instruction in speech reading (lipreading), auditory (listening) training , and speech production.
Each patient is given instruction in a home practice program designed to continue the graining progress with the implant. It must be kept in mind that learning to effectively and comfortably use the signals provided by the implant takes considerable time and effort, perhaps up to a year. Patience is required by the implant user, the family, and acquaintances.
Six months and one year after beginning use of the implant, the patient returns for further testing and training. These follow up visits continue annually (every 6 months for young children).
Risks and complications of implant surgery
There are risks and complications associated with any operation. With regard to implant surgery, these are related to the mastoid surgery and to long term use of the implant.
The long term risks of electrical stimulation are unknown. One patient has had an implant since 1973 and many others for lesser periods of time. There have been no discernible or obvious problems related to this stimulation, but long term tolerance remains to be proven.
There have been failures of the internal coil. Fortunately, design changes over the years of experience with the implant have eliminated most of these failures. If problems do occur, or if new devices are later developed, the implant can be removed and another inserted.
Taste Disturbance and Mouth Dryness
Taste disturbances and mouth dryness are not uncommon for a few weeks following surgery.
Numbness of the Ear
Temporary loss of skin sensation in and about the ear is common following surgery. This numbness may involve the entire outer ear and may last for six months or more.
Weakness of the Face
The facial nerve travels through the ear bone in close association with the middle ear bones, eardrum and the mastoid. An uncommon post operative complication of ear surgery is temporary paralysis of one side of the face. This may occur as a result of an abnormality or a swelling of the nerve.
This is a remote possibility, but should it happen, the eye on the side of surgery would fail to close and the mouth would pull over to the opposite side. Further treatment would be required.
This is a rare complication that usually resolves spontaneously.
A hematoma (collection of blood under the skin) develops in a small percentage of cases, prolonging hospitalization and healing. Reoperation to remove this clot may be necessary if the complication occurs.
Complications Relating to Anesthesia
Anesthesia complications are very rare, but can be serious. You may discuss these with the anesthesiologist if desired.