A Discuslinesion of Cochlear Implantation

Background

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 patients with severe to profound hearing loss in both ears.

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 is designed to directly stimulate the auditory nerve fibres. It provides sound information to adults and children who have severe to profound nerve hearing loss in both ears, and who demonstrate little or no ability to understand speech through hearing aids or tactile devices.

Function of the Normal 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 membrane.

The middle ear chamber lies between the external ear and the inner ear and consists of an eardrum membrane and three small ear bones (ossicles): malleus (hammer), incus (anvil) and stapes (stirrup). These structures transmit sound vibrations to the inner ear. In so doing they act as a transformer, converting sound vibrations in the external ear canal 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. Movements of these hair cells generate an electrical current in the auditory nerve. This current is transmitted, through various complicated inter-connections in the brainstem, to that portion of the brain (auditory cortex), which recognises these electrical stimulations as sound.

Types of Hearing Impairment

The external ear and the 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 usually correctable by medical or surgical treatment.

When the hearing impairment is due to some problem in the inner ear, a sensorineural (nerve) impairment results. Sensorineural hearing impairment is not correctable by medical or surgical treatment but can be helped by amplification or in some cases by a cochlear implant.

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 brain stem or in the auditory cortex (hearing centre of the brain). Here, amplification is of no benefit.

Diagnosis

Hearing impairments are measured in decibels (dB). Normal hearing individuals have thresholds between 0 and 25 dB. A threshold of 26 to 40 dB is considered to be a mild impairment, a 41 to 55 dB threshold to be a moderate impairment, a 56 to 70 dB to be a moderate-severe impairment, 71 to 90 dB impairment is called severe, and a threshold of 91 dB or more is considered to be a profound impairment.

Candidates for a cochlear implant have severe or profound hearing impairment in both ears.

In most cases of hearing loss it is not difficult to determine the type of impairment. Carefully administered hearing tests (of pure tones and speech) and tuning fork tests allow the otologist (ear specialist) to decide whether the problem is conductive or sensorineural.

Sensorineural Deafness

Fortunately few hearing impairments progress to deafness (profound or total loss of hearing). Deafness may result though from a variety of causes such as infection, life-saving drugs or head injury. Congenital deafness (present at birth) usually results from unknown causes but may result from hereditary factors or 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 accident does not regrow.

If fluid waves in the cochlea have no hair cells to stimulate the nerve fibres they do not transmit an electrical impulse. It is exactly as if there was a telephone wire but no receiver; no amount of shouting at the wire would result in transmission of sound.

The Cochlear Implant

All cochlear implants consist 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 directly stimulate the auditory nerve, current that cannot he provided by damaged hair cells.

I. Who is a Candidate?

A. Children (18 months-17 years)

  • •Severe to profound sensorineural hearing loss in both ears
  • Little or no useful benefit from hearing aids
  • No medical contraindications
  • High motivation and appropriate expectation (both child and family)
  • Educational program with auditory/oral component

B. Adults

  • Severe to profound sensorineural hearing loss in both ears
  • Limited benefit hearing aids
  • No medical contraindication
  • Hearing loss acquired after using oral speech and language
  • Desire to be part of hearing world

II. Expected Benefits from Cochlear Implant Use

A. Children (18 months-17 years)

  • Children are able to detect environmental sounds, including speech, at comfortable loudness levels.
  • Some children can identify everyday sounds such as car horns, doorbells and birds singing from a set of alternatives. Many children can distinguish among different speech patterns.
  • Many children can identify words from a set of alternatives, without lip reading.
  • Some children can recognise speech without lip reading.
  • After training and experience with the device, many children demonstrate improvements in speech production.

B. Adults

  • Adults are able to hear conversation and environmental sounds at comfortable loudness levels.
  • Almost all adults experience improved lip reading ability.
  • Almost all adults improve their communication abilities when using the implant in conjunction with lip reading.
  • Some adults are able to understand speech in quiet and noise without lip reading.
  • Some adults have a limited ability to use the telephone.

Tinnitus

Severe tinnitus (head noise) often has been decreased by the implant.

BECOMING A COCHLEAR IMPLANT PATIENT

Becoming a cochlear implant patient involves an evaluation which includes otological (ear surgeon), audiological, radiological and psychological tests.

Cochlear implant surgery is performed under general anaesthesia. The surgery takes one to three hours to perform and the patient may go home the same day or stay overnight in hospital. Implant programming begins about six weeks following surgery.

Otological (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.

Audiological (Hearing) Evaluation

An audiological evaluation is performed to determine the level of hearing loss. A hearing aid evaluation or trial use of high-powered hearing aids is also performed to determine if conventional amplification will provide adequate benefit.

Radiological (X-ray) Evaluation

Special x-rays are taken to evaluate the condition of the inner ear.

If the candidate and the cochlear implant team agree to proceed with implantation, members of the team will assist in determination of insurance coverage and will schedule surgery.

Psychological Evaluation

Is undertaken to ensure that patients have realistic expectations and goals.

Cochlear Implant Surgery

The poorer hearing of the two ears is usually selected for surgery.

Cochlear implant surgery is performed under general anaesthesia through an incision behind the ear and involves opening the mastoid and middle ear. Surgery lasts about two to three hours. Hair is shaved several centimetres 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 of surgery (or the following morning) and can return home by air or ground transportation. The typical recovery period is 5 to 7 days.

Rehabilitation and Training with the Cochlear Implant

Five to six weeks following surgery the patient returns and is fitted with the external components: a body-worn and/or ear level speech processor and a head set consisting of a microphone and transmitter coil, 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, these currents stimulate the inner ear fluids and the nerve fibres, which results in a sensation of sound.

The audiologist programs the speech processor, setting the appropriate levels of stimulation for each electrode, from soft to comfortably loud. The program can easily be adjusted as the user becomes accustomed to the new sound information.

Patients (especially children) need to be seen more frequently during the initial six months for reprogramming. After that, follow-up visits are recommended every six months or as needed. It must be kept in mind that learning to effectively and comfortably use the signals provided by the implant takes considerable time and effort, especially for pre-linguistically deafened children. Patience is required by the implant user and the family. A consistent program of speech, language and auditory skill development is necessary for children to make maximum use of the cochlear implant. Adults will typically achieve their maximum performance after six to twelve months implant use.

Risks and Complications of Surgery

There are risks and complications associated with any operation. In regard to implant surgery these are relate 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 discernable or obvious problems related to this stimulation, but long-term tolerance remains to be proven.

There have been failures of the internal coil (less than 2% of the over 15000 implanted patients worldwide); 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 disturbance 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. The numbness may involve the entire outer ear and may last for six months or more.

Infection

The risk of infection is quite small, but should this occur it would require treatment and could cause the operation to fail.

Weakness of the Face

The facial nerve travels through the ear bone in close association with the middle ear bones, eardrum and mastoid. An uncommon post-operative complication of ear surgery is temporary or permanent 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 the surgery would fail to close and the mouth would pull over to the opposite side. Further treatment would be required.

Haematoma

A haematoma (collection of blood under the skin) develops in a small percentage of cases, prolonging hospitalisation and healing. Re-operation to remove this clot may be necessary if complication occurs.

Complications Relating to Anaesthesia

Anaesthetic complications are very rare but can be serious. You may discuss these the anaesthesiologist if desired.

Should any question arise regarding your hearing impairment or cochlear implantation assessment, please feel free to call or write to Dr Nirmal Patel.

Adapted from the Patient Discussion Booklet

Series of the University of Utah Otolaryngology.

Reprinted by Permission.

Reproduction in part or whole of this material is restricted by copyright law unless express permission is granted by the Norwest ENT Group.

For more information on our cochlear implant services please visit our hearing implant centre:

http://www.northsidehearing.com.au/index.html

Northside Implantable Hearing.


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