Cochlear Implants

About Sensorineural Hearing Impairment and Cochlear Implants

Cochlear implants can help restore hearing loss which is caused by a congenital abnormality of the cochlea, part of the hearing apparatus of the inner ear. This is a common type of deafness in children who are born deaf.

Cochlear implants (or bionic ears) are also commonly used in adults where their hearing aids are not working well for them. In 2010, worldwide, more cochlear implants were placed in adults than children, and it has been predicted that this is the catagory of patinets who will need the most bionics ears in the future.

The Adult criteria for becoming a cochlear implant candidate are changing all the time. It may surprise many to know that if their hearing aids are not working well for them they can dramatically improve their hearing and quality of life by considering cochlear implants.

Cochlear implants are electronic devices which stimulate the auditory nerve fibres directly, to allow the sensation of sound to reach the brain. Part of the device is implanted in the temporal bone of the skull, and part is worn on the body like a behind the ear hearing aid. It is suitable for people of all ages who have little or no ability to understand speech through hearing aids.

Healthy ear function

The outer, middle and inner ear each play a different role in providing hearing.

The auricle and ear canal make up the outer ear. They are designed to gather sound from as wide an angle as possible, directing it towards the tympanic membrane (eardrum).

The eardrum and ear bones or ossicles make up the middle ear. There are three small ear bones – the hammer (malleus), the anvil (incus) and the stirrup (stapes). The eardrum and the ear bones act as a transformer, converting sound waves in the air to fluid vibrations in the inner ear.

The inner ear contains structures both for hearing and balance. The auditory (hearing) part of the inner ear is called the cochlea, a Latin term which means snail shell (which the cochlea resembles).

The cochlea contains over 16,000 delicate hair cells. The middle ear bones transmit sound to the hairs of the cochlea via fluid waves, and the movement of the hair cells generates an electric stimulation of the auditory nerves. Eventually this nerve stimulation reaches the auditory cortex of the cerebrum, which interprets them as sound.

Conduction versus Sensorineural Hearing Impairment

There are two main types of hearing impairment: conduction and sensorineural.

Conduction hearing impairment occurs when there is an obstruction in some part of the ear which prevents sound transmission to the inner ear. Medical and surgical treatments are usually very effective for conduction hearing loss.

Sensorineural hearing impairment occurs when there is a problem with the sensory or neural structures which transmit sound from the inner ear to the brain. Amplification devices may help in some cases, and others can be corrected with a cochlear implant. These days there are even hybrid (half hearing aid/ half cochlear implant) devices which may help your hearing.

A third and uncommon type of hearing loss is central hearing impairment. This occurs due to problems with the complex interconnections that transmit nerve signals from the cochlea to the auditory cortex. Amplification is of no benefit in central hearing loss.

How will my hearing loss type be diagnosed?

Your audiologist will need to measure the severity and the type of your hearing loss.

Sound is measured in decibels (dB), as are hearing impairments. The following thresholds apply:

  • 0-25 dB for normal hearing individuals
  • 26-40dB threshold is considered a mild hearing impairment
  • 41-55 dB threshold is a moderate hearing impairment
  • 56-70dB is considered a moderate-severe hearing impairment
  • 71-90 dB threshold is considered a severe hearing impairment
  • 90dB+ threshold is called a profound impairment

If you have severe or profound hearing impairment in both ears, you may be a candidate for cochlear implant surgery.

Your audiologist will also perform pure tone and speech hearing tests, as well as tuning fork tests to determine whether you have conductive or sensorineural hearing impairment.

Sensorineural Deafness

Profound or total hearing loss (deafness) is fortunately quite a rare condition. It can result from serious infection, or may be a complication of life-saving drugs or head injury. Hereditary factors or a prenatal viral infection can cause congenital hearing loss, which is present from the time of birth.

Sensorineural deafness is most often due to damage to the hair cells in the cochlea. These cells do not heal or regenerate in the same way that other somatic (body) cells can. Without the hair cells to stimulate the auditory nerves, it is as if the microphone has been removed from a telephone – the hardware is there to transmit sound, but the sound cannot reach it. If this is the case, the impairment can be improved with a cochlear implant.

How do Cochlear Implants work?

Cochlear implants have two main components – an internal coil which is implanted under the skin, and an active electrode which is placed in the fluid-filled coils of the cochlea. The internal coil converts sound waves into electrical current to directly stimulate the auditory nerves, bypassing the malfunctioning cochlear hair cells.

Am I a candidate for Cochlear Implants?

We recommend that those considering cochlear implant surgery have the following qualities:

Adults over the age of 18

  • Hearing loss is classified severe to profound and is bilateral
  • Hearing aids are providing little or no benefit
  • There are no medical reasons not to go ahead with the surgery
  • Acquired hearing loss after learning to use language
  • High motivation and appropriate expectations
What benefits can I expect from Cochlear Implants?

Adult Recipients of cochlear implants can expect the following improvements after surgery:

  • The ability to hear conversations and ordinary environmental noises at comfortable volumes
  • Improvements in lip reading accuracy, and therefore improved communicability
  • The possibility of learning to talk on the phone
  • Many adults may be able to understand speech without lip reading
  • Tinnitus symptoms are usually improved by the cochlear implant.
The Cochlear Implant process

You’ll need to complete a range of tests related to your hearing, ear anatomy and head anatomy and psychology in preparation for getting a cochlear implant. These are carried out with an otologist, audiologist, radiologist and psychologist.

Ear anatomy examination

An otologist (hearing specialist) will examine you to ensure that you are free of any active infection or anatomical problems that contraindicate implanting a cochlear device.

Hearing examination

Your level of hearing loss will also need to be evaluated – severe and profound bilateral hearing loss is a criteria for cochlear implant suitability. Before the decision is made to go ahead with surgery, your audiologist will trial a conventional amplification device like a hearing aid to see if it provides adequate benefit. We recommend a cochlear implant audiology assessment at Northside Audiology (www.northsideaudiology.com.au).

Inner Ear x-ray examination

A radiologist will examine your inner ear structures by CT and MRI to evaluate their condition.

Psychological examination

A psychologist evaluates your preparedness, motivation and expectations regarding cochlear implant surgery.

If all the members of the medical team agree that surgery will be a suitable option for you, the next step is to determine insurance coverage and schedule the actual surgery.

What is Cochlear Implant surgery like?

Your medical team will usually select the ear with less hearing ability to receive the implant. This reduces the risk of you losing hearing ability through surgical complications, and provides greater net benefit.

The actual surgery is performed under general anaesthesia and takes between 2 and 3 hours. An overnight stay in hospital may be required for some patients. The incision is made behind the ear, and a section of hair will need to be shaved for access. During the surgery, the active electrode wire is placed into the fluid filling the cochlea, with an internal receiver (or coil) inserted under the skin posterior to the outer ear. You can return home by plane or ground transport, but will need somebody to drive you home. It usually takes around 5-7 days to resume normal activities. To find out more about the cochlear implant process visit:

www.northsideaudiology.com.au

Learning to use your Cochlear Implant

The first step in learning to use your cochlear implant is to have the external components fitted, including ear-level speech processor, a headset which consists of a microphone and sound transmitter. This will occur around 4 weeks after the initial surgery.

The headset is placed over the top of the internal coil. These devices receive speech and other incoming environmental sounds through the microphone; the transmitter coil transforms them into electrical currents; and these are transferred to the inner ear active electrode through a magnetic coupling between the inner and outer receiver. The active electrode stimulates the auditory nerve directly, and this message is delivered to the brain as the sensation of sound.

Your audiologist will program the speech processor, setting the appropriate level of sound stimulation for each electrode (which is interpreted as volume). As you become accustomed to the new sound information, the program will be adjusted – this process is called Mapping the Implant.

Reprogramming will need to occur several times. Usually 3 to 4 trips are required to the implant audiologist at Northside Audiology for reprogramming to provide an optimal learning environment for previously hearing impaired people to learn (or re-learn) to interpret the sound signals. Adults usually reach their optimal usage level in 6-12 months after implantation, however, recent research from New York University suggests that even implant recipients over the age of 80 continue to learn and gain new hearing for up to 2 years after implantation.

Does Cochlear Implant surgery have any risks?

All operations carry some degree of risk, which patients must weigh against the benefits for themselves. Specifically with regard to cochlear implants, risks relate to mastoid surgery and the long term use of the device.

Cochlear implants are still relatively new in the medical world. Long term tolerance to direct electrical stimulation of the auditory nerve must still be proven, though there are many examples of implant recipients that have had the device for upwards of two decades.

In some very rare cases the internal coil has failed. If this occurs, it is possible to remove the malfunctioning implant and insert a new one.

Dry mouth and taste disturbances

Your mouth may feel dry and taste may be disturbed for several weeks after the device is implanted.

Loss of surface sensation

You may lose surface sensation on your outer ear following cochlear implant surgery, in small areas or across the entire ear. This usually resolves spontaneously within 6 months after surgery.

Infection

Infection can occur in a very small number of cases. If you develop symptoms, it is critical that you get prompt treatment – infection can impact on the success of the implant.

Facial weakness

Swelling of the facial tissues or abnormalities of the facial nerve itself can sometimes cause weakness or temporary or permanent paralysis of one side of the face. For people who experience this complication, the eye on the affected side may fail to close and the mouth corner can pull to the side. This is a very rare complication of cochlear implant surgery.

Haematoma

In a very small percentage of cases, blood may pool under the skin in a clot known as a haematoma. The haematoma can prolong healing and may require further hospitalisation or re-operation for its removal.

Anaesthesia complications

In very rare cases, there may be serious complications related to the anaesthesia. Your anaesthetist can help you make an informed decision about the risks and benefits.

Should any question arise regarding your hearing impairment or cochlear implantation, feel free to get in touch with us. You can also find out more at:

www.northsideaudiology.com.au

Videos

How hearing works

Video by Associate Professor Nirmal Patel.