Cochlear Implants

Hearing loss is a relatively common problem, particularly in the older population. There are about 9 million individuals suffering from some degree of deafness in the UK, about 700,000 of these being severely or profoundly deaf.¹ Over 800 babies are born in the UK with significant deafness¹ and about 1 in every 100 children is profoundly deaf by the age of 3.² It is categorised as conductive (interference of transmission of sound from the outer to the inner ear) or sensorineural (damage within the cochlea, auditory nerve or auditory centres within the brain). Go to the following related articles for more information about:

Deafness
Deafness in adults
Deafness in children

Some individuals with hearing problems choose to identify themselves with a cultural model of deafness and become members of the 'Deaf Community'; they may not consider their deafness as a disability. Others may have grown up in a hearing environment (e.g. late onset deafness or a deaf child born into a hearing family) and therefore deafness for them is a significant disability.

There are a number of approaches to the management of deafness which are discussed in the above articles. This record will give you an overview of cochlear implants and the issues surrounding them.

A cochlear implant is an electronic device that stimulates cells of the auditory spiral ganglion to produce a sense of sound in a person with hearing impairment. There is an external and internal component:

• External component - this is a microphone and word processor which is worn externally behind the ear. The processor is connected to a transmitter coil worn on the side of the head.
• Internal component - a receiver/stimulator package is surgically implanted in the mastoid bone. It receives and translates data into electrical signals that are delivered to electrodes placed in the cochlea.These electrodes stimulate the spiral ganglion cells that innervate the fibres of the auditory nerve. This provides a sensation of hearing but does not restore hearing.

Various cochlea implant systems exist. For example, there is a totally implantable device available (although it has been found to be less effective than devices with internal and external components).³

During the year ending March 2008, about 700 patients had unilateral cochlear implants in England and Wales. Almost 80 bilateral implants (mostly in children) were inserted.

General practitioners need to be aware of this rapidly advancing field so that they can refer patients who may benefit from this exciting innovation. If a patient who has had an implant joins the list it is important to ascertain that there is a full supportive team in place in the new locality. This may include a social worker for the deaf.

Inclusion criteria: the patient

Referral is to the local ENT consultant who makes an initial assessment and decides whether to refer on to one of the national cochlea implant centres.

Providing a unilateral implant for an adult who became deaf after achieving language, has been the earliest indication and remains the most common. However, implants may be used for adults who became deaf before they developed language and more recently, for children younger than one year old.

NICE recommends the following categories of patients as being suitable for consideration:²

• Unilateral implants - patients with severe to profound deafness who do not experience adequate benefit from hearing aids after a valid 3 month trial (providing that there are no contraindications and that the hearing aid option is appropriate).
• Simultaneous bilateral implants - children (infant to 18 years) with severe to profound bilateral deafness or adults who have an additional impairment (e.g. blindness) that increases their reliance on auditory stimuli as a primary sensory mechanism for spatial awareness. Children must additionally be in an education programme of hearing and listening.
• Sequential bilateral implants - these are generally not recommended although those patients who have received a unilateral implant prior to the latest NICE guidance may occasionally be offered a second implant after specialist assessment.

For the purpose of inclusion criteria, severe to profound deafness is defined by NICE as hearing only sounds that are louder than 90 dB HL (roughly: shouting) at frequencies of 2 and 4 kHz without acoustic aids. Specialist tests define the degree of benefit provided by acoustic aids. In addition to the inclusion criteria, there must be no medical contraindications such as cochlear aplasia or active middle ear infection. Adults who have had meningitis may get some return of hearing over the next 6 months and this should be awaited before proceeding.

The team

A successful outcome requires collaboration from patients, families, schools, audiologists, speech and hearing therapists and surgeons. Preoperative expectation shapes postoperative satisfaction and use of the implant. Therefore, all patients and families require counselling from an implant team before such a major undertaking and a good line of communication is essential.

Assessment

Potential candidates will be referred to one of the cochlear implant centres of England and Wales where they receive a multidisciplinary assessment to determine their suitability for cochlear implantation.²

Functional

Preliminary tests are important to predict outcome.⁴ For adults and children who can cooperate, standard pure-tone and speech audiometry tests are used for screening. For children, the speech reception threshold (SRT) and/or pure-tone average (PTA) should be at least 90 dB. For adults, the SRT/PTA should be at least 70 dB. A number of speech recognition tests are currently in use.

One of the most used speech recognition tests is the Hearing In Noise Test (HINT), which tests speech recognition in the context of sentences. HINT measures word recognition abilities to determine suitability for cochlear implant. HINT consists of 25 equivalent 10-sentence lists that may be presented in either quiet or noisy environments to assess the understanding of the sentences.⁵ The first test is in a quiet surrounding, using 2 lists of 10 sentences that are scored for the number of words correctly identified. Then in a noisy situation the other sentences are tested. For adults, the current threshold for cochlear implant suitability is a HINT score of under 40%. For children, the threshold is a score less than 20%. See Further Reading for a link to audiometry testing.

Anatomical

It is necessary to have pre-operative CT scan or MRI to evaluate the cochleovestibular apparatus and internal auditory canals. It may reveal absence or abnormal calibre of the internal auditory canal and/or cochlear dysplasia. In children or young adults with progressive hearing loss, MRI is required to exclude neurofibromatosis type 2. MRI is better at revealing fluid spaces of the cochlea and is increasingly the primary imaging of choice.

The operation⁶

The operation is carried out in a specialist centre. It is done under a general anaesthetic and takes between 2 and 5 hours. Risk factors associated with the procedure include:

• Temporary tinnitus or vertigo.
• Numbness over the site of the scar.
• Damage to the facial nerve/change in taste sensation.
• CSF leakage or infection.

Pneumococcal otitis media is frequent in young children and could lead to labyrinthitis after implantation. For this reason pneumococcal vaccination is recommended.

The length of in-patient stay varies between units but can be expected to be in the region of 2 or 3 days with a further fortnight's rest at home (after which hair can be washed). The processor is only switched on a few weeks after the operation. It will then be 'tuned' by the team to suit the patient who may take a while to become accustomed to the new sounds.

Post implant issues⁶

Generally, once the patient is used to the implant, they can carry on life as usual. The only thing to avoid is contact sports (such as boxing or rugby). Swimming is fine once the external piece is removed but scuba diving should be avoided as pressure changes can affect the unit. The external unit may be replaced as technology progresses. If the internal part fails, this can also be replaced.

General points

The benefit is highly variable, depending on a number of physical, psychological and social factors. Improved performance, with increasingly sophisticated electrodes and programming strategies, has widened indications for operation. Overall however, studies consistently show statistically significant benefits in various specific areas (such as hearing and speech production) as well as in more general areas (such as quality of life).²

• Over 90% of implants survive beyond 11 years.⁷
• The quality of life in adults (including the elderly³) given cochlear implants improves with less isolation and depression.⁸
• A number of studies have shown that cochlear implants can reduce tinnitus, even in the contralateral ear (although occasionally, it can get worse too).
• Benefit increases with time and the mode of action appears to be multifactorial.⁹
• There is evidence that bilateral implants produce a better result than unilateral implants, especially when listening in adverse conditions.¹⁰

Implants in children

A number of factors affect the outcome of cochlear implants in children in particular.

• Cochlear implants are associated with improvement of hearing, speech perception and speech production.² The quality of life as perceived by the child and by the parents has also been noted to improve.
• The age at onset of deafness and duration of deafness before implantation are important. Earlier surgery favours a better outcome in terms of communication.³ Progressive hearing loss allows time for development of lip-reading skills and favours performance.
• Sometimes children have multiple handicaps which may not be evident until after implantation. Even if results are less than perfect such children seem to obtain benefit.¹¹
• Children with learning difficulties can benefit.³
• The school should emphasise oral rather than sign language to maximise listening and language development even though they continue to need specialist support.³

It can be very difficult to predict outcome. The expectations of the families must be realistic. Counselling should stress the need for long-term therapy, variable outcome and limitations of the technology.

The function of electrodes will continue to improve which will broaden indications for operation. Studies of bilateral implants suggest improved ability to localise sounds and improved ability in noise.

Short implant electrodes are being developed to allow implantation of patients with good low-frequency hearing and poor high-frequency hearing. The electrode is placed atraumatically in the basal turn of the cochlea through a small cochleostomy to preserve low-frequency hearing and a conventional hearing aid for low-frequency amplification can also be worn. This approach would help patients who are not suitable for conventional implants because their low-frequency hearing exceeds current guidelines. As the technology improves it may be acceptable to use implants where there is just unilateral hearing loss.

Normal auditory nerves fire asynchronously. In implanted ears, synchronous stimulation of the spiral ganglion occurs. Preliminary reports suggest that tonic stimulation of neurons at 5000 Hz may improve performance of implants. Patients given tonic stimulation report improved perception of sound timbre and some improvement in speech discrimination.

It is now possible to place a device in the malformed cochlea. When an auditory nerve is absent or when implantation failed despite a functional device, auditory brainstem implants can restore some form of hearing to the deaf.¹²