Cochlear Implants

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. In 2005 around 85,000 people worldwide were thought to have received a cochlear implant. In England in the 2 accounting years 2004/5 and 2005/6 there were 1,406 operation performed. The number of operations performed is increasing.

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. The indications for use expand as the technical results become better. Originally they were used only for adults with profound loss of hearing but as they have improved they have become accepted in children and now are inserted at just 1 or 2 years old. There is limited experience of inserting implants before the first birthday.¹

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.

Preliminary tests are important to predict outcome.² 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.³ 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%.

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 for children younger than one year old.

Candidates for implants must fulfil the following criteria:

• If between 18 months and 18 years old they must have profound sensorineural hearing loss in both ears. This means SRT/PTA rated as "not useful" at 90 dB hearing loss or worse.
• They must benefit from hearing aids.
• Children over 5 years must score 20% or less on sentence recognition tests under optimal conditions. This includes use of a hearing aid.
• They must be in an education programme of hearing and listening.
• There must be no medical contraindications such as cochlear aplasia or active middle ear infection.
• There must be good communication between patients, families, schools, audiologists, therapists and surgeons.
• Sometimes children have multiple handicaps and sometimes these handicaps are not diagnosed until after implantation. Even if results are less than perfect such children seem to obtain benefit.⁴
• Adults must have severe or profound sensorineural hearing loss in both ears. The SRT/PTA must be rated at less than 70 dB hearing loss, or they score under 40% on sentence recognition tests with hearing aids.
• Adults also need no medical contraindications along the same line as children.

The cause of hearing loss is a single gene mutation in a third to half of cases in childhood. About a quarter to a third are environmental causes and another quarter to a third are of unknown aetiology.

Many of the last group are probably spontaneous genetic mutations. Meningitis causes nearly 10% of childhood deafness and can make implantation difficult. Nearly a third of such cases are caused by S pneumoniae. The use of dexamethasone in meningitis may reduce the incidence of subsequent hearing loss, especially with infection from H. influenzae but results are controversial.⁵ Sometimes meningitis results in calcification of the cochlear. Congenital rubella does not seem to feature prominently but that could change if uptake of MMR is not improved.

In adults the commonest cause of hearing loss is noise damage. Other causes include Meniere's disease, otosclerosis, temporal bone trauma, autoimmune hearing loss, and ototoxic drugs. Adults who have had meningitis may get some return of hearing over the next 6 months and this should be awaited before proceeding.

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

• The age at onset of deafness and duration of deafness before implantation are important. Earlier surgery favours a better outcome in terms of communication. Progressive hearing loss allows time for development of lip-reading skills and favours performance.
• The school should emphasise oral rather than sign language.
• Despite this 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.

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.

• Over 90% of implants survive beyond 11 years.⁶
• The quality of life in adults given cochlear implants improves with less isolation and depression.⁷ This is noted in the elderly too.⁸
• Even children with learning difficulties can benefit.⁹
• A potential post-operative complication is leakage of CSF and meningitis. Pneumococcal otitis media is frequent in young children and could lead to labyrinthitis after implantation. For this reason pneumococcal vaccination is recommended.
• A number of studies have shown that cochlear implants can reduce tinnitus, even in the contralateral ear. Benefit increases with time and the mode of action appears to be multifactorial.¹⁰
• There is evidence that bilateral implants produce a better result than unilateral implants, especially when listening in adverse conditions.¹¹

General practitioners need to be aware of this rapidly advancing field so that they can refer patients who may benefit from this exciting innovation.¹² 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.

Earlier age of implantation may be beneficial. This was illustrated by a study of Chinese children.¹³ Mandarin has 4 different tones. Other Chinese dialects and other Asian languages also use tones. Children with implants were correctly able to identify tones in just under 50% of tests compared with around 80% for age matched controls with normal hearing. Those who had received the implant at a younger age and those who had had it longer did better. A study from Croatia also found that early implantation and good rehabilitation in children resulted in better speech.¹⁴

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.¹⁵