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Welcome to Entclinic.co.uk
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| Digital Hearing Aids |
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The introduction of digital hearing aids by the National Health Service has rekindled general interest in the rehabilitation of patients with hearing loss. Whilst the hardware contained in the new aids can compete with those in the private sector cosmetic advantages of the “in the canal aid” and the “in the ear aid” are still significant. Unfortunately the “digital revolution” has not produced the panacea for hearing loss. Some patients still remain dissatisfied; the aid is not strong enough, it irritates the ear canal, leads to infection, causes feedback and of course doesn’t significantly improve the patients ability to discriminate speech especially in noisy environments.
Over the past decade auditory implants have been introduced that complement and sometimes challenge the role of the conventional aid.
In cases of profound bilateral sensorineural hearing loss the Cochlear Implant has become the mainstay of management. The implant surgery has become standardised although only performed in a few centres. Transcranial scars have been replaced by minimally invasive 5cm scars in the retroauricular sulcus. Indications for surgery have broadened although ‘ length of the profound hearing loss’ and ‘the score obtained in a word discrimination test’ still remain the best predictive factors. While it is not possible to predict the outcome in any one case,using data collected nationwide, an Odds Ratio can be constructed to indicate the chance of a particular outcome e.g. a 6:1chance of improving over the best preoperative hearing would suggest that if 7 patients profoundly deaf for exactly the same length of time and having the same discrimination score were to be implanted 6 would score as well or better and 1 worse. Changes in electrode design and speech processing software have led to improvements in hearing acuity such that nowadays nearly 50% of implanted patients can have a telephone conversation. Those candidates with recordable low tone hearing may benefit from shorter electrodes that conserves the intra cochlear structures and allow a conventional hearing aid to stimulate the apical turns by acoustic energy . More children are being implanted than adults and they are being implanted earlier and earlier. Their responses monitored using neural response telemetry. In the congenitally deaf child the best results follow implantation before the age of 3 yeas. In cases of meningitis urgent action is taken to access and implant prior to cochlear ossification consequent on ascending infection in the cochlea.
More recently bone anchored hearing aids have been introduced to help those patients with a conductive hearing loss who due to pain or recurrent discharge are unable to wear a conventional aid. The BAHA supersedes the old band bone conductor being more stable and acoustically a good deal more efficient. A surprising new indication for the BAHA is in patients with a single sided deafness (complete sensorineural hearing loss in one ear with a normal hearing in the other). These patients who have always been told there is nothing that can be done have difficulty localising sound, hearing in a noisy environment and of course hearing in the deaf ear. The hearing aid output is passed to the hearing cochlear by bone conduction and is then perceived by the brain as to come from the deaf side.
The baby of auditory implants is the middle ear implant. An extraordinary bioengineering innovation that employs an electromagnet to vibrate the ossicular chain The electromagnet called the floating mass transducer is fixed to the long process of the incus; its design characteristics are such that they mimic the natural transfer function of the middle ear ossicles. Using a technique called Laser Doppler Vibromatory the movement of the ossicles can be studied in detail. These studies have shown that at near normal thresholds to sound the movement of the stapes is of the order of 1-2 microns, near molecular size. The middle ear implant is used for patients with moderate to severe sensorineural hearing loss who cannot use a conventional aid and consist of an external auditory processor that communicates through the skin with a modulator which electrically stimulates the floating mass transducer. The advantages of the system are increased amplification of the middle and higher frequencies where speech is understood. As there is no acoustic imput there is no feedback and the occlusion effect ( the sensation in the ear, usually to the patients own voice, resulting from a hearing aids physical occlusion of the ear canal) is lost. Substantial gains in auditory performance can be achieved and an improvement in the quality of sound may be achieved.
What for the future?
Improving technology will lead to better speech discrimination for cochlear implantees utilising every last spiral ganglion cell active in the cochlea. Totally implanted device both in the cochlea and middle ear are just around the corner. Digital hearing aids are being made available to all on the NHS (just remember digital does not always mean best!). We are on the cusp of a revolution in the care of patients with hearing loss.
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London Bridge Hospital
27 Tooley Street
London, SE1 2PR
Tel: 020 7407 3100
Fax: 020 7407 3162 |
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