Monday, 25 July 2011

Bone Anchored Hearing Aid

Published in City Times

The BAHA offers hope for the hearing impaired

There’s hope yet, for those suffering from congenital hearing loss. And the heartening news is that age is no bar to this treatment and children as young as two years of age, with congenital hearing impairment can be successfully habilitated with BAHA(Bone Anchored Hearing Aid) Technology, says Dr Sunil Narayan Dutt, ENT Specialist and Departmental Coordinator, Canadian Specialist Hospital, Dubai.

“The BAHA is now a well-established form of hearing rehabilitation in most parts of the western world. It is for patients who will benefit with conventional hearing aids but cannot use them for various reasons”, explains Dr.Dutt

Air conduction hearing aids and bone conduction hearing aids are the two categories of hearing aids that offer hope to the hearing impaired, depending upon the way in which they conduct sound. BAHA promises to be an improvement, even over the conventional bone conduction hearing aids hitherto being used. Since the BAHA sound processor works without pressure on the skin, it avoids the headaches and soreness associated with the conventional bone conductor. Also, since the BAHA device is placed behind the ear, under the hair, it is not perceptible to others. Because it is held in place by a clip and directly integrated with the skull bone, there is no need for a head band and pressure against the skin of the head.

Hearing impairment or hearing loss may be either sensorineural, that is, involving the inner ear or it may be conductive in nature, involving the external and middle ear. When hearing loss is associated with the inner ear, the only treatment option in most cases may be to fit unilateral or bilateral air conduction hearing aids or under select circumstances, cochlear implantation. The use of air conduction aids may be precluded in instances where the conductive loss results from congenital malformations of the external and middle ear structures. The best treatment option in such cases might be to use bone conduction hearing aids which directly stimulate the skull and transmits sound to the cochlea.

The BAHA is an option for patients with maximal conductive hearing loss but normal bone conduction thresholds. The clinical indications for individuals who can benefit from BAHA, explains Dr.Dutt, include the groups of patients who will benefit but cannot wear conventional hearing aids.
1. Those with congenital ear abnormalities that cannot be fitted with conventional air conduction aids.
2. Those with a conductive or mixed hearing loss who are unable to wear conventional aids because of discharge, irritation or feedback (chronic suppurative otitis media and chronically draining mastoid cavities).
3. Those with otosclerosis (a condition that fixes the tiny bone in the middle ear), who cannot or will not wear conventional aids and who will not contemplate stapedectomy.
4. Those with single-sided deafness of a profound variety who wish to reap the benefits of binaural hearing such as, directional hearing (sound localization), better speech recognition in the presence of background noise and (pseudo) stereo effect.
5. Those with unilateral conductive or mixed loss when the second ear is profoundly deaf and correctional surgery carries too great a risk.

Patients with severe hearing loss on one side, but normal hearing in the other ear have difficulty understanding speech amidst background noise; they are unable to determine the direction of the sound. This type of unilateral deafness can result from trauma, viral infections, acoustic neuromas and other ear tumors and ear surgery. For such individuals, the BAHA device, placed on the side of the deaf ear, transfers sound through bone conduction and stimulates the cochlea of the normal hearing ear, thus resulting in a sensation of hearing from the deaf ear.

What is the basis on which decision is made to ensure that an individual is a candidate for BAHA? Dr.Dutt explains, “The primary investigation is audiological. Questionnaires can be helpful to exemplify the problems followed by pure tone audiometry, aided and unaided thresholds and speech audiometry. The recommendations are for bone conduction thresholds no worse than 45 dB for the ear level device (BAHA Compact) and 60 dB for the body worn one (BAHA Cordelle). The decision as to the suitability and advisability of the BAHA for a particular patient is made in the multidisciplinary clinic and a treatment plan formulated to include long-term management and maintenance.”

How exactly does BAHA help sound transmission? The BAHA transmits sound directly to the skull without interference from intervening tissue which can dampen the acoustical signal by as much as 10-20dB (Decibells). The BAHA consists of three parts: a titanium implant, an external abutment and a sound processor. Sound travels to the inner ear by bypassing the external auditory canal and the middle ear. This type of hearing aid is attached to a titanium screw implanted into the skull which vibrates in response to sound, and sends the vibrations to the cochlea via the bones of the skull. The vibrations are then converted into sound in the usual way.

Highlighting the beginnings of BAHA, Dr.Dutt explains, “The concept of direct bone conduction with hearing aids, using the principle of osseointegration, was introduced by Prof Tjellstrom from Sweden and is achieved by using a skin penetrating coupling from a titanium implant in the mastoid bone to an impedance-matched transducer. That is, the patient can apply and remove the Processor at will. The titanium oxide surface is highly biocompatible and integrates with the osteocytes to form a stable interface that can withstand large stresses without displacement and this osseointegration phenomenon takes about 6 weeks to 3 months to be complete. Direct bone conduction is achieved by the absence of interposing soft tissues and this gives better sound quality, requires less energy and offers much greater comfort than conventional bone conduction hearing aids.”

What does the implant procedure involve? Dr.Dutt explains that the titanium implant is placed during a short surgical procedure and over time it naturally integrates with the skull bone. Surgery must be meticulous the site chosen should be planned using a template and the side decided with the patient depending on their requirements such as handedness and car driving. In the vast majority of adults the surgery is performed under local anaesthesia with some preferring light sedation; children however will require general anaesthesia with a two-staged procedure but it always can be performed as a day case. The preferred site invariably falls within a hair-bearing region. 6 mls of local anaesthetic is infiltrated after shaving the area.

An inferiorly based split thickness skin graft is raised using a dermatome and the soft tissues down to the periosteum excised. The periosteum is minimally opened using a cruciate incision and then the special equipment needed to insert the titanium 'fixtures' used. The essence of this part is the production of a threaded hole at 90 degrees to the bone surface into which the titanium fixture (3 mm or 4 mm) is inserted with absolutely minimal trauma to the living bone cells (osteocytes). This would then constitute the end of the first stage in a child but in an adult the 'abutment' which is the attachment for the aid is now fitted and the skin closed with only the abutment protruding. More recently, a single self tapping pre-mounted fixture-abutment assembly is fitted in a one stage procedure. The wound is dressed using ribbon gauze soaked in steroid antibiotic cream and 3 months later when osseointegration has occurred the aid is ready to be fitted.

What are the post surgical measures required to be followed by the patient? The ENT BAHA nurse is responsible for the after-care and follow-up of the patients' initially until fitting of the BAHA. This includes regular dressings and inspection and care of the skin graft/flap and cauterisation of small granulations, removal of crusts and topical antibiotic-steroid cream application. The nurse is responsible later, that is, after aid-fitting, for any wound or flap related issues on a 'when required' basis. The patient is ready to snap-on or clip-on the hearing aid processor after 3 months (osseointegration time), explains Dr.Dutt.

Is it possible to determine the benefits from the procedure? Dr.Dutt assures that evidence of benefit can be obtained from audiological testing and questionnaire feedback.

Long term outcome analyses from several pioneering centres have shown good stability of the device, audiological benefit, reduction of otorrhoea, increased comfort, patient satisfaction and improved quality of life. The device reduces the risks of conventional hearing restoration surgery, is predictable and is suitable for use in children, adds Dr.Dutt.


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