Bone Anchored Hearing Aids
Since the late 1970’s the use of Bone Conduction Hearing Devices (BCHD) have been a possible way to treat conductive hearing losses (CHL) i.e. where the sound struggles to conduct through either the middle and/or outer ear. Possible situations where this may happen is when someone has a malformed ear, chronic ear canal infections, a hearing loss due to a perforation or even a condition like otosclerosis (fusion of the bones in the middle ear).
In recent years Bone Anchored Hearing Aids (BAHA) have been used for conducive losses with good success rates in treating physical/mechanical hearing losses.
How a BAHA works:
A BAHA is a special hearing aid that is attached to the skull through a special titanium screw called an abutment. It converts vibrations of sound into electrical information which it then amplified and converted back into mechanical vibrations that travel through the skull and directly to the cochlear. The vibrations then stimulate/move the fluid in the cochlear to create an electro-chemical signal. This form of hearing aid by passes the outer and middle ear and is different to BTE hearing aids as it doesn’t use air conduction.
Surgery and different abutments:
For the BAHA to work successfully it needs to be attached to the skull. This can be done in 2 different ways:
Percutaneous abutment - this is where a titanium abutment is attached to the skull. A small hole is drilled into the side of the skull (roughly above and behind the outer ear) where a screw thread is inserted. The abutment is then screwed into this thread and can be changed when necessary. Overall the sound travels best through this method as there is no attenuation (loss) of sound due to the skin, but will require regular maintenance and there is a possible risk of infection.
Transcutaneous abutment – this is where a magnet is inserted under the skin in the skull and the BAHA is attached to another magnet and then, when the 2 magnets meet then the BAHA is secured. This method is normally used where the skull is a little weaker or attaching the percutaneous abutment is not appropriated. One advantage of using this method is that the strength of the magnet can be varied, although in some cases there is a possible reduction of sound as the skin around the skull can create resistance for the sound.
Current NHS England situation:
Currently both adults and children who meet the following criteria are entitled to a BAHA device on the NHS in England:
Where traditional BTE hearing aids have been tried for a minimum of 4 weeks including the CROS/Bi Cros system have not been successful.
Have a hearing loss that falls within the manufacturers guide lines, this should be either a unilateral/bilateral conductive loss. This could also be a single sided SNHL.
Have stable bone conduction thresholds i.e. 15 dB deterioration in >2 frequencies in a 2 year period.
Have tried a bone conduction hearing device (BCHD) on a softband or headband for a minimum of 14 days and show benefit in speech tests.
In the UK the 2 main manufacturers are Oticon and Cochlear.
Bone Conduction V’s Air Conduction Hearing aids:
Below are some of the advantages of using a BAHA’s
Patients who have any middle or outer ear abnormalities such as microtia or atresia can be fitted with a BAHA to bypass a conducive hearing loss.
Infections – people who get persistent ear infections may find a BAHA more suitable as the ear canal is not occluded (blocked) with an ear mould.
Single sided deafness (SSD) – if a patient has a hearing loss on one side but has good hearing on the other, then a BAHA’s may be appropriate. The sound from the deaf side is then transmitted through the bone to the good side.
Below are some advantages of Air Conduction Hearing aids:
BTE hearing aids require no surgery and if the patient decides not to wear it then there are no side effects of surgery.
BTE hearing aids are a cheaper clinical option for audiology departments. To purchase a BAHA it will cost the NHS approximately £2500, where as a BTE hearing aid will cost £75 approx. This cost of surgery, fitting and follow up appointments will be extra.
For a young child a BAHA would not be considered before the age of 4 years old as the skull is not thick enough and therefore a BCHD on a head band or BTE hearing aid would be used.
It is important to point out that before any BAHA is fitted the decision will be made by a multi-disciplinary team (MDT) and extensive testing will be done to make sure that it is the right decision for the patient. Although the surgery is minor, and the procedure has been performed many times, there are still risks and complications as with any surgery.
Why two aids, not one?
Audiologists who fit BAHA devices must consider various factors when deciding if a BAHA is going to be successful for the patient. This can include things like lifestyle and dexterity. Due to current NHS funding adults are only entitled to 1 BAHA and children can be entitled to 2 depending on their level of hearing loss.
Studies have shown that bilateral (one on each side) BAHA’s can help in the development of language and speech as it improves localisation (the brains ability to detect where sound is coming from). This is especially important during school years as hearing loss can have a negative impact on cognitive and social development.
Other factors for BAHA selection can include whether the hearing loss is temporary or just on one side. If the loss is more permanent, then the patient will benefit more from a BAHA, but if it is temporary due to a condition such as glue ear then bone conduction hearing aids which are on a head band may be more appropriate.
Other research carried out suggests that for people that have other sensory disabilities such as a visual impairment, can have their quality of life improved through using a BAHA.
Overall for people with a specific CHL, a BAHA can be an appropriate solution in auditory rehabilitation. Later models now include wireless/Bluetooth capabilities which means devices such as mobile phones and MP3 players are now able to connect to the BAHA meaning that people who have a CHL can still enjoy activities that people with a normal level of hearing take for granted