Non-routine Hearing Aid/s
Bone Anchored Hearing Aid (BAHA)
A bone anchored hearing aids (BAHA) is a type of hearing device that is surgically implanted. It delivers acoustic output to the mastoid process of the temporal bone, rather than through the outer ear like conventional hearing aids. This is a process known as direct bone conduction. The system works by enhancing natural bone conduction, which in turn stimulates the inner ear allowing hearing.
How does it work?
A BAHA consists of three main components:
- Titanium implant - This is surgically implanted and eventually bonds with the surrounding tissue, this is called osseo-integration. This is responsible for the vibration of the skull and inner ear.
- External abutment - This is the connection between the sound processor and the implant. It has a snap lock coupling which allows the sound processor to fix securely into place.
- Sound processor - This sits comfortably on the outside of the abutment, picking up sound and transmitting it through to the skull and inner ear by vibration.
There are various reasons for why a BAHA may be more useful and beneficial than a conventional hearing aid. For those individuals that have abnormalities with the outer ear such as atresia or microtia, a BAHA may be the only sensible option. For those that suffer from chronic outer or middle ear infections again a conventional hearing aid more not be appropriate.
As a BAHA works on natural bone conduction it can be fitted to conductive loss as well as sensori-neural and mixed losses, as long as the bone conduction thresholds are within criteria.
Research suggests that a BAHA can greatly aid localisation for those with a single sided hearing loss. This can also aid in speech discrimination and help to hear in the presence of background noise. Therefore people with single sided deafness are also considered for a BAHA.
- Helps in localisation of sounds.
- Ear canal not occluded.
- People with congenital conductive hearing loss reported improved speech discrimination.
- Significant benefit especially hearing in background noise.
Cochlear Implant (CI)
A cochlear implant (CI) is a small complex device that can help to provide a sense of sound to a person who is profoundly deaf or has a severe hearing loss. The implant does not restore normal hearing, instead it gives a useful representation of sounds by directly stimulating the auditory nerve, bypassing the inner ear. It commonly consists of an external device that sits behind the ear and an internal portion that is surgically placed under the skin.
How does it work?
Initially the external portion which consists of a microphone and sound processor picks up incoming sound and analyses. The speech processor converts the sound to an electrical signal coded for transmission to the internal device. The coded signal is carried along the cable and delivered across the skin by radio wave transmission to a receiver implanted under skin. The circuit within the CI receives the information, decodes the signal, and delivers electrical stimulation to the implanted electrodes via an electrode array. The current stimulates fibers of the auditory nerve which results in the perception of sound.
A CI is considered for those that have very minimal or no benefit from a conventional hearing aid/s.
Limited benefit is usually due to the severity of the hearing loss hence why a CI has an audiological requirement. This can vary from department but most have a severe/profound hearing loss criterion. Addition to this speech testing is performed to further assess how much benefit a conventional hearing aid is providing.
General anaesthetic is administered during the procedure so various medical checks are performed. There is no set age restriction on who can be considered for a CI. As long as one is medically suitable to undergo the operation a CI is a possibility. Also, as rehabilitation post operation is vital and key to the success of a CI, a positive attitude and realistic expectations of the benefits as well as the possible disadvantages are a must.
- Increased perception of sound pitch and loudness levels
- Avoid problems with acoustic feedback and occlusion sensation
- Bypasses the cochlear which is likely the source of the hearing loss
- Ability to use a telephone and speech without the use of lip reading
Middle Ear Implant (MEI)
A middle ear implant (MEI) is an effective treatment that leaves the ear canal completely open. It involves a prosthesis that is surgically implanted in the middle ear that mechanically stimulates the ossicles (three small bones in the middle ear). This type of hearing device can used to fit hearing loss that range from mild to severe sensori-neural, conductive and mixed loss. Unlike a CI there is no electrical simulation but similar to a CI it does consist of an external audio processor.
How does it work?
An audio processor, sited on mastoid bone, detects environmental sound and converts this to electrical signals. This in turn transmits sound to the internal portion of the hearing device. The implant works by directly stimulating the bones in the middle ear which is done by a floating mass transducer. The vibrations can be amplified and adjusted to optimally compensate for different types and degrees of hearing losses. A MEI is designed so that the natural hearing pathway is utilised and relies on a functioning cochlear and auditory nerve.
A MEI device is an alternative to conventional hearing devices, so similar to a BAHA it can be used to overcome any outer ear pathologies.
Research shows that a MEI can be fitted to those with a mild up to a severe hearing loss, whether this be a conductive loss, sensori-neural loss or even a mixed hearing loss. As the system utilises the natural pathway there is evidence that a MEI can be particularly useful for those that suffer from a steeply sloping high frequency loss. Each department however usually has a strict audiometry criterion which is required before considering a MEI.
As the ear canal is free of any blockage it is mostly useful for those that require the ability to hear harmonics free and undistorted by the occlusion effect e.g. physicians, musicians, singers. Also some professions require the ability to use headphones or stethoscopes so a MEI may be the only suitable option.
- Natural speech and sound quality.
- Open ear canal that can be considered more comfortable
- No feedback or whistling.
- Excellent speech understanding, particularly in noisy surroundings.
- Transmission of high pitches.
The function a contralateral routing of signals (CROS) and bilateral contralateral routing of signals (BiCROS) system is to transfer sound from one side of the head to the other so that sound can be heard better regarding of the direction of the source of sound.
How do they work?
Typically a CROS hearing aid system consists of a small microphone that detects the incoming signal that is placed on the poorer hearing year. This is then transmitted wirelessly to a BTE type hearing aid to the better hearing ear. A Bi-CROS system works in a similar manner as it consists of a microphone on the poor ear which transmits sound to the better ear wirelessly. The only difference is that a Bi-CROS amplifies the incoming sound from the poorer ear to help with any hearing difficulty on the better ear.
CROS Hearing Aids
- Patient with a unilateral hearing loss
- The worst hearing aid is not aidable i.e. wearing a hearing provides little benefit.
- The better hearing ear is within normal limits or a mild hearing loss.
BICROS Hearing Aids
- Patient with an asymmetric bilateral hearing loss.
- The worst hearing aid is not aidable or where amplification adversely affects speech identification ability.
- Sound can be heard in the better ear no matter which direction the sound is coming from.
- Improves speech intelligibility in noise if speech is directed to the poorer ear.
- Microphone and receiver are well separated, less chance of feedback.