By Associate Professor Alexander Saxby
While traditional hearing aids are effective for most people with hearing loss, there is a subset for whom they are not suitable. These patients can sometimes benefit from implantable solutions which deliver acoustic stimulation in a novel way.
What conditions might render the ear canal unsuitable for conventional hearing aids?
• Recurrent or chronic infection
• wax impaction in extremely narrow canals
• bony exostoses with associated debris
• fibrosis of the tympanic membrane and deep canal (medial canal stenosis)
• congenitally absent ear canal (canal atresia)
• wide meatoplasty and mastoid cavity after mastoidectomy.
This article covers implantable solutions that bypass the ear canal, delivering the acoustic signal to the cochlea.
One common device is a bone anchored hearing implant (BAHI) of which several different brands and models exist. Less commonly seen are middle ear implants which direct the vibrational sound signal through the ossicular chain or round window rather than via the skull bone. Both have advantages and disadvantages.
How do BAHIs attach?
Several different products are available that achieve vibration of the skull bone to deliver sound energy to the cochlea.
Optimal placement is generally above and behind the auricle which achieves good bone vibration while placing the microphone in an ideal position. They can be placed onto the side of the head using a headband or adhesive patch.
This provides a useful trial of whether the device is adequate for audiological needs. This removable non-operative solution can be a more permanent solution for those too young for surgery, or in whom an operation is contraindicated.
In most cases it precedes a more definitive operation in which a secure attachment is made to the bone. BAHIs are classified according to this bone attachment as active or passive.
Passive bone conduction implants
The vibrating component of the implant is outside the body. Connection to the bone is directly via an abutment which is screwed into the skull bone or via a set of magnets (one under the skin and one attached to the processor) with the internal magnet firmly screwed to the skull bone.
Active bone conduction implants
• The vibrating component is implanted under the skin with a magnet/coil set up to communicate with an exterior processor which houses the battery and microphone.
• There is no attenuation of the signal by the skin so they tend to be more powerful.
• Acoustic feedback is practically eliminated due to lack of external moving parts.
• Placing the vibrational unit internally means the outer processor is smaller and more discreet.
Why not just use BAHIs on everyone?
Firstly, in most cases a conventional hearing aid works well and doesn’t require any operation. Secondly, BAHIs have a maximum output. For single-sided deafness candidates, they generally need to have near normal bone conduction thresholds in the contralateral ear for the device to be useful due to attenuation of the signal crossing over.
If used for conductive or mixed hearing loss ipsilaterally, bone conduction thresholds should ideally be less than 55dB but in some newer more powerful devices they can accommodate hearing loss in a higher range.
This leads to a subgroup of patients in whom conventional hearing aids are contraindicated but who have hearing loss beyond what a BAHI can deliver. This niche is where middle ear implants can be useful.
Middle ear implants
These implants have a vibrating component which is generally attached to the ossicular chain directly, to the short or long process of the incus. They can also be placed onto the round window membrane. Sound energy is more directly coupled to the cochlea so a stronger output can be achieved, in the moderate to severe range. If hearing loss is worse than that, and hearing aids are still contraindicated, patients become candidates for a cochlear implant.
There is nuance and complexity to all this, and exceptions occur. It is always appropriate in difficult cases where the conventional pathway to hearing aids does not seem in the patient’s best interests, to refer to an otologist who will explain alternative options, many of which can be managed by the referring audiologist post-surgery.
About the author: Associate Professor Alexander Saxby is a paediatric and adult ENT surgeon from Sydney who specialises in hearing implants.
