Patients with mild-moderate hearing loss, caused by:
- Noise induced hearing loss
- Drug reactions e.g. gentamicin
- Head injury
- Genetic conditions
A hearing aid is basically a micro-sound system:
- A microphone picks up sound
- An amplifier makes it louder
- Most modern hearing aids have a digital processor to filter out feedback or alter sound to improve the quality.
- A speaker sends the amplified sound into the outer ear canal.
Patients with moderate to profound hearing loss (usually bilaterally) and who receive little or no benefit from hearing aids. They must score <50% on sentence recognition tests in the ear to be implanted and <60% in the non-implanted ear (or in both ears with hearing aids).
NB Patients must still have some function of their cochlear nerve.
- A sound processor worn behind the ear or on the body, captures sound and turns it into digital code. The sound processor has a battery that powers the entire system.
- The sound processor transmits the digitally-coded sound through the coil to the implant.
- The implant converts the digitally-coded sound into electrical impulses and sends them along the electrode array placed in the cochlea (the inner ear).
- The implant’s electrodes stimulate the cochlea’s hearing nerve, which then sends the impulses to the brain where they are interpreted as sound.
Bone conduction Implants
Bone-anchored hearing aid (Baha) may be useful in patients with
- Chronic ear disease where the insertion of a hearing aid may be problematic or cause further damage. Most cases are a conductive hearing loss, which the BAHA will improve (it uses bone conduction).
- Unilateral deafness (or indeed hearing impairment of the only hearing ear, where risk of cochlear implant surgery is deemed too great)
- Malformations of the ear canal.
- Patients with mixed hearing loss
- A sound processor captures sounds and converts them into vibrations.
- A connecting abutment transfers the sound vibrations from the sound processor to the titanium implant
- The small titanium implant is placed in the bone behind the ear where it fuses with the bone to form a very strong bond. The implant transfers the sound vibrations through the bone directly to the cochlea, bypassing the outer or middle ear completely.
- These sound vibrations cause the fluid in the inner ear to move hair cells and send sound signals to the brain. We then hear sound.