Help for the hearing impaired
Advances in technology can now help the aurally challenged to overcome the stigma associated with the use of hearing aids. A look at the latest developments.
HEARING loss, unlike blindness, does not attract immediate attention although it affects an individual's performance and capability. A deaf person, more often than not, delays seeking medical help, partly due to the wrong notion that his condition is incurable. The other reason is the fear of having to wear a hearing aid for the rest of his life. But thanks to technological breakthroughs, we have now overcome the stigma associated with the use of hearing aids. Here is an account of the latest developments and the role of new technology in treatment.
Anatomically, the ear is divided into three sections: the outer, middle and inner ear. In combination, the outer and middle ear function as part of the conductive hearing system. Sound waves picked up by the outer ear vibrate the eardrum, which then causes movement of the ossicles. The ossicles conduct the sound vibrations to the inner ear. Any disease or obstruction in the outer and/or the middle ear results in "conductive deafness". This impairment may be due to a variety of reasons and may be corrected by medical or surgical treatment.
The inner ear includes the "cochlea" (shaped like a snail) which contains fluid and hair cells. The vibrations transmitted by the ossicles to the cochlea make the fluid within it move which in turn moves the hair cells. These cells generate electrical signals which are sent to the brain via the auditory nerve which is the nerve of hearing. Damage to the hair cells results in "sensorineural deafness" (nerve deafness). Although many of the auditory nerve fibres may be intact and capable of transmitting electrical impulses to the brain; without the presence of functioning hair cells, the nerve fibres remain unstimulated. Sensorineural deafness cannot be corrected medically and the patient requires a hearing aid. A combination of conductive and sensorineural hearing loss results in "mixed deafness".
Just as there are varying degrees and reasons for hearing impairment, there are various treatment options to help amplify sound and improve the quality of life for those with hearing loss.
Problems like ear discharge, conductive deafness, dizziness and facial nerve disorders are now being routinely treated and cured by simple surgeries. The introduction of the laser has become an asset to the surgeon.
When hearing loss cannot be corrected medically or surgically, the patient is forced to wear a hearing aid. Although technical improvements and modifications have aided the performance of the conventional hearing aids, these can still be uncomfortable to wear and have a considerable "feedback effect" which is one of the most annoying adverse effects of their use. In addition, there is the issue of visibility. These problems have now been overcome by various hi-tech devices which are approved for the rehabilitation of conductive, sensorineural and mixed hearing loss.
These new devices include:
The cochlear implant
It is a revolutionary assertive listening device for people who are stone deaf.
Children and adults with severe to profound nerve deafness (they cannot discriminate sound in words and language) with little to no benefit with standard hearing aids are candidates for cochlear implant surgery.
A profoundly deaf ear is typically one in which the hair cells of the cochlea are damaged or reduced in number. As such, the cells are not able to transfer the signals to the brain. Consequently, conventional hearing aids that merely amplify sound do not work in such cases. In contrast, cochlear implants are surgically implanted devices that bypass the damaged hair cells and directly stimulate the hearing nerve with an electric current, allowing individuals who are profoundly or totally deaf to receive sound.
All cochlear implant systems consist of a microphone and speech processor (worn outside the body like a standard hearing aid), a signal coupler (receiver cum transmitter worn over the skin behind the ear) and an internal part from which one or more electrodes emanate which are implanted into the cochlea. The receiver cum transmitter is placed on the skin behind the ear and is held magnetically to the internal part. A depression is created surgically beneath the skin in the bone behind the ear into which the internal part is placed. The sound signals picked up by the microphone are processed and converted into electrical impulses which are sent via the electrodes into the cochlea. The electrical signals stimulate the hearing nerve and the signals are then sent to the brain. Following surgery, the patient returns to the implant clinic after one month for fitting of the external portion of the device.
After surgery, the candidate (especially a child) has to undergo rigorous training for periods extending upto a few years in order to recognise and understand speech. It is important to clearly understand the time commitments involved before making a decision about the cochlear implant.
There are a number of different cochlear implants currently available: Clarion, Nucleus and MED-EL being the pioneers. The most consistent results are obtained with the Clarion device.
The Bone Anchored Hearing Aid (BAHA) is a device available for people having the following problems:
1. A deformed ear
2. An absent ear canal
3. Persistent ear infection and ear discharge not responding to medicines
4. A genetically acquired deformity in the bones of the middle ear
The BAHA bypasses the defective external and/or middle ear and directly stimulates the cochlea. The BAHA can be used in children as young as five years of age. It improves hearing, allows recognition of speech and detection of quiet sounds. A small part of the device that is present outside the skin behind the ear gets hidden easily by the hair.
Also called middle ear implants, they have been developed for patients with conductive hearing loss and sensorineural hearing loss with substantial remaining hearing. In general, there are two types of implantable middle ear devices: partially implantable and completely implantable.
The types of devices available are:
1. The vibrant soundbridge (Symphonix, U.S.)
It was the first implantable hearing aid for the sensorineural type of hearing loss and received the Food and Drug Administration (FDA) approval in 2000. It is made up of both implantable (internal) and external parts.
2. The middle ear transducer (MET) ossicular stimulator
Over 20 years of extensive research conducted primarily at Washington University School of Medicine, St.Louis, led to the development of this device. It is made up of two parts; an external transmitter and an internal receiver.
The clinical trials for the MET have been successful and it has been proved that it is safe.
Although, the two devices explained above have an external part that is to be worn behind the ear, it is so small that it gets hidden beneath the hair and is easily detachable making it convenient to wear. The results are also far superior to the results achieved after the use of conventional hearing aids.
Many challenges remain in developing the ideal implantable hearing device. Costs associated with the development of these devices as well as surgical implantation make these devices considerably more expensive than conventional hearing aids.
We are now on the threshold of the completely implantable hearing aid. Although in the early stages, it holds great promise as after being implanted, nothing is apparent outside.
The Totally Integrated Cochlear Amplifier (TICA) (Implex American Hearing Systems) and The Envoy System are examples of completely implantable hearing aids. They consist of the main body (surgically implanted into the bone behind the ear), a microphone for picking up sound (implanted under the skin of the ear canal near the ear drum or onto the eardrum itself) and a direct connector to the bones in the middle ear. Sound is picked up by the eardrum, amplified and passed directly to the bones in the middle ear, from where it is sent to the cochlea. The nerve of hearing further carries the sound to the brain. These devices have a built-in battery, which can be charged using a headphone-like device and have a remote control for control of sound quality.
Advantages of middle ear implants:
The single most important advantage is cosmesis, since they are either completely or mostly invisible. When compared to conventional hearing aids, they provide much better perception of hearing by allowing the ear canal to remain open. They also eliminate feedback and provide better tolerance to loud sounds. Devices such as the TICA and Envoy System, allow the patient to continue receiving amplification even while swimming or bathing.
The disadvantages are the cost factor, limited availability and limited experience with the use of these devices.
Although several hurdles still exist, the potential advantages and increasing number of people who would benefit from such devices continue to fuel their development.
Dr. K.K. RAMALINGAM
Dr. RAVI RAMALINGAM
Dr. VINOD NADIG
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