Know the fascinating Mechanism Behind Hearing Aids 

Before knowing about the hearing aid and its types, let us first know about the deafness and hearing impairment.

DEAFNESS:

Deafness is a profound hearing loss and this indicates the complete loss of hearing ability in one or both ears. Even when the sound is amplified, the person can’t be able to understand the speech through hearing. So deaf people often use sign language or lip reading for communication. However, deafness and hearing impairment are not the same. Deafness is a form of hearing impairment. The types of deafness includes:

• Pre-lingual deafness
• Post lingual deafness
• Unilateral deafness
• Bilateral deafness

HEARING IMPAIRMENT

According to PWD Act 1995, hearing impairment is described as loss of 60 dB or more in the better ear in the conversational frequencies. In simple terms, it is the reduced ability to hear sounds and also these people can get benefitted from hearing aids, cochlear implants and other assistive devices. These are generally classified into three such as:

1. Conductive impairment
2. Sensorineural impairment
3. Mixed impairment

HEARING AID:

A hearing aid is a device that amplifies the sound effectively to the ear of the listener. The major types of hearing aids are:

1. Conventional hearing aids
2. Bone Anchored hearing aids
3. Implantable hearing aids

A hearing aid majorly consists of three parts:

• Microphone: The microphone helps to pick up the sound and converts it into electrical impulse.
• Amplifier: It boosts up the electrical impulse.
• Receiver: Here electrical impulse is converted back to sound. Then this amplified sound wave reaches the tympanic membrane.

TYPES OF CONVENTIONAL HEARING AIDS:

Air conduction hearing aid:

Here the amplified sound reaches the tympanic membrane through ear canal. There are five types of air conduction;

1. BODY WORN TYPES:
   • It is the most common type.
   • Microphone, amplifier along with battery will be in one case and it is placed at chest level, and receiver is situated at the ear level, and this allows high degree of amplification.
   • Used by people who are severely affected by deafness and also used by children with congenital deafness.
2. BEHIND THE EAR (BTE) TYPES:
   • Here, the microphone, amplifier, receiver and battery all are present together as a single unit and it is worn behind the ear. This type of hearing aid is coupled to the ear canal with the tubing and the earmould.
   • Used by the people with moderate hearing loss.
3. SPECTACLES TYPES:
   • It is the modification of BTE type
   • The hearing unit is housed in the auricular part of the spectacle frame
   • Used by the people who need both eye glasses for vision and hearing aid.
4. IN THE EAR TYPE:
   • In this type the hearing aid is mounted in a ear mould which can be worn in the ear, and this is a popular one because of its cosmetic appeal.
   • Used by people with moderate hearing loss
5. CANAL TYPES (ITC & CIC)
   • This type of hearing aid is so small and can be worn in the ear canal without projecting into the concha.
   • If one want to use this, their ear canal should be large and wide.
   • Used by people with mild to moderate hearing Loss
   • Canal types are available in two models:
     • ITC – In the canal. This is very small and invisible type
     • CIC – Completely in the canal

Bone conduction hearing aid:

• In bone conduction hearing aids, instead of a receiver it has a bone vibrator, which fits on the mastoid bone and stimulates the cochlea directly.
• Used by the people with actively draining ears, otitis externa or atresia of ear canal when ear inserts cannot be worn.

CROS AIDS

• The microphone is placed on the side of deaf ear and the sound is passed to the better ear where the receiver is placed.
• Used by persons with one ear severely impaired.

BONE ANCHORED HEARING AIDS (BAHA)

BAHA is based on the principle of bone conduction. It has 3 components- Titanium fixture, titanium abutment and sound processor.

INDICATIONS FOR BAHA:

• Canal atresia
• Ear discharge
• Discomfort in ear
• Single sided hearing loss
• Conductive or mixed hearing loss e.g., otosclerosis and tympanosclerosis

PROCESS:

Surgically, the titanium fixture is embedded in the skull bone and the abutment is exposed outside the skin. With osseointegration process, the fixture gets bonded with the surrounding tissue. Finally, the sound processor is attached to the abutment once the osseointegration process is completed. The process of osseointegration usually takes 2 to 6 months after the implantation.

BAHA replaced the usage of CROS (Contralateral Routing of Signals) aids. This BAHA device is implanted on the side of deaf ear, and this transmits sound to the contralateral cochlea by bone conduction. The sound waves are collected from the deaf side and transmitted to the healthy cochlea of the other side. Head-shadow effect is eliminated and allows hearing from both sides of the head.

Complication:

• Failure of osseointegration.
• Local infection and inflammation on the Implant site.

IMPLANTABLE HEARING AIDS:

This works on a direct drive principle. Instead of delivering the acoustic energy into the external auditory canal, direct drive middle ear implant system uses mechanical vibration delivered directly to the ossicular chain, while leaving the ear canal open. These are available in 2 types:

• Piezoelectric devices – The operation of these devices is done by passing the electric current to piezoceramic crystal that changes the volume and produces a vibratory signal. The piezoelectric transducer is coupled to the ossicles and drives the ossicular chain by vibration.
• Electromagnetic hearing devices – Here the function is carried out by passing the electric current into the coil that creates magnetic flux which drives the adjacent magnet. The vibration is passed to cochlea through a small magnet that is attached to one of the ossicles.

VIBRANT SOUNDBRIDGE DEVICE-SEMI IMPLANTABLE DEVICE:

There are two components – internal and external.

• INTERNAL COMPONENT – It is called as VORP (Vibrating Ossicular Prosthesis). This is made up of 3 parts – receiver, FMT (Floating Mass Transducer) and a conductor link between these two.
• EXTERNAL COMPONENT – This is called as audio processor and is worn behind the ear. It contains a microphone from which is sound is collected from the environment and transmitted to the internal receiver through the skin by radiofrequency waves.

Here the internal component is surgically placed under general anaesthesia. The receiver is placed under the mastoid bone. The ossicular chain is visualised and FMT is attached to incus. The patient is fitted with external component after 6 to 8 weeks of the procedure.

Complication:

• Displacement of device
• Pain at the site of extrusion.

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