Auditory rehabilitation

Introduction:

Deaf children needs to be rehabilitated audiologically. These rehabilitation procedures help the child to communicate. The various rehabilitation procedures available include:

1. Instruments

   Hearing aids:

       Conventional hearing aids
       Bone anchored hearing aids
       Implantable hearing aids

    Implants:

       Cochlear implants
       Auditory brainstem implants

2. Training

     Lip reading
     Auditory training
     Speech conservation

Devices:

Hearing aids:

Conventional hearing aids - Hearing aid is a device that is used to amplify sounds reaching the ear. Hearing aids consists of three parts:

Microphone - Picks up the sounds and converts them into electrical impluses.

Amplifier - Magnifies the electrical impulses

Receiver - Converts electrical impulses back to sound

The amplified sound is carried through an earmould to the ear drum.

Types of hearing aids:

Air conduction hearing aid:

In this type of hearing aid the amplified sound is transmitted via the external auditory canal to the ear drum. Majority of hearing aids belong to this category. The various types of air conduction aids include:

Body worn hearing aid:

This is the most common type used. The microphone, amplifier and battery are housed in a single casing which is kept inside the pocket and worn at chest level. This type of aid allows for high degree of amplification with very minimal feedback. This type of aid is very useful in patients with severe deafness and in a congenitally deaf child.

Behind the ear type:

In this type microphone, amplifier, receiver and battery are all in one unit and is worn behind the ear. It is coupled to the ear canal with a tubing and an earmould. This type of hearing aid is useful for slight to moderate cases of hearing loss, that too particularly in high frequencies.

Spectacle types:

This is actually a modification of the behind the ear model. The hearing aid unit is housed in the articular part of the spectacle frame. It is useful in individuals who need glasses as well as hearing aid. This is currently not popular these days.

In the ear type of hearing aid:

In this type the entire hearing aid is housed within an earmould which can be worn in the ear. This is useful to persons who have mild to moderate hearing loss with flat configuration. This is also cosmetically acceptable to the patient.

Canal types:

This type of hearing aid is very small and the entire aid can be worn within the external auditory canal without projecting into the concha. In order to use this aid the external canal should be large and wide to house the aid. Patient should have the manual dextirity to handle the aid. This is useful for managing mild - moderate hearing loss in the high frequency range (1-4kHz).

Indications for hearing aid:

Any person who has a hearing loss that cannot be managed either medically / surgically is an ideal candidate for hearing aid.

1. Sensorineural hearing loss

2. Deaf child

3. Conductive deafness

Factors to be considered while prescribing a hearing aid:

1. Degree of hearing loss

2. Frequencies affected (pattern of hearing loss)

3. Type of hearing loss (sensorineural / conductive / mixed)

4. Presence / absence of recruitment

5. Uncomfortable loudness level

6. Age and dexterity of the patient

7. Outer / middle ear condition

8. Acceptance

9. Whether to be fitted to single or both the ears

Disadvantages of conventional hearing aids:

1. Cosmetically unacceptable due to visibility

2. Acoustic feed back

3. Spectral distortion

4. Ear canal occlusion

5. Accumulation of wax

6. Ear canal sensitivity to ear moulds

7. Difficult to use in discharging ears

CROS hearing aids:

Cros is an acronym for Contralateral routing of signals. This type of hearing aid is very useful in

treating patients with unilateral severe deafness. The principle behind this type of hearing aid is that the good ear

is made use of to hear sounds from the bad ear.

In this type of hearing aid the microphone is placed in the bad ear to pick up sounds directed towards the bad ear.

The receiver and amplifier is fixed to the good ear. The sound picked up by the microphone is transmitted to the amplifier fixed

to the good ear, it is amplified and projected to the good ear.

Advantages:

1. Since hearing is in the good ear, amplification need not be maximum

2. There is virtually no feed back

3. Directionality of sound is not lost

Modifications of CROS hearing aids:

BiCROS: This type of aid is used to treat bilateral asymmetrical hearing loss.

SteroCROS: This device is a recent innovation in cros technology. This type of hearing aid restores binaural hearing functionality of ear.

Bone anchored hearing aid (BAHA):

Bone anchored hearing aids (BAHA) are implantable hearing aids that can be used to treat hearing loss of a severe magnitude. This device works directly by stimulating the inner ear via bone conduction. It has been in use from 1977 in Europe. It got approved by the FDA only in 1996.

Indications for use of BAHA:

1. Bilateral canal atresia is an absolute indication for the use of BAHA. This is because canal atresia prevents insertion of ear mould for an air conduction aid.

2. In chronic ear infections when insertion of ear moulds is a problem.

3. Unilateral deafness which cannot benefit from use of regular hearing aids.

Components of BAHA:

BAHA has 3 components. They are :

1. Titanium screw that becomes integrated with the skull bone behind the ear.

2. Titanium abutment is fitted to the titanium screw which is already integrated to the skull bone.

3. Ear level sound vibrator.

Requirements for BAHA implant: The prime requirement for a successful BAHA implant is that the patient should have adequate bone conduction thresholds. It has been suggested that bone conduction threshold should be atleast 45dB for effective functioning of BAHA. Preopertaive speech audiometry should be performed in all patients before BAHA implant.

Implantation procedure:

BAHA implantation is a surgical procedure. Surgery is performed under local / general anesthesia. It is a relatively simple surgical procedure. The bone posterio-superior to the ear canal is usually of sufficient thickenss over the age of three to take an implant, and allow osseointegration. In adults mostly a single stage procedure is preferred.

In this procedure bone over the skull just postero superior to the ear canal is drilled and the titanium screw is inserted into it. Three month period is allowed to elapse for osseointegration to take place. Abutment is introduced after the osseointegration is complete. The ear level sound vibrator can be attached to the abutment.

Complications of BAHA insertion :

1. Infection

2. Crusting

3. Screw falling out

How BAHA works: It works by by taking the sound from the outside and transmitting it to the inner ear through the bone. This bypasses the ear canal and the middle ear.

BAHA care:

1. Clean the area around the abutment DAILY: Washing your hair will soften any crust. Use the supplied Entific soft cleaning brush and gently wipe the bristles against the side of the abutment, not the skin. Remove any debris around or inside the abutment. Antibacterial soap is recommended. Dry the area gently.

2. Do not allow hair to wrap itself around the abutment.

3. Do not keep hot air from a hair dryer on the abutment for a long period.

4. No hair will grow under the processor.

5. Whenever strong chemicals, such as hair dying solutions, are being applied to your hair, please cover the abutment and skin graft site with plastic to protect your skin and the abutment from the chemicals. When exposed to strong chemicals, the skin surrounding the abutment may become red, swollen, infected, or burned.

6. BAHA should be removed before undergoing MRI imaging.

7. For best benefit it should be worn througout the day.

8. The processor will whistle when touched or when it comes into contact with other objects. Whistling can be reduced by simply repositioning the processor.

9. During windy conditions outdoors, the directional microphones may pick up wind sounds. Simply rotate the processor on the abutment 90 degrees or until the wind sound stops.

Implantable hearing aids:

These type of devices deliver sound energy to the middle ear ossicular chain leaving the external canal completely open. There are two types of implantable middle ear devices. They are:

Piezoelectric devices:

These devices operate by passing electric current into a piezoelectric crystal, which changes its volume producing a vibratory signal. This piezoelectric transducer in turn is coupled to the ossicles. It drives the ossciular chain by vibration.

Electromagnetic hearing devices:

These devices function by passing electric current into a coil which creates a magnetic field that drives a magnet. The small magnet is attached to the ossicles of the middle ear to convey vibrations to the cochlea.

Vibrant sound bridge device:

This is a semi-implantable device comprising an internal and external units. The internal unit is known as vibrating ossicular and is made up of the following components (receiver, floating mass transducer, and a conducting link between these two).

The external component is also known as audio processor is worn behind the ear. The audio processor contains a microphone that picks up sound from the environment and transmits it across the skin by radiofrequency waves to the internal receiver.

Surgical procedure:

The internal device is surgically implanted. This surgery is ideally performed under general anesthesia. The receiver of the implant is positioned under the skin over the mastoid bone via standard mastoidectomy approach. Posterior tympanotomy is performed and the floating mass transducer is attached to the long process of the incus. The middle ear structures are not modified. Since middle ear is not tampered the residual hearing is preserved. Six to eight weeks following the procedure the patient is fitted with the external audio processor that attaches magnetically to the back of the ear. The processor is programmed.

Implants:

Cochlear implants:

This is an electronic device that provides useful hearing along with improved communication abilities to the patient with severe to profound sensorineural hearing loss.

The cochlear implant works by producing meaningful stimulation of eht auditory nerve in patients in whom degeneration of cochlear hair cells prevented sound transmission to the auditory nerve.

Cochlear implant has two components:

External component - This consists of an external speech processor and a transmitter. This speech processor may be body worn or behind the ear type.

Internal component - It is surgically implanted and comprises of receiver / stimulator package with an electrode array. This is passed into the cochlea via the round window membrane which can be accessed through facial recess approach.

Sound is picked up by the microphone present in the speech processor. the speech processor analyses and codes sounds into electrical impulses. It uses a variety of coding stratergies.

These electrical impulses are sent from the processor to the transmitting coil which in turn sends the signal to the surgically implanted receiver via radiofrequency. The receiver / stimualtor decodes the signal and transmits the same to the electrode array which directly stimulates the auditory nerve.

Indications:

1. Profound sensorineural hearing loss not responding to hearing aids

2. Deaf mute children under the age of 6

Auditory brainstem implant:

This implant is designed to directly stimulate the cochlear nuclear complex present in the brain stem by placing the implant in the lateral recess of the fourth ventricle. These implants are needed when auditory nerve has been severed or hopelessly degenerated.

Assistive devices:

These are not hearing aids but they help the hearing impaired person to listen efficiently in the presence of background noise, use phone, view a movie in a theatre. These devices can be hard wired ones or induction loops, AM or FM or infrared signals.

Alerting devices:

These devices alert the individual to answer the door bell, phone, crying babies etc.

Telecommunication devices:

Telephone amplifier can be used along with the telephone hand set to facilitate hearing.

Training:

Lip reading

Auditory training

Speech conservation - useful in patients with profound hearing loss which has occurred suddenly. The patient hereby loses the ability to monitor his speech. Speech conservation educates this patient to use his tactile / proprioceptive feed back systems to monitor his speech production.