Hearing Aids & Implantable Hearing Devices

HEARING AIDS

Conventional hearing aids

Bone anchored hearing aids (BAHA)

Implantable hearing aids (vibrant sound bridge)

1) Conventional hearing aids A hearing aid is a device to amplify sounds reaching the ear.

Consists of 3 parts :

a) Microphone: picks up sound & converts them to electrical impulses.

b) Amplifier: magnifies electrical impulses.c) Receiver: converts electrical impulses back to sound.This amplified sound is then carried through the earmould to the tympanic membrane.

Types of Hearing AidsTypes of Hearing Aids

1) Air conduction hearing aid- the amplified sound is transmitted via the ear canal to the tympanic membrane.

Most of the aids are air conduction type.

They can be of 5 types.

Body-worn types:Body-worn types: most common; microphone and amplifier along with the battery are in one case worn at the chest level while receiver is situated at the ear level.

allows high degree of amplification.

useful in severely deaf persons or children with congenital deafness.

Behind-the-ear (BTE) types:Behind-the-ear (BTE) types: microphone, amplifier receiver and battery are all in one unit which is worn behind the ear.

It is coupled to the ear canal with a tubing and an earmould.

useful for slight to moderate cases of hearing loss.

Spectacles types:Spectacles types: it is a modification of “behind-the-ear” type & the unit is housed in the auricular part of the spectacle frame.

useful to persons who need both eye glasses for vision and a hearing aid.

In-the-ear (ITE) types:In-the-ear (ITE) types: The entire hearing aid is housed in an earmould which can be worn in the ear.

useful in mild to moderate hearing loss.

very popular because of their cosmetic appeal.

Canal types (ITC & CIC):Canal types (ITC & CIC): The hearing aid is so small that the entire aid can be worn in the ear canal without projecting into the concha.

For using this aid, it is required that the ear canal should be large and wide and the patient should have dexterity to manipulate the minute controls in the aid.

useful in mild to moderate hearing loss. 2 types available- in the canal (ITC) &still smaller and

invisible type, completely in the canal (CIC).

2) Bone conduction hearing aid-

instead of a receiver, it has a bone vibrator which snugly fits on the mastoid & directly stimulates the cochlea.

useful in persons with actively draining ears, otitis externa or atresia of the ear canal when ear-inserts cannot be worn.

Indications for Hearing AidIndications for Hearing Aid

Any individual who has a hearing problem that cannot be helped by medical or surgical means is a candidate of hearing aid.

Fitting a Hearing AidFitting a Hearing AidConsideration is given to :

Degree of hearing loss Configuration of hearing loss (type of frequencies

affected) Type of hearing loss (conductive or sensorineural) Presence of recruitment Uncomfortable loudness level

Age & dexterity of patient Condition of the outer and middle ear Cosmetic acceptance of the aid Type of earmould The type of fitting; whether it is monoaural (one aid

only), binaural (one aid for each ear), binaural with y-connection (one aid but two receivers, one for each ear) or the CROS type.

CROS (contralateral routing of signals) – microphone is fitted on the side of the deaf ear and the sound thus picked up is passed to the receiver placed in the better ear.

This is useful for persons with one ear severely impaired & helps in sound localisation coming from the side of the deaf ear.

Now bone-anchored hearing aids (BAHA) are being preferred for single-sided deafness & have replaced the use of CROS aids.

2) Bone-anchored Hearing Aids (BAHA)

Based on the principle of bone conduction. Has 3 components:1)1) Titanium fixtureTitanium fixture2)2) Titanium abutmentTitanium abutment3)3) Sound processorSound processor The titanium fixture is surgically embedded in the

skull bone with abutment exposed outside the skin. The titanium fixture bonds with the surrounding

tissue in a process called osseointegration. The sound processor is attached to the abutment

once osseointegration is complete which usually takes 2 to 6 months after implantation.

Indications for BAHAIndications for BAHA when air-conduction hearing aid cannot be used.

- canal atresia, congenital or acquired, not amenable to trtmnt.

- c/c ear discharge, not amenable to trtmnt

- excessive feedback & discomfort from air-conduction hearing aid.

Conductive or mixed hearing loss, e.g. otosclerosis & tympanosclerosis where surgery is contraindicated.

Single-sided hearing loss

BAHA have replaced the use of CROS aids.

The BAHA device can be implanted on the side of deaf ear, and it transmits the sound by means of bone conduction to the contralateral cochlea.

The BAHA is fixed on the deaf side & collects sound waves to transmit to healthy cochlea of the other side.

This process eliminates the head-shadow effect and allows for hearing from both sides of the head

SurgerySurgery typically performed in a single stage in adults. Abt 3 months are allowed for osseointegration bfr the

sound processor can be attached. 2 stage procedure is recommended in children in

whom the fixture is placed into the bone in the first stage. After abt 6 months to allow for osseointegration, a second stage operation is done to connect the abutment through the skin to the fixture.

ComplicationsComplications

• Few

• Failure to osseointegrate the implant

• local infections and inflammation at the implant site.

3) Implantable Hearing Aids

Works on a direct drive principle.

Rather than delivering acoustic energy into the external auditory canal (as with traditional hearing aid systems), direct drive middle ear implant systems use mechanical vibrations delivered directly to the ossicular chain, while leaving the ear canal open.

Implantable middle ear devices are generally available in 2 types :

Piezoelectric devices: operates by passing an electric current into a piezoceramic crystal, which changes its volume and thereby produce a vibratory signal. This piezoelectric transducer in turn is coupled to the ossicles and drives the ossicular chain by vibration.

Electromagnetic hearing devices: function by passing an electric current into a coil, which creates a magnetic flux that drives an adjacent magnet. The small magnet is attached to one of the ossicles of the middle ear to convey vibrations to the cochlea.

Vibrant soundbridge deviceVibrant soundbridge device• Semi-implantable device• 2 components – internal & externalinternal & external• The internal component is called VORP (Vibrating VORP (Vibrating

Ossicular Prosthesis)Ossicular Prosthesis) and is made up of 3 parts- receiverreceiver, FMTFMT (Floating Mass Transducer) and a conductor linkconductor link between the two.

• The external component is called the audio audio processorprocessor which is worn behind the ear. It contains a microphone that picks up sound from the environment and transmits it across the skin by radiofrequency waves to the internal receiver.

Candidacy profileCandidacy profile

Adults aged 18 yrs and older with moderate to severe sensorineural hearing loss.

Candidates should have experience of using traditional hearing aids and should have a desire for an alternative hearing system.

ProcedureProcedure The internal device is surgically implanted. Conducted under general anaesthesia. The receiver of the implant is positioned under the skin

over the mastoid bone via a std cortical mastoidectomy and posterior tymapanotomy approach.

The ossicular chain is visualised and the FMT is attached to the long process of incus.

6 to 8 weeks after the procedure, the patient is fitted with the external audio processor.

AdvantagesAdvantages A direct drive system provides mechanical energy

directly to the ossicles, bypassing the ear canal and the tympanic membrane.

Eliminates occlusion, feedback, discomfort and wax related problems.

Provide improved sound quality to the hearing-impaired subjects.

Disadvantages of conventional hearing aidsDisadvantages of conventional hearing aids Cosmetically unacceptable due to visibility Acoustic feedback Spectral distortion Occlusion of external auditory canal Collection of wax in the canal and blockage of insert Sensitivity of canal skin to earmoulds Problem to use in discharging ears

IMPLANTS

Cochlear implants

Auditory brainstem implants

1) Cochlear implants

Electronic device that can provide useful hearing and improved communication abilities for persons who have severe to profound hearing loss and who cannot benefit from hearing aids.

Works by producing meaningful electrical stimulation of the auditory nerve.

Components- Components- external external

internalinternal External component:External component: consists of an external speech external speech

processorprocessor and a transmittertransmitter.. Internal component: Internal component: it is surgically implanted and

comprises the receiver/stimulatorreceiver/stimulator package package with an electrode array.

Candidacy profileCandidacy profileUsed both in children and adults.Used both in children and adults. Bilateral severe to profound sensorineural hearing

loss. Little or no benefit from hearing aids. No medical contraindication for surgery Realistic expectation Good family & social support toward habilitation Adequate cognitive function to be able to use the

device.

Outcomes of cochlear implantationOutcomes of cochlear implantationFactors that predict a successful clinical outcome are : Previous auditory experience (post-lingual pts or prior

use of hearing aids) Younger age at implantation ( especially for pre-lingual

children) Shorter duration of deafness Neural plasticity within the auditory system

SurgerySurgery Carried out under general anaesthesia There are broadly 2 surgical techniques:

i) The facial recess approachfacial recess approach where a simple cortical mastoidectomy is done first & the short process of the incus and the lateral semicircular canal are identified.

The facial recess is opened by performing a posterior tympanotomy.

The stapes, promontory and round window are identified.

Cochleostomy is performed antero-inferior to the round window membrane to a diameter of 1 to 1.6 mm depending on the electrode used.

ii) The pericanal techniquepericanal technique where a tympanomeatal flap is elevated to perform a cochleostomy either by endaural or postaural approach.

a bony tunnel is drilled along the external canal towards the middle ear.

Complications of Cochlear Implant SurgeryComplications of Cochlear Implant SurgeryEarly complications Facial paralysis Wound infection Wound dehiscence Flap necrosis Electrode migration Device failure CSF leak Meningitis

Late complications Exposure of device and extrusion Pain at the site of implant Migration/displacement of device Late device failure Otitis media

2) Auditory Brainstem Implant (ABI)

Designed to stimulate cochlear nuclear complex in the brainstem directly by placing the implant in the lateral recess of the fourth ventricle.

Such implant is needed when CN VIII has been severed in surgery of vestibular schwannoma.

ABI help in communication, awareness and recognition of environmental sounds; however they are not efficient as multichannel cochlear implants.

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