Cochlear Implants – An insight

Dr. Atul JainSr. Consultant and Head,

Department of ENT Head & Neck Surgery

Pushpanjali Crosslay Hospital, NCR

Understanding the human ear

Cochlea cross-section & organ of Corti

No Hair Cells

• Conductive and Sensorineural components• Conductive hearing loss

– Causes in external ear– Causes in middle ear

• Sensorineural hearing loss– Cochlear deafness– Auditory nerve deafness

• Hearing impairment can be due to deficiency in either or both components

Hearing Loss

Treatment of SNHL

• Sensorineural hearing loss can be treated by– Hearing Aids– Speech therapy / Sign language– Cochlear Implants

• Cochlear Implants : Boon in profound bilateral deafness when hearing aids are unable to rehabilitate

Basics of a cochlear implant• Cochlear Implants : First true bionic

sense organs• The human cochlea: An

electromechanical transducer• Cochlear implants, like the human hair

cell, also receive mechanical sound energy and convert it into a series of electrical impulses

• Very different from hearing aids, which only amplify the mechanical sound waves

• Internal Device - surgically implantedunder the skin– Electronics package

(receiver-stimulator) with magnet– Electrode array placed inside the

cochlea

• External Device– Worn on the body or at ear level– Sound/speech processor– Microphone– Coil with magnet

Cochlear Implant Components

An Implantee with the external parts of the implant in position

Mechanism of the implant• Hair cells of the cochlea if damaged, do

not function and hearing becomes impaired.

• Intact auditory nerve fibres capable of transmitting electrical impulses to the brain become unresponsive because of hair cell damage.

• A cochlear implant makes use of this intact auditory pathway by bypassing the damaged hair cells of the cochlea.

Electrode Array in the Cochlea

Electrode in Scala Tympani

– Sounds are picked up by a microphone.

– A speech processor filters, analyzes and digitizes sound into coded signals.

How a Cochlear Implant Works?

Pulses are sent to the coil and transmitted trough the skin to the implant.

How a Cochlear Implant Works?

The implant sends the pulses to the electrodes in the cochlea.

How a Cochlear Implant Works?

– The auditory nerve picks up the signal and sends it to the auditory centre in the brain.

– The brain recognizes these signal as sound

How a Cochlear Implant Works?

• Cochlear implants as we know them now are the result of intensive research over the last four decades.

• However, there is a long history of attempts to provide hearing by the electrical stimulation of the auditory system.

• The centuries old interest in the biologic application of electricity was the basis for the development of cochlear implants.

History

Volta (1800)• First electrical stimulation of

the auditory system.• As part of his experiments he

connected two metal rods to a 50V battery and closed the circuit by placing one rod in each ear

• Heard a noise like "the boiling of thick soup“

History

• Djourno and Eyries(1953)- directly stimulated Auditory nerve- the patient heard sounds that resembled a “roulette wheel” ora “cricket”

• House and Doyle(1961) - Scala tympani approach• Simmons(1966)

- electrodes were placed directly into the modiolar segment of the auditory nerves- able to discern the length of signal duration, some degree of tonality could be achieved

• House(1972) - first commercially available device - The House 3M Single-Electrode Implant

History

Candidates for a cochlear implant • Pre-lingual candidates

– Onset of deafness before the development of speech usually congenital

– Ideal candidates should be less than 6 years of age– Older pre-lingual candidates are not ideal due to

neural plasticity and inability to understand sound• Post-lingual candidates – More suitable

– Onset of deafness after the development of speech– E.g. Bilateral deafness due to meningitis/viral

infections

Audiologic Criteria

Post–lingual Deafness• Pure tone average greater than 70 dB (FDA)• Standard Speech Discrimination score less than

20 to 30%

Pre–lingual Deafness• Brainstem Evoked Response Audiometry

Evaluation and Planning• Otologic Evaluation: Rule out congenital or

acquired abnormalities and/or infections• Audiologic Evaluation: PTA and/or BERA• Radiological Evaluation: HRCT Temporal

bones or MRI • General, Cardiovascular and Neurological

Examination• Psychological evaluation and counselling

Syndromic candidates• Usher’s syndrome- Retinitis pigmentosa with

SNHL• Pendred’s syndrome- Profound SNHL with

colloid goitre• Jervell-Lange-Nielsen Syndrome –

Congenital deafness with ECG abnormalities, sudden fainting attacks and sudden death

• Alport’s Syndrome – Nephropathy with SNHL

Look at the candidate as a whole, not just the deafness and plan accordingly.

Selection of the side of implant• Ear with lesser duration of deafness • Better hearing ear – More residual neural

elements – Better performance• Anatomic factors: Dysplastic, hypoplastic ear

not preferred• Ear with no previous middle ear surgeries

preferred

Selection of the side of implant• Ear with more CNS activation preferred by

using brain-imaging techniques like single-photon emission CT, functional MRI, refined cortical auditory electrophysiology

• Ear with better labyrinthine function not operated, other ear preferred

• If no differences between ears on any parameters, surgeon’s choice: Left ear for right-handed surgeon

Age of implantation

• Youngest age can be even 9 – 12 months

• Earlier fears of electrode migration or extrusion secondary to skull growth have been proven wrong

• Younger age of implantation associated with better outcomes

• General anesthesia preferred• Big C shaped, or inverted U or inverted L

or inverted J shaped incision in postaural area

• Flaps elevated over the mastoid cortex• Well created for placement of the

stimulator-receiver• Conduit made to pass the electrodes into

the mastoid cavity

Surgical Procedure

Surgical Procedure• Mastoidectomy cavity created

• Facial recess identifed and posterior tympanotomy done

• Facial recess widely opened

• Cochleostomy of 1 to 1.5 mm done anteroinferior to the round window niche

• Electrode Array introduced into the scala tympani

Surgical Procedure

• Position of all electrodes confirmed by intra-operative mapping

• Cochleostomy sealed with tissue

• Receiver/Stimulator anchored in the well

• Ground electrodes inserted under the temporalis muscle

• Wound closure in layers

Post-operative course• Antibiotics, Labyrinthine sedatives,

Analgesics

• Can be discharged as day care

• Implant activation done about 4 weeks after surgery when wounds are well healed

• Intensive and meticulous rehabilitation by a speech-language pathologist

Complications

• Facial nerve injury• Taste alteration• Wound infection,

hematoma• Flap necrosis• Wound dehiscence• CSF leak• Balance

disturbances

• Postoperative meningitis

• Long standing pain• Displacement of

electrodes• Extrusion of device• Poor rehabilitation• Device Failure

Newer dimensions

• Cochlear implantation in old age– Working solution for profound hearing loss in old age

• Bilateral implantation– Advantages of binaural hearing– Better localization of sound– Better speech development

• Controversies:– 2nd ear can be kept reserved for better technology– Increased mapping difficulties– ? Bilateral vestibular dysfunction– Cost effectiveness

• Absent or thin auditory nerve bilaterally

• Neurological damage impeding auditory processing

• Medical risks of surgery that exceed the expected benefits of the cochlear implant

Contraindications

• Length of profound deafness• Age at implant• Etiology• Use of hearing aids prior to implantation• Amount and quality of re/habilitation

before and after implant• Family support and commitment• Educational methods and communication

mode

Factors that influence outcomes

• Coordinator/ Cochlear Implant Audiologist

• Administrative Assistant

• Aural Rehabilitation Audiologist

• Speech/Language Pathologist

• Surgeon• Social Worker• Developmental

Pediatrician• Representative from

school• Candidate & family

The Cochlear Implant Team

“After a lifetime in silence and darkness, to be deaf is a greater affliction than to be blind. Hearing is the soul of knowledge and information of a high order. To be cut off from hearing is to be isolated indeed."

– Helen Adams KellerDeaf-Blind American author, activist and lecturer

(1880-1968)