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Cochlear Implantation: Correcting Profound Deafness

Dr Sunil Narayan Dutt

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Who can have a cochlear implant?
The cochlear implant is for a patient who is stone-deaf or profoundly deaf in both ears, and who does not benefit with hearing aids.

The foremost criterion to be considered is the age of the patient who wishes to undergo implantation. For instance, a congenitally deaf-mute adult (more than 18 years of age), who has never used a hearing aid, uses sign-language, and is well-integrated into the deaf community is NOT a candidate for CI. On the other hand, deaf born children should be ‘conditioned’ i.e., rehabilitated as soon as possible to exploit the benefits of ‘neural plasticity’ of the auditory pathways. This plasticity (the ability to be stimulated  by extrinsic sound stimuli and hence allowing for processing and adaptation) of the brain and its connections, in relation to hearing, speech and language, diminishes after the age of 7 years.

The process called ‘conditioning’ is essential especially with children that are prelingually deaf, wherein the child is optimally aided, and is trained in therapy sessions to acclimatize to the aided environment throughout the wakeful hours of the day. This process may take six months or longer, and is a true ‘patience and endurance test’ for all concerned: the child, the parents and the speech therapist.

The audiological tests that are performed include Pure Tone Audiometry (PTA), Speech Audiometry, Impedance Audiometry, Brainstem Evoked Response Audiometry (BERA), Otoacoustic Emissions (OAE) and more recently, Auditory Steady-State Evoked Responses (ASSR).

The radiological investigations include a high resolution Computerized Tomographic scan (CT scan) of the inner and middle ears and Magnetic Resonance Imaging (MRI) of the hearing pathways. This is essential to study the anatomy of the cochlea, rule out any congenital anomalies of the auditory pathways and ensure that the cochlear nerve is structurally present.

Psychosocial criteria include ensuring normal mentation, family commitment, social support and realistic expectations. It is also important to ensure that the candidate does not have any other medical illness that would make him/her a poor anaesthetic risk.
The audiological criteria for cochlear implantation include:
For adults
  1. Moderate to profound hearing loss in the low frequencies, and profound hearing loss of worse than 90 decibels in the mid and high frequencies.
  2. Post-lingually deafened adults (rendered deaf after normal speech and language development).
  3. Little or no benefit from hearing aids as demonstrated by less than 30% speech discrimination scores using standard open-set speech recognition tests

For children

  • Severe profound to profound hearing loss; 90 decibels or worse thresholds in the frequencies 2 kHz and above.
  • Prelingually (before speech and language development occurs) or perilingually deaf children, preferably before 7 years of age.
  • Adequate period of hearing aid trial and conditioning, with little or no benefit.
 
Surgical procedure for cochlear implant
Cochlear implantation surgery involves performing a mastoid operation from behind the ear and opening into the middle ear space (cortical mastoidectomy and posterior tympanotomy).

A shallow bed is made on the side of the skull to place the receiver-stimulator package and the implanted electronics. Following this, a tiny opening is made into the first (basal) turn of the cochlea (cochleostomy) and the delicate active multi-channel electrode is inserted into the turns of the cochlea. Modern day state-of-the-art technology is employed to test the device soon after its insertion to check for neural responses with respect to each of the channels, a process called ‘Neural Response Telemetry’ (NRT). It is also useful to check the correct position of the electrode with an x-ray performed immediately after the surgery. Post-operative care includes adequate broad-spectrum antibiotic cover to prevent infection. Suture removal is performed after seven days, and the complete healing of the wound and the flaps take about 3 weeks. After this period, the patient is ready for programming.

Rehabilitation program after cochlear implant
The first ‘switch-on’ is done after the wound and the flap have completely healed. This usually takes about three to four weeks after surgery. Some basic threshold levels and comfort levels are ‘mapped’ for each of the electrodes.

This process of mapping and programming is carefully and meticulously done in children over several months. It is important to realize in a child born deaf, the day of the switch-on would be the first day of hearing.

From there on, it would take anywhere from 6 months to 18 months to develop speech and language. Parents’ expectations have to be realistic in this regard.

The multidisciplinary expertise, technology and the rehabilitative backup for this complex surgery is available at Apollo Hospitals. The facilities include state-of-the-art deafness screening and diagnostic equipment such as:
  • Multiple Auditory Steady-State Responses (MASTER)
  • Otoacoustic Emissions (OAE)
  • Auditory Brainstem Responses (ABR) and
  • Neonatal hearing screening
 

Dr Sunil Narayan Duttis Senior Consultant and Coordinator, Department of Otolaryngology (ENT) and Head & Neck Surgery Apollo Hospitals, Bangalore.

 
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