Cochlear Implant

COCHLEAR IMPLANT
Cochlear implants are neural prostheses that convert sound energy to electrical signals and can be used to stimulate the auditory division of the eighth nerve directly.
  • Bilateral cochlear implants are increasingly common and serve to enhance sound localization and improve understanding of speech in background noise.
General criteria for cochler implant candidacy
  • B/L severe to profound sensorineural hearing loss(with non-functional Cochlear Hair Cells).
  • PTA of 70 dB or more
  • Little or no benefit from hearing aids.
  • No medical contraindication for surgery.
  • An intact and functional auditory processing pathway, from Spiral ganglion cells to - Auditory Nerve to -Auditory cortex.
  • Realistic expectation
  • Good family and social support
  • Adequate cognitive function to be able to use the disease.
Contraindications for Cochlear Implants:
  • Deafness caused by lesions of the acoustic nerve or the central auditory pathway
  • Active middle ear infections
  • Cochlear ossification that prevents electrode insertion
  • Tympanic membrane perforation
  • Cochlear malformation is not a contraindication for cochlear implantation.
Pre-op investigation before a Cochlear Implant is placed:
  • HRCT Temporal Bone and Complete Audiological Examination.
  • MRI may be needed.
Parts of Cochlear Implant:
  • Microphone, Speech-processor and Transmitter are the part of external component of cochlear implant, which remain outside the body.
  • Receiver/Stimulator (Implanted under the skin) and Electrode array (implanted in the scala tympani of the cochlea) are the part of internal component of cochlear implant, which are fitted inside the body.
Candidates are defined as pre-lingual or post-lingual depending on whether they were deafened before or after the acquisition of speech & language.
  • In children with pre-lingual deafness (at birth or in early child hood) implantation at an early age ensures better results and children can be implanted at 12 months of age.
  • Minimum age for Cochlear Implant Surgery is 1 year. 
  • Post lingual patients with shorter duration of deafness have better outcome.
In cochlear implant surgery the electrode array is placed within the scala tympani of the cochlea(at Round window).
  • The electrode thus stimulates the cochlear nerve which is a part of the vestibulo-cochlear nerve (CN VIII).
There are two techniques to approach cochlea:
  • Facial Recess approach (Posterior tympanotomy) which involves doing a cortical mastoidectomy.
  1. Facial recess is a suprapyramidal recess,middle ear can be approached through it and important in Cochlear Implant Surgery.
  2. From the middle ear the electrodes are then introduced into the scala tympani through the round window.
  • Cochleostomy is performed antero-inferior to the round window membrane.
  1. The pericanal technique in which cochleostomy is performed either by endaural or post-aural approach for e.g. Veria and suprameatal recess approach.
  2. Recently Veria technique (Non-Mastoidectomy technique) is gaining popularity for cochlear implantation. It uses transcanal approach.
  • Advantage of Vera technique
  • Simple
  • Less chances of injuring facial nerve
  • Suitable in young children where mastoid has not developed fully.
Use of nitrous oxide is contraindicated in Cochlear Implant Surgery.
  • When nitrous oxide is inhaled in large quantities N20 replaces the N2 in the middle air cavity thereby increasing the pressure in the cavity.
  • After discontinuation of N20 the gas is rapidly reabsorbed from the cavity thus creating negative pressure in the cavity this may cause: Serous otitis media,Disarticulation of stapes,Impaired hearing.
Switch on is done after 3 weeks.
Exam Question
  • Cochlear malformation is a surgical challenge but not a contraindication for cochlear implantation.
  • Use of nitrous oxide is contraindicated in Cochlear Implant Surgery.
  • In Cochlear implants, the electrodes are most commonly placed at Cochlea.
  • Auditory Nerve should be intact for Cochlear Implantation.
  • Patients of profound binaural SNHL-PTA of 70 dB or more(with non functional cochlear hair cells) who have intact auditory nerve functions and show little or no benefit from hearing aids are the candidate for Cochlear Implant surgery.
  • Facial recess is a suprapyramidal recess,middle ear can be approached through it and important in Cochlear Implant Surgery.
  • Minimum age for Cochlear Implant Surgery is 1 year
  • Electrode is kept in Scala Tympani in Cochlear Implant surgery
  • In cochlear implants, electrodes are most commonly placed through Round window.
  • Switch on of Cochlear Implant is done after 3 weeks.
  • Pre-op investigation before a Cochlear Implant is placed:HRCT Temporal Bone and Complete Audiological Examination.MRI may be needed.
  • Receiver/Stimulator and Electrode array are the part of internal component of cochlear implant, which are fitted inside the body.

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