Cochlear Implant: Parts, Indications, Contraindications and Criteria

• A cochlear implant is a device used for hearing rehabilitation to improve the hearing for those who have impaired hearing.
• The insertion of a cochlear implant is a surgical process. The electrodes are inserted into the scala tympani of the cochlea via the round window membrane.
• It stimulates the 8^th nerve.
• Signals travel through the organ of Corti and reach the 8^th nerve which further stimulates the Brain.

Cochlear Implant Parts

• It has two components: external and internal.
• The external component contains a microphone, speech processor, and transmitter.
• The microphone receives the sound from the external environment and transfers it to the speech processor. This speech processor works by converting the sound energy into electrical energy and then transmits it to the internal component.
• The internal component includes the receiver and stimulator.
• It receives the energy from the external component via the electrode array and transfers it to the 8^th nerve.

Also Read: Otitis Media : Types, Causes, Symptoms and Treatment

Indications

• Bilateral severe to profound sensorineural hearing loss (70-90 dB hearing loss).
• Child who is born deaf and has not yet developed speech (Prelingual deafness).
• Child or adult who has become deaf but has already developed speech (Postlingual deafness).
• It is very important to make the diagnosis earlier because the treatment will also be started earlier and hence there is a better chance to get good results.
• The crucial window period is zero to three years of age.

Contraindications

• Cochlear aplasia or absence of cochlea.
• Retro-cochlear pathology

Criteria for Implant

• Age should be in prelingual deafness
• No evidence of absence of cochlear nerve
• Functioning central auditory pathway
• Failed trial of hearing aid

Preoperative Evaluation

• Audiometry is done in children five years old or above.
• Otoacoustic Emissions (OAE) is a screening test used for the preoperative evaluation of the patient. If this test is failed then the patient will be referred to brainstem evoked response audiometry.
• Brainstem evoked response audiometry is done to check the cochlear pathology or retro cochlear pathology.
• MRI is done to look for the cochlea and 8^th nerve structure.
• CT scan

Cochlear Implant Surgery

• The most common is posterior tympanostomy or facial recess approach.
• The mastoid route is used to reach the middle ear opening and from there, bone is removed in facial recess. After this the round window becomes visible and an electrode is inserted into the scalar tympani of the cochlea.
• Electrode should be covered with a soft seal because the facial recess approach can lead to Perilymph leak, perilymph fistula and it can also injure the vertical segment of facial nerve.
• Cochlear implants can cause meningitis as they connect scala tympani to the subarachnoid space through the cochlear aqueduct.
• It is very important to recommend the patient about pneumococcal and meningococcal vaccinations before cochlear implant surgery to prevent meningitis.
• X-ray, CT scan, and MRI should be done for post-operative assessment to confirm the position of the implant.

Other Hearing Rehabilitation Methods

• Bone Anchored Hearing Aid (BAHA)
  • In this technique, bone conduction follows the sensory neural pathway, bypassing the external and middle ear to reach the cochlea.
  • It is used when the air conduction method of rehabilitation cannot be used. The reasons might be external ear deformities like external auditory canal atresia or Microtia.
• Auditory Brainstem Implant
  • It is used when the auditory nerve has a lesion.
  • The indications are: bilateral acoustic neuromas, cochlear aplasia and cochlear nerve aplasia.
  • Electrode bypasses the cochlea and its nerve and it directly stimulates the brainstem.
  • The position of the implant is lateral recess of the 4^th ventricle.

Hope you found this blog helpful for your ENT Residency Otology Preparation. For more informative and interesting posts like these, keep reading PrepLadder’s blogs. 

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