Cochlear Implant Surgery

Cochlear Implant Surgery

Cochlear implants are advanced electronic devices designed to provide hearing for children and adults with severe to profound sensorineural hearing loss. The cochlear implant provides direct electrical stimulation to the auditory nerve, bypassing the damaged parts of the inner ear and degenerated hair cells in the cochlea, which are damaged to such a point that amplification provided by hearing aids is no longer effective. The auditory nerve then carries the signals to the auditory cortex in the brain for hearing perception.

Components and Mechanism of a Cochlear Implant. A cochlear implant consists of two main components: an external unit and an internal unit.

1. External Component: It consists of
   • Speech Processor: This can be worn behind the ear or on the body, with the behind-the-ear type being more common. It captures sound through a microphone, processes it, converts it into digitally coded signals, and sends them to the transmitter coil. The speech processor converts sound into electrical pulses using a variety of advanced coding strategies, such as the Simultaneous Analogue Strategy (SAS), Continuous Interleaved Sampling (CIS), Spectral Peak (SPEAK), and Advanced Combination Encoder (ACE).
   • Transmitter coil: The external transmitter sends digitally coded signals to the receiver-stimulator package via radiofrequency waves.
     
     
2. Internal Component:
   • Receiver-Stimulator package: It consists of a magnet, which holds the external transmitter in place and an electronic system that decodes signals received from the external transmitter, converts them into electrical impulses and transmits them to the electrode array by using a speech strategy. It is surgically implanted behind the ear beneath the skin.
   • Intracochlear electrode Array: It is a thin, flexible wire inserted into the scala tympani of the basal turn of the cochlea (the entire length of the cochlear duct). This array contains multiple electrodes that deliver electrical impulses to the spiral ganglion cells in the cochlea. These cells are bipolar neurons that connect the cochlea to the auditory nerve. When the electrodes stimulate the spiral ganglion cells, the auditory nerve carries these signals to the auditory cortex in the brain, where they are interpreted as sound. For effective speech perception, it is essential to stimulate at least 10,000 of the approximately 35,000 nerve fibres in the auditory nerve. This ensures that the brain receives sufficient information to get good speech perception and distinguish speech sounds, especially in noisy environments.

Candidacy for Cochlear Implants. Cochlear implants are suitable for both children and adults who meet specific criteria:

1. Severe to Profound Sensorineural Hearing Loss.
2. Fracture of the cochlea following temporal bone fracture.
3. Auditory neuropathy or dyssynchrony (ANDS) patients.
4. Inability to achieve functional hearing with conventional devices.
5. No medical contraindications for fitness for surgery and general anaesthesia.
6. Realistic Expectations: Understanding the potential outcomes and limitations.
7. Strong family and social support for post-implantation rehabilitation.
8. Adequate Cognitive Function: Ability to use and adapt to the device.

Candidates are categorized as prelingual (deafened before acquiring speech) or postlingual (deafened after acquiring speech). Early intervention is critical for prelingual children, as auditory deprivation during early development can lead to degeneration of central auditory pathways, limiting the benefits of implantation.

Outcomes of Cochlear Implantation. The success of cochlear implantation depends on several factors:

1. Prior Auditory Experience: Postlingual patients or those with prior hearing aid use tend to achieve better outcomes.
2. Age at Implantation: Younger children, especially those implanted before 12 months, show improved speech and language development.
3. Duration of Deafness: Shorter periods of deafness correlate with better outcomes.
4. Neural Plasticity: Early diagnosis and rehabilitation (ideally before age 5) are critical for prelingually deaf children due to neural plasticity. Neural plasticity is the brain’s ability to adapt to new auditory stimuli. Without early stimulation, the brain’s auditory areas may be repurposed for other functions, limiting the potential for speech and language development.

• Postlingual patients often achieve significant benefits, including the ability to understand speech without visual cues and use the telephone.
• Prelingual children also develop speech and language skills over time, though this requires consistent auditory-verbal therapy.
• Prelingual adults with no prior auditory experience may gain only sound awareness.

Evaluation for Cochlear Implantation. A thorough evaluation is essential to determine candidacy and set realistic expectations. The evaluation process includes:

1. Medical Evaluation:
   • Detailed history and physical examination to assess fitness for surgery.
   • Preoperative tests and vaccinations, particularly against meningitis (e.g., Haemophilus influenza, meningococcus).
   • Check for any ear infections. Pt should be free from all ear infections. Myringoplasty or mastoid exploration if required, should be 03 months before the cochlear implant surgery.
2. Imaging Studies: 
   • HRCT of Temporal Bone: Identifies inner ear abnormalities, cochlear lumen obliteration, middle ear pathology, and anatomical variations (e.g., low-lying dura, anterior sigmoid sinus). It detects conditions like cochlear hypoplasia, enlarged vestibular aqueduct, and labyrinthitis ossificans, which can impact surgical planning and candidacy for cochlear implants (CI).
   • MRI: Diagnoses cochlear nerve aplasia, a contraindication for CI, necessitating an Auditory Brainstem Implant (ABI) instead.
3. Audiological Evaluation:
   • Pure tone audiometry, speech discrimination tests, tympanometry, otoacoustic emissions (OAE), auditory brainstem responses (ABR), and auditory steady-state responses (ASSR).
   • A mandatory hearing aid trial to assess the extent of benefit from conventional devices.
4. Speech and Language Evaluation: Assesses the patient’s current communication abilities and identifies any developmental delays or disorders.
5. Psychological Evaluation: Evaluates cognitive function and identifies any additional disabilities, helping to set realistic expectations for post-implantation outcomes.

Surgical Procedure. Cochlear implant surgery is performed under general anaesthesia and involves the following steps:

1. Patient Positioning: The surgery is performed under general anaesthesia, with the patient in a supine position and the head turned 45-60 degrees from the surgeon.
2. Incision: A C-shaped postauricular incision is made.
3. Flap Elevation and Mastoidectomy: The skin flap , subcutaneous tissue, and part of the temporalis muscle (palva flap) are elevated. A cortical mastoidectomy is performed, preserving overhanging edges.
   
4. Formation of the well. A subperiosteal pocket is made in the posterior and superior to the mastoidectomy cavity. A bony well is created for the receiver-stimulator placement.
   
   
5. Posterior Tympanotomy: The middle ear is accessed via the facial recess, and the round window niche is visualized.
   
   
   
   
6. Cochleostomy: A cochleostomy is done with Rosen’s pick instrument. Cochleostomy is created anteroinferior or inferior to the round window, ensuring the electrode enters the scala tympani.
   
   
7. Receiver-Stimulator Placement: The receiver-stimulator is placed tightly in the subperiosteal pocket and the electrode array is passed through the bony well, which is secured with non-absorbable sutures.
   
   
   
8. Electrode Insertion: The electrode array is inserted into the cochlea, either through the cochleostomy or the round window (the latter is preferred for reduced trauma and postoperative complications).
   
   
9. Cochleostomy sealing. Temporalis muscle pieces or fat are used to seal the cochleostomy site. Electrophysiological testing is done to confirm proper functioning.
   
   
10. Verification and Closure: Neural response telemetry checks electrode functionality. The incision is closed in layers, and a postoperative X-ray (Stenver’s view) is done to confirm the electrode position.
    
    

Postoperative Mapping and Rehabilitation

1. Device Activation: The implant is activated 3–4 weeks post-surgery.
2. Mapping (Programming): The speech processor is programmed to optimize sound perception. Regular adjustments are made during follow-up visits.
3. Habilitation: Auditory-verbal therapy is essential for all patients, particularly prelingual children. This therapy focuses on developing listening and speaking skills without relying on visual cues. Consistent effort from the patient, family, and therapists is crucial for successful adaptation.

Cochlear implants have transformed the lives of individuals with severe to profound hearing loss, offering them the opportunity to experience sound and develop communication skills. With careful patient selection, precise surgical techniques, and dedicated postoperative rehabilitation, cochlear implants can provide life-changing benefits, particularly for children who receive early intervention. This technology continues to evolve, promising even greater outcomes for future recipients.

 

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Download full PDF Link: Cochlear Implant Surgery Best Lecture Notes Dr Rahul Bagla ENT Textbook

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of Surgery of the Ear.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Rob and smith, Textbook of Operative surgery.
• Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.
• Gordon B. Hughes, Myles L. Pensak, H. B. Broidy. Textbook of Clinical Otology.
• Mario Sanna. Textbook of Color Atlas of Endo-Otoscopy Examination–Diagnosis–Treatment.

Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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• Please read. Anatomy of External Ear. https://www.entlecture.com/anatomy-of-ear/
• Please read. Anatomy of Temporal Bone. https://www.entlecture.com/anatomy-of-temporal-bone/
• Please read. Stenger’s, Chimani Moos, Teal test. https://www.entlecture.com/special-tuning-fork-tests/

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