Cochlear Implants — Comprehensive Overview

Hybrid cochlear implant system: external sound processor (1), transmitter coil (2), earmould (3), implanted receiver/stimulator (4, 5), electrode array (6) within the cochlea (7), external auditory meatus (8)

1. Definition and Principle

2. Components

External Components

Internal Components

3. How It Works (Signal Processing)

1. Sound is picked up by the microphone.
2. The sound processor performs frequency analysis (Fourier), compression, and filtering.
3. The digitised signal is transmitted via radiofrequency (RF) across the skin to the internal receiver.
4. The receiver/stimulator decodes the signal and delivers electrical pulses to specific electrodes along the array.
5. Electrodes are positioned tonotopically in the scala tympani — basal electrodes stimulate high-frequency fibres; apical electrodes stimulate low-frequency fibres.
6. The auditory nerve is activated, and signals travel to the brainstem and auditory cortex.

4. Electrode Array Design

• Precurved perimodiolar arrays (Contour Advance) and straight lateral-wall arrays
• Nucleus Hybrid: shortened 19.5 mm array for combined electric-acoustic stimulation (EAS)

• 1j electrode: banana-shaped with flat contacts oriented toward the modiolus; raised partitions between contacts reduce electrode interactions
• Helix electrode: same flat-contact configuration, precurved for perimodiolar positioning
• Mid-Scale electrode: shorter length but achieves 420° insertion depth due to precurved configuration

• Titanium-housed receiver/stimulator, MR Conditional at 1.5 T
• Standard 31.5 mm array with contacts every 2.4 mm
• FLEX electrode family (FLEX23 to FLEXSOFT) — 20–31.5 mm options
• Custom common-cavity electrode for severe malformations
• Compressed array (15 mm) for ossified cochleas

• Perimodiolar arrays: closer to the modiolus → lower stimulation thresholds, reduced current spread, better frequency specificity
• Straight/lateral-wall arrays: less traumatic insertion, better hearing conservation for EAS candidates

5. Candidacy

Adults

• Bilateral moderate-to-profound SNHL
• Aided sentence scores ≤50% in the ear to be implanted and ≤60% in the best-aided condition
• CMS (Medicare/Medicaid) uses stricter criterion: ≤40%
• FDA guidelines per device — generally considered conservative relative to current evidence

Children

• 12–24 months: profound SNHL (>90 dB HL) with limited hearing-aid benefit (MAIS score criteria)
• >24 months: severe-to-profound SNHL (>70 dB HL); aided open-set word recognition ≤30% (MLNT or LNT)
• Minimum 3-month hearing-aid trial (waived for meningitis-related ossification)
• Earlier implantation (<12 months) is increasingly common off-label — studies show language outcomes comparable to normal-hearing children

Cochlear Nucleus Hybrid System (EAS Candidacy)

• Low-frequency thresholds ≤60 dB HL at 500 Hz
• PTA (2, 3, 4 kHz) ≥75 dB HL in ear to be implanted
• Aided CNC word score 10–60% in the implant ear; contralateral ear ≤80%

Special Populations

Contraindications

• Complete labyrinthine aplasia
• Cochlear aplasia
• Complete cochlear ossification
• Absent cochlear nerve
• Retrocochlear pathology along the auditory pathway (relative contraindication)

6. Preoperative Evaluation

• MRI (preferred): confirms cochlear nerve presence, identifies labyrinthine malformations
• HRCT of temporal bone: evaluates bony anatomy, cochlear malformations, narrow IAC (<3 mm); assesses surgical anatomy for:
  • Poorly pneumatised mastoid / facial recess
  • Anterior sigmoid sinus
  • High-riding jugular bulb
  • Anomalous facial nerve
  • Risk of CSF gusher (thin cribriform area, widened IAC, enlarged vestibular aqueduct)

• Children: OAE, ABR, behavioural audiometry, parental questionnaires (age-dependent)
• Adults: AzBio sentences, CNC monosyllabic words, BKB-SIN test

7. Surgical Procedure

1. Postauricular incision and creation of a tight periosteal pocket for the receiver/stimulator
2. Mastoidectomy — drilling of the mastoid cortex to expose the antrum and posterior tympanum
3. Posterior tympanotomy / facial recess approach — opening between the facial nerve and chorda tympani to access the middle ear
4. Cochleostomy or round window membrane access — entry into the scala tympani
5. Electrode array insertion — gentle, atraumatic ("soft surgery") insertion to preserve residual hearing

• Avoid direct suctioning of perilymph from the scala tympani
• Use non-ototoxic topical antibiotics, corticosteroids, and hyaluronate lubricant
• Immediately seal cochlea with fascia after insertion
• Use round window approach where possible

8. Bilateral Cochlear Implantation

• More spiral ganglion cell populations stimulated — ensures the better ear is implanted
• Reduces risk of auditory deprivation from single-device failure
• Confers binaural hearing advantages:
  • Head shadow effect: preferential use of the ear with better signal-to-noise ratio
  • Binaural summation: central processing draws more information from two ears than one
  • Squelch effect ("cocktail party effect"): focus on one talker among competing noise
  • Improved sound localisation: brain uses interaural time and level differences

• Simultaneous bilateral implantation is preferred over sequential implantation with >12-month inter-implant delay
• Sequential implantation with <12-month delay achieves comparable outcomes
• Critical "window of opportunity": birth to 2 years, when neural plasticity and language receptivity are at their peak
• If this window is missed, cross-modal plasticity occurs — auditory cortex is reorganised for vision or somatosensation

9. Electric-Acoustic Stimulation (EAS / Hybrid)

• Short electrode array (10–20 mm) inserted atraumatically to stimulate the basal (high-frequency) cochlea
• Acoustic component via hearing aid earmould provides low-frequency amplification
• Outcomes in EAS condition exceed either electric or acoustic stimulation alone
• Shorter arrays → better hearing conservation; longer arrays → more effective electrical stimulation — a trade-off managed per patient

10. The CI Team

• Core: CI surgeon, audiologist, speech-language pathologist
• Extended: psychologist, educational specialist, social worker
• Community: early interventionists, deaf/hard-of-hearing itinerant teachers, educational audiologist
• In complex cases: physical/occupational therapists, developmental paediatricians, behavioural interventionists

11. Outcomes and Rehabilitation

• Adults (including those aged 65–80) achieve significant improvements in speech recognition pre- to post-implantation across varied stimulus presentation levels
• Auditory training (rehabilitation) is an important component of post-CI care — especially in children where habilitation directly drives language development
• Spiral ganglion cell (SGC) survival directly correlates with word recognition scores — supporting the case for earliest possible implantation

12. Complications

• Wound dehiscence / infection (most common)
• Otitis media
• Incomplete electrode insertion
• Facial nerve injury (rare but monitored intraoperatively)
• CSF gusher (labyrinthine dysplasia)
• Subdural haematoma (rare)

• Hard failure: device no longer meets manufacturer specifications; sudden significant change in auditory perception → requires revision
• Soft failure: performance decline and adverse symptoms with normal device imaging → heavily dependent on clinical judgement

• Rare but serious — predominantly Streptococcus pneumoniae
• Mandates universal antipneumococcal vaccination

13. Vaccination (Meningitis Prophylaxis)

Children

• PCV13 at least 2 weeks before implantation
• PPSV23 at least 8 weeks after completing PCV13 series (after age 2)
• Children 6–18 years: single dose PCV13 regardless of prior history

Adults (>19 years)

• Single dose PCV13 followed by PPSV23 at least 8 weeks later
• Second PPSV23 dose recommended if >65 years
• If previously received PPSV23: give PCV13 at least 1 year later; then PPSV23 no sooner than 8 weeks after PCV13

AOM management in CI patients

• Higher meningitis risk after AOM
• No observation period allowed — treat promptly
• AOM <2 months post-surgery: parenteral antibiotics
• AOM >2 months post-surgery: high-dose amoxicillin-clavulanate
• Lumbar puncture with CSF cultures if meningitis suspected

14. MRI Compatibility