Cochlear Implants - MS ENT PG Level Theory

Definition

Basic Principle

Components of the CI System

Figure: Components of the cochlear implant system (KJ Lee's Essential Otolaryngology, p. 407)

External Components

Internal Components

Speech Processing Strategies

Electrode Arrays

• Number of electrodes: MED-EL - 12 electrodes; Advanced Bionics - 16 electrodes; Cochlear (Nucleus) - 22 electrodes
• Despite the number, most users have fewer than 10 perceptually unique channels active at any one time
• Modern electrodes are shorter, thinner, and more flexible to minimize insertion trauma and preserve residual hearing

Candidacy Criteria (FDA-Based)

Pediatric Candidacy

• Children 2-17 years: Moderate to profound SNHL (>40 dB) with sentence recognition ≤50% in the ear to be implanted and ≤60% in the contralateral ear (or ≤40% by CMS criteria)
• Special circumstances (e.g., post-meningitis with risk of cochlear ossification) may warrant implantation before 12 months (off-label)
• Off-label implantation <12 months is increasingly common - studies show language comprehension and expressive development comparable to normal hearing children

Adult Candidacy

Non-Traditional / Expanding Indications

• Single-sided deafness (SSD)
• Retrocochlear hearing loss
• Asymmetrical SNHL
• Auditory Neuropathy Spectrum Disorder (ANSD)

Electroacoustic Stimulation (EAS) / Cochlear Nucleus Hybrid System

• Shortened electrode arrays (20 mm for MED-EL; 10 mm for Nucleus Hybrid) are inserted via the round window to preserve low-frequency residual hearing
• Combined electric (basal cochlea) + acoustic (apical cochlea) stimulation provides better outcomes than conventional CI alone
• FDA criteria: aided CNC word recognition score 10%-60% in the ear to be implanted; contralateral ear score equal to or better but ≤80%

Pre-operative Assessment

Medical Assessment

• Determine etiology of hearing loss (affects prognosis)
• Screen for factors causing cochlear ossification: meningitis, temporal bone fracture, otosclerosis
• Duration of deafness: longer duration predicts poorer outcomes
• Chronic otitis media: may require staged surgery (treat COM first, then CI)
• Recurrent AOM should not unduly delay implantation in children

Audiometric Assessment

• Children: OAE, ABR, behavioral audiometry, parental questionnaires (IT-MAIS), minimum 3-month HA trial
• Adults: AzBio sentences, CNC monosyllabic word testing, BKB-SIN (Bamford-Kowal-Bench Speech-in-Noise)
• ANSD evaluation: requires ear-specific data; MRI to rule out cochlear nerve deficiency (CND)

Radiologic Assessment

• Gold standard for cochlear patency (distinguishes fluid vs. fibrosis vs. ossification)
• Superior for visualizing the cochlear nerve (heavily T2-weighted - CISS or FIESTA sequences, sagittal plane)
• Identifies retrocochlear pathology (e.g., vestibular schwannoma)
• Should be done pre-operatively as last opportunity for high-quality brain imaging without artifact

• Diagnoses bony cochlear malformations
• Evaluates bony cochlear nerve canal (BCNC) in cases of narrow IAC (<3 mm)
• Identifies surgical obstacles: poorly pneumatized mastoid, anterior sigmoid sinus, high-riding jugular bulb, aberrant facial nerve course
• Risk of CSF gusher in: thin cribriform area, widened IAC, enlarged vestibular aqueduct, IP3 deformity

Contraindications

• Absence/aplasia of the cochlear nerve (cochlear nerve deficiency)
• Complete cochlear ossification (relative - partial insertion may still be possible)
• Active middle ear infection

• Severe cognitive impairment
• Significant medical comorbidity precluding general anesthesia
• Unrealistic patient/family expectations
• Lack of adequate rehabilitation support

The CI Team (Multidisciplinary)

Surgical Approach

1. Cortical mastoidectomy
2. Posterior tympanotomy (via the facial recess, between the facial nerve and chorda tympani)
3. Cochleostomy or round window membrane approach into scala tympani
4. Electrode array insertion
5. Receiver-stimulator placed in a bony well posterior to the mastoid

Bilateral Cochlear Implantation

• Binaural summation (improved SNR)
• Directional hearing / sound localization
• Better speech in noise performance
• Benefit for visually impaired patients
• Incidental/background learning through listening
• Logistical benefit: one device remains functional if the other fails

Activation and Programming ("Mapping")

• Begins 3-4 weeks post-surgery
• Impedance testing at each visit to check electrode functionality
• Programming involves setting T-levels (threshold/detection) and M/C-levels (maximum comfort)
• Electrophysiologic aids: eCAP (electrically evoked compound action potential) and eSRT (electrically evoked stapedial reflex threshold) - useful when behavioral responses are unreliable in infants

Outcomes

Adults

• Primary goal: improved open-set speech perception
• Mean open-set word recognition (CNC): 25-55% at >6 months
• With modern processing strategies: mean sentence recognition >80% (range up to 100%)
• Continued improvement up to 3 years post-implantation; smaller improvements beyond 3 years
• Telephone speech recognition and music appreciation occasionally reported
• Multichannel implants clearly superior to single-channel designs

Children

• Earlier implantation = better language outcomes
• CI before 12 months: language comprehension and expressive development comparable to normal hearing peers
• Auditory training post-CI further improves outcomes

Factors Predicting Better Outcomes

• Shorter duration of deafness
• Postlingual (vs. prelingual) onset of deafness
• Younger age at implantation
• Higher pre-implant residual hearing
• Good cognitive function
• Active rehabilitation and family involvement

Factors Predicting Poorer Outcomes

• Long-standing profound deafness
• Cochlear ossification/malformation
• Cochlear nerve deficiency
• Significant neurodevelopmental comorbidities

Post-implant Complications

Special Considerations in CI Patients

Meningitis Risk

• CI recipients are at higher risk for meningitis after AOM
• No observation period permitted for AOM in CI patients (unlike standard children)
• AOM <2 months after surgery: parenteral antibiotics
• AOM >2 months after surgery: high-dose amoxicillin-clavulanate
• Suspected meningitis: LP with CSF culture

Vaccination (Mandatory Pre-/Post-implantation)

• PCV13 at least 2 weeks prior to implantation
• Children 6-18 years: single dose PCV13 regardless of prior history
• Children >2 years: PPSV23 after completing PCV13 series

• Single dose PCV13 followed by PPSV23 at least 8 weeks later
• Second dose PPSV23 for CI recipients >65 years

Tympanostomy Tubes in CI Patients

• CIs can safely coexist with PE tubes
• Children susceptible to RAOM may keep PE tubes without placing CI or patient at undue risk
• Replacement of PE tubes after CI is acceptable for treatment of RAOM
• If history of recurrent AOM: consider subtotal petrosectomy with ET ablation and EAC closure with second-stage CI

MRI Safety

• Modern CI devices are MRI-conditional (typically up to 1.5 T with precautions)
• Pre-implant MRI should be obtained as this is the last opportunity for high-quality brain imaging without artifact

Auditory Neuropathy Spectrum Disorder (ANSD) and CI

• Defined by: cochlear microphonic (wave I) present + distal ABR waveforms absent ± OAEs present
• Cannot predict hearing thresholds by electrophysiology alone
• Worse discrimination scores than expected from thresholds
• Stepwise approach: observation → hearing thresholds + discrimination → amplification or alternate communication → CI if severe/profound range
• MRI mandatory to rule out cochlear nerve deficiency before CI in ANSD
• IT-MAIS used for candidacy evaluation in young children with ANSD