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AUDITORY BRAINSTEM IMPLANTATION (ABI)

A Comprehensive Discussion — 50 Marks

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1. DEFINITION & CONCEPT

"The ABI consists of a multielectrode surface array placed over the cochlear nucleus within the lateral recess of the fourth ventricle." — Cummings Otolaryngology, p. 3119

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2. HISTORICAL BACKGROUND

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3. ANATOMY OF TARGET SITE

• Dorsal Cochlear Nucleus (DCN)
• Anteroventral Cochlear Nucleus (AVCN)
• Posteroventral Cochlear Nucleus (PVCN)

• Cranial nerves IX, X, XI (lower cranial nerves — inferolateral guide)
• Cranial nerves VII & VIII (acoustic-facial bundle — superior border)
• Foramen of Luschka (entry point into lateral recess)
• Anterior inferior cerebellar artery (AICA) — to be preserved

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4. DEVICE HARDWARE

┌──────────────────────────────────────────────────────────────┐
│                    ABI SYSTEM COMPONENTS                     │
├─────────────────────┬────────────────────────────────────────┤
│ EXTERNAL            │ INTERNAL (implanted)                   │
├─────────────────────┼────────────────────────────────────────┤
│ Microphone          │ Receiver/Stimulator (titanium housing) │
│ Speech Processor    │ Electrode array (Dacron mesh paddle)   │
│ Transmitter coil    │ 12–21 active electrode contacts        │
│ Battery             │ Ground electrode (separate)            │
└─────────────────────┴────────────────────────────────────────┘

• Electrode array: Flat paddle, 3 × 8 mm, 12–21 platinum disk electrodes on silicone + Dacron mesh backing
• The mesh allows suture fixation to prevent electrode migration
• Commercially available devices: Cochlear Corporation (Nucleus ABI), MED-EL (MAESTRO ABI), Oticon Medical (Digisonic SP ABI)
• Penetrating ABI (pABI): Experimental — intraneural penetrating electrodes to stimulate tonotopic layers within CN (not yet FDA approved)

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5. INDICATIONS

5A. ESTABLISHED (FDA-APPROVED) INDICATIONS

Neurofibromatosis Type 2 (NF2) — patients ≥12 years undergoing first- or second-side vestibular schwannoma removal

NF2 Patient with Hearing Loss
           │
           ▼
   Cochlear nerve intact?
    ┌──────┴──────┐
   YES            NO
    │              │
    ▼              ▼
Cochlear      ABI at time of
Implant       schwannoma
first         removal
    │
    ▼
Cochlear nerve
preserved but
non-functional?
    │
    ▼
ABI (next step)

5B. EXPANDED (NON-NF2) INDICATIONS

• Functioning cochlear nerve (CI preferred — superior outcomes)
• Unstable NF2 with multiple cranial tumors (high surgical risk)
• Severe medical co-morbidities
• Brainstem invasion by tumor at CN level
• Inability to cooperate with post-operative mapping/rehabilitation
• Patient/family with unrealistic expectations

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6. CANDIDACY CRITERIA & SELECTION

CANDIDACY EVALUATION FLOWCHART
════════════════════════════════════════════════════════════
                     Patient referred for ABI
                              │
                  ┌───────────▼───────────┐
                  │  Comprehensive        │
                  │  Audiological         │
                  │  Assessment           │
                  └───────────┬───────────┘
                              │
              ┌───────────────▼───────────────┐
              │  CI candidacy evaluation:      │
              │  - MRI cochlea/CN              │
              │  - CT temporal bones           │
              │  - EABR / ECAP testing         │
              └───────────────┬───────────────┘
                              │
                   CI possible and CN intact?
                    ┌─────────┴──────────┐
                   YES                   NO
                    │                    │
               CI preferred         ABI candidacy
                    │                    │
                    ▼              ┌─────▼─────────────────┐
             CI trial 6 mos        │ Multidisciplinary team│
                    │              │ - Neurotologist        │
            Adequate benefit?      │ - Neurosurgeon         │
            ┌────────┴───────┐     │ - Audiologist          │
           YES               NO    │ - Speech therapist     │
            │                │     │ - Psychologist         │
         Continue CI         │     │ - Radiologist          │
                             └─────┴─────────────────┘
                                         │
                             ABI confirmed: Pre-op workup
════════════════════════════════════════════════════════════

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7. PRE-OPERATIVE PREPARATION

7A. HISTORY AND PHYSICAL EXAMINATION

• Detailed otological and neurological history
• Family history of NF2
• Prior ear surgeries, radiation history
• Systemic co-morbidities (cardiovascular, coagulation disorders)
• Psychological assessment and counseling

7B. AUDIOLOGICAL ASSESSMENT

• Pure Tone Audiometry (PTA) — baseline bilateral thresholds
• Speech Audiometry — speech discrimination scores
• Auditory Brainstem Response (ABR) — wave V absent → cochlear nerve non-functional
• Electrically Evoked Auditory Brainstem Response (EABR) — promontory stimulation to assess central pathway integrity (CN and beyond)
• Otoacoustic Emissions (OAE)
• Aided audiometry with best hearing aids

If EABR is present at promontory stimulation but no auditory nerve function, this confirms central (CN) pathways are intact → ABI will likely yield benefit

7C. IMAGING

7D. MEDICAL & SURGICAL PREPARATION

• Coagulation profile (PT, aPTT, platelet count), CBC, blood group
• Cardiac evaluation — ECG, echocardiogram if needed
• Anaesthetic fitness — neurological co-morbidities
• Anti-epileptic medications — review pre-operatively
• Steroid protocol — perioperative dexamethasone to reduce brainstem edema
• Antibiotic prophylaxis — standard craniotomy regimen
• Shaving and scalp preparation — retrosigmoid region
• Informed consent — detailed counseling on realistic outcomes, risks of non-auditory stimulation, CSF leak, cranial nerve palsies

7E. PRE-OPERATIVE COUNSELING (Critical)

1. ABI provides sound awareness not normal hearing in most cases
2. Outcomes are highly variable and inferior to cochlear implants
3. ABI aids lip-reading and environmental awareness
4. Extensive post-operative rehabilitation and mapping sessions are required
5. NF2 patients may have additional CNS burdens affecting outcomes

7F. TEAM PREPARATION

• Intraoperative neurophysiology monitoring team (electrophysiologist)
• Neurosurgeon + Otologist — dual surgical team
• Manufacturer representative — for device programming
• Equipment: ABI device, intraoperative electrophysiology unit, operating microscope, neurophysiologic monitoring (CMAP for CN IX, X; BAEP)

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8. SURGICAL TECHNIQUE

8A. APPROACHES TO THE COCHLEAR NUCLEUS

8B. STEP-BY-STEP OPERATIVE TECHNIQUE (Translabyrinthine Approach)

• Supine with head turned 30–45° away from operative side
• Alternatively: lateral decubitus (park bench) position
• Head fixed in Mayfield/Gardner-Wells tongs
• Neurophysiology monitoring electrodes applied (EEG, CMAP for CN IX, X, XI)

1. C-shaped retroauricular incision
2. Cortical mastoidectomy
3. Complete labyrinthectomy → removal of cochlea, vestibule, semicircular canals
4. Identify and skeletonize the sigmoid sinus
5. Expose the posterior fossa dura posterior to sigmoid sinus
6. Identify jugular bulb, posterior semicircular canal, and the internal auditory canal (IAC)
7. Dural incision — L-shaped or T-shaped opening of posterior fossa dura

ELECTRODE PLACEMENT SEQUENCE
════════════════════════════════════════════
1. Remove any CSF, blood, and cottonoids
   from the lateral recess
        │
        ▼
2. Gently open the foramen of Luschka with
   a fine hook or suction irrigator
        │
        ▼
3. Insert electrode paddle into lateral
   recess with smooth forceps or stylet
        │
        ▼
4. INTRAOPERATIVE ELECTROPHYSIOLOGY:
   - Electrically Evoked Auditory Brainstem
     Response (EABR) — confirm cochlear
     nucleus stimulation
   - EABR waves III and V must be present
   - Test for non-auditory stimulation
     (check for facial twitch, shoulder
     elevation → CN IX, X, XI activation)
        │
        ▼
5. Optimize electrode position until EABR
   is maximal and non-auditory stimulation
   is minimized
        │
        ▼
6. Secure electrode array:
   - Dacron mesh sutured to dura/fibrous
     tissue at lateral recess with 4-0
     Prolene or titanium clips
   - Tie-down suture through mesh holes
════════════════════════════════════════════

8C. INTRAOPERATIVE ELECTROPHYSIOLOGY — EABR CRITERIA

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9. POST-OPERATIVE MANAGEMENT

Immediate (0–2 weeks):

• ICU monitoring for 24–48 hours
• Neurological observations — level of consciousness, cranial nerve function
• Head of bed elevated 30–45°
• Monitor for CSF leak, wound dehiscence
• Corticosteroids (dexamethasone) — prevent brainstem edema
• Antiemetics — post-craniotomy nausea
• Pain management
• Early ambulation

Device Activation (4–6 weeks post-op):

• Initial programming (mapping) session by audiologist
• Electrical thresholds (T-levels) and comfort levels (C-levels) established per electrode
• Non-auditory stimulation channels identified and deactivated
• Multiple sessions over months required to optimize MAP

Rehabilitation:

• Auditory training
• Speech reading support
• Communication therapy
• Regular audiological follow-up

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10. OUTCOMES

Performance Levels (Variable spectrum):

ABI PERFORMANCE SPECTRUM
═══════════════════════════════════════════════════════════
  POOR                    MODERATE               GOOD
  ←───────────────────────────────────────────────────────→
  
  Environmental          Enhanced               Open-set
  sound awareness        lipreading             speech
  only                                          recognition
  
  (~20–30%)            (~40–50%)              (~10–20%)
═══════════════════════════════════════════════════════════

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11. COMPLICATIONS

11A. INTRAOPERATIVE

11B. EARLY POST-OPERATIVE

11C. DELAYED / DEVICE-RELATED

Complication Summary Table (Cummings):

MOST COMMON ABI COMPLICATIONS
══════════════════════════════════════════════
1. Non-auditory stimulation      ← MOST COMMON
   (facial twitch, parasthesia)
2. CSF leak
3. Electrode migration
4. Lower cranial nerve dysfunction
5. No auditory benefit
══════════════════════════════════════════════

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12. SPECIAL CONSIDERATIONS IN PEDIATRIC ABI

• Retrosigmoid approach preferred in children (avoids labyrinth, allows bilateral future access)
• Cochlear nucleus is proportionally smaller — electrode placement technically demanding
• Imaging critical — MRI for CN aplasia vs. hypoplasia vs. cochlear aplasia
• Outcomes superior to NF2 adults — up to 80% gain open-set speech in non-tumor children
• Age at implantation critical: best outcomes if implanted <2–3.5 years (critical period)
• Must coordinate with neurosurgeon experienced in pediatric posterior fossa surgery

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13. RECENT ADVANCES

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14. COMPARISON: ABI vs. COCHLEAR IMPLANT

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15. COMPREHENSIVE FLOWCHART SUMMARY

COMPLETE ABI MANAGEMENT FLOWCHART
═══════════════════════════════════════════════════════════════════
INDICATION CONFIRMED (NF2 or non-NF2 non-CI candidate)
                          │
         ┌────────────────▼────────────────┐
         │     PRE-OPERATIVE WORKUP        │
         │ • Audiometry / EABR             │
         │ • MRI (CN anatomy, brainstem)   │
         │ • HRCT (temporal bone)          │
         │ • Multidisciplinary assessment  │
         │ • Counselling & consent         │
         └────────────────┬────────────────┘
                          │
         ┌────────────────▼────────────────┐
         │     SURGICAL APPROACH           │
         │ NF2 → Translabyrinthine (TL)    │
         │ Non-NF2/Child → Retrosigmoid    │
         └────────────────┬────────────────┘
                          │
         ┌────────────────▼────────────────┐
         │     INTRAOPERATIVE STEPS        │
         │ 1. CPA exposure                 │
         │ 2. Tumour removal (NF2)         │
         │ 3. Identify foramen of Luschka  │
         │ 4. Electrode placement in       │
         │    lateral recess (CN surface)  │
         │ 5. EABR confirmation            │
         │    (waves III + V present)      │
         │ 6. Adjust if non-auditory stim  │
         │ 7. Secure with Dacron suture    │
         │ 8. Receiver in bony well        │
         └────────────────┬────────────────┘
                          │
         ┌────────────────▼────────────────┐
         │     POST-OPERATIVE CARE         │
         │ • ICU 24–48h                    │
         │ • Steroids, antibiotics         │
         │ • CSF leak surveillance         │
         └────────────────┬────────────────┘
                          │
         ┌────────────────▼────────────────┐
         │  DEVICE ACTIVATION (4–6 weeks)  │
         │ • Audiological mapping (T & C   │
         │   levels per electrode)         │
         │ • Non-auditory channels OFF     │
         │ • Multiple mapping sessions     │
         └────────────────┬────────────────┘
                          │
         ┌────────────────▼────────────────┐
         │       REHABILITATION            │
         │ • Auditory training             │
         │ • Lipreading support            │
         │ • Speech therapy                │
         │ • Long-term follow-up           │
         └─────────────────────────────────┘
═══════════════════════════════════════════════════════════════════

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16. KEY EXAMINATION POINTS (RGUHS 50-mark focus)

1. ABI = central neural prosthesis stimulating cochlear nucleus directly
2. Primary indication = NF2 ≥12 years (FDA-approved); CI always preferred if nerve intact
3. Target = lateral recess of fourth ventricle via foramen of Luschka
4. EABR intraoperatively = mandatory to confirm correct placement (look for waves III and V)
5. Non-auditory stimulation = most common complication (CN IX/X/XI current spread)
6. CSF leak = most common surgical complication
7. Translabyrinthine = approach of choice for NF2; Retrosigmoid for non-NF2/pediatric
8. Outcomes in non-NF2 children are superior to NF2 adults (intact central pathways)
9. Recent advances: penetrating ABI, flexible arrays, AMI (auditory midbrain implant)
10. Device components: microphone → speech processor → transmitter coil → receiver/stimulator → 12–21 electrode paddle on Dacron mesh

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• Cummings Otolaryngology Head and Neck Surgery (6e), Chapter 163 — Auditory Brainstem Implantation, pp. 3119–3210 (Choudhury, Deep, Wilkinson, Roland)
• Scott-Brown's Otorhinolaryngology, Head & Neck Surgery — Volume 3, Cochlear Implants chapter
• KJ Lee's Essential Otolaryngology (10e)
• Dhingra PL — Diseases of Ear, Nose and Throat
• Hazarika P — Textbook of Ear, Nose and Throat
• Brackmann DE et al., Otolaryngol Head Neck Surg 1993 — original ABI surgical technique
• Colletti V et al., Laryngol Otol 2000 — Retrosigmoid approach for ABI
• Otto SR et al., Arch Otolaryngol 2004 — ABI in adolescents
• Lenarz M et al., Otol Neurotol 2002 — subjective outcomes