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AUDITORY BRAINSTEM IMPLANTS (ABI)

A Comprehensive Discussion — 50 Marks

Based on Cummings Otolaryngology, Scott-Brown's, Stell & Maran's, and Related Literature

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1. INTRODUCTION & HISTORICAL BACKGROUND

Historical Milestones

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2. ANATOMY RELEVANT TO ABI

Target: The Cochlear Nucleus Complex

┌─────────────────────────────────────────┐
│        COCHLEAR NUCLEUS COMPLEX         │
│                                         │
│   ┌─────────────────────┐               │
│   │  Anteroventral CN   │ (AVCN)        │
│   │  Posteroventral CN  │ (PVCN)        │
│   │  Dorsal CN          │ (DCN)         │
│   └─────────────────────┘               │
│                                         │
│  Location: Lateral recess of 4th        │
│  ventricle / Foramen of Luschka         │
│                                         │
│  Bordered by:                           │
│  • CN VII & VIII (anterosuperior)       │
│  • CN IX, X, XI (posteromedial)         │
│  • AICA (vascular landmark)             │
│  • Choroid plexus (surface marker)      │
└─────────────────────────────────────────┘

• Foramen of Luschka — the key entry point; found between the roots of CN VII–VIII and CN IX
• Choroid plexus — surface marker identifying the lateral recess
• AICA (Anterior Inferior Cerebellar Artery) — important vascular landmark to preserve
• Tenia — arachnoid band overlying the foramen that is opened for access

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3. DEVICE HARDWARE

SOUND → [Microphone] → [Speech Processor] → [Transmitting Coil]
                                                      ↓
                              (Transcutaneous RF transmission)
                                                      ↓
                              [Receiver/Stimulator] (implanted)
                                                      ↓
                              [Electrode Array on Cochlear Nucleus]

Available Devices

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4. INDICATIONS

Primary Indications

┌─────────────────── ABI INDICATIONS ───────────────────┐
│                                                         │
│  A. TUMOR ETIOLOGY (Primary/Traditional)               │
│     • Neurofibromatosis Type 2 (NF2) — bilateral       │
│       vestibular schwannomas → cochlear nerve loss      │
│     → FDA-approved: NF2 patients ≥12 yrs undergoing    │
│       1st or 2nd vestibular schwannoma removal          │
│                                                         │
│  B. NON-TUMOR ETIOLOGY (Expanding)                     │
│     • Bilateral cochlear nerve avulsion (trauma)        │
│     • Complete cochlear ossification (post-meningitis)  │
│     • Severe cochlear malformations:                    │
│         - Cochlear aplasia                              │
│         - Cochlear nerve deficiency/aplasia             │
│         - Michel deformity                              │
│     • Failed cochlear implantation                      │
│                                                         │
│  C. PEDIATRIC (Emerging)                               │
│     • Congenital cochlear nerve aplasia                │
│     • Bilateral cochlear agenesis                      │
│     • Severe inner ear malformations                   │
│     → Stimulation during critical auditory             │
│       developmental period                             │
└─────────────────────────────────────────────────────────┘

Decision Algorithm: CI vs. ABI in NF2

NF2 Patient with Hearing Loss
          │
          ▼
  Cochlear nerve intact?
  ┌────────┴──────────┐
 YES                  NO
  │                    │
  ▼                    ▼
Consider CI first    ABI indicated
(up to 70% open-set
speech recognition)
  │
  ▼
Tumor stable /
radiation treatment?
  │
  ▼
CI preferred over ABI

Contraindications

• Absent/destroyed cochlear nucleus (rare brainstem lesions)
• Active infection/meningitis
• Unable to comply with rehabilitation
• General anaesthesia risk too high
• Severe psychiatric illness
• Absence of realistic patient expectations

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5. PREOPERATIVE EVALUATION

Multidisciplinary Team

• Neurotology / Otolaryngology
• Neurosurgery
• Audiology
• Speech-language pathology
• Neuropsychology
• Paediatrics (for children)

Investigations Required

Preoperative Counselling Points (Cummings, p. 3121):

1. Some patients receive no auditory sensations
2. ABI does not provide normal sound quality
3. Most ABI patients do not achieve open-set speech recognition
4. Regular follow-ups required for ABI optimization
5. Extended time and use needed to develop maximum benefit

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6. OPERATIVE TECHNIQUE

6A. Surgical Approaches

┌──────────────────────────────────────────────────────┐
│           SURGICAL APPROACHES FOR ABI                │
│                                                      │
│  1. TRANSLABYRINTHINE APPROACH                       │
│     • Preferred in NF2 (simultaneous tumor removal)  │
│     • Better lateral view of brainstem               │
│     • Better direct view of foramen of Luschka       │
│     • Sacrifices residual hearing (already lost)     │
│                                                      │
│  2. RETROSIGMOID (SUBOCCIPITAL) APPROACH             │
│     • Preferred in nontumor patients                 │
│     • Craniotomy made as far ANTERIOR & INFERIOR     │
│       as possible                                    │
│     • Sigmoid sinus skeletonized & retracted fwd     │
│     • Allows direct access to foramen of Luschka     │
│       with minimal cerebellar retraction             │
└──────────────────────────────────────────────────────┘

6B. Step-by-Step Surgical Technique (Cummings, pp. 3122–3123)

• General anaesthesia — no long-acting paralytics (for nerve monitoring)
• Supine with head turned to contralateral side
• Continuous EMG facial nerve monitoring (orbicularis oris and oculi)
• Subdermal electrodes for intraoperative EABR: placed at vertex, C7 level, and occipital hairline
• Endotracheal tube with recurrent laryngeal nerve monitoring (CN X monitoring)

• Post-auricular / C-shaped incision down to temporalis fascia
• Reflected anteriorly and posteriorly
• Periosteum elevated off bone posteriorly and superiorly

• Bony well drilled to house the receiver/stimulator
• Silicone replica of receiver/stimulator used as guide for size and contour
• Trough drilled for electrode lead wire before it enters the dura

• Craniotomy site is anterior and inferior to the receiver well
• Mastoidectomy ± labyrinthectomy (translabyrinthine) OR posterior fossa craniotomy (retrosigmoid)

• Standard approach to cerebellopontine angle (CPA)
• Identify lower cranial nerves (IX, X, XI)
• Identify CN VII and VIII (if present)
• Stimulation probe confirms CN VII and X positions

• Found between roots of CN VII–VIII (anterosuperior) and CN IX (posteromedial)
• Choroid plexus identified at the foramen — key surface marker
• Tenia (arachnoid band) opened to enlarge access
• Any overlying vein carefully dissected away
• Valsalva manoeuvre performed → CSF outflow from foramen confirms correct identification

• Electrode paddle gently inserted into the lateral recess of the fourth ventricle
• Array positioned flat against the cochlear nucleus surface
• Electrode stylet used for insertion, then withdrawn to deploy the paddle
• Dacron mesh backing helps secure the electrode to adjacent structures

• Once electrode is in place, Electrically Evoked Auditory Brainstem Response (EABR) is recorded
• Stimulating current applied to individual electrodes
• Presence of EABR confirms contact with cochlear nucleus
• If EABR absent: electrode repositioned
• Test also identifies electrodes causing nonauditory stimulation (facial twitching, neck jerk)

• Electrode lead wire secured in trough
• Receiver/stimulator placed in bony well
• Permanent suture closes soft tissue pocket
• Abdominal fat graft packed around electrode at lateral recess to prevent migration and CSF leak
• Layered wound closure

Diagram: ABI Electrode Placement

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7. POSTOPERATIVE MANAGEMENT

• Device activation: Usually delayed 4–6 weeks postoperatively to allow wound healing
• Programming (Mapping): Audiologist maps each electrode individually — threshold, comfort level, pitch
• Nonauditory electrodes deactivated: Electrodes causing facial twitching, tingling, vertigo, or neck sensation are switched off
• EABR-guided mapping used to assist programming
• Aural rehabilitation: Intensive speech therapy and auditory training
• Regular follow-up: For reprogramming and performance optimization

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8. COMPLICATIONS

Classification of Complications

┌──────────────────────────────────────────────────────────┐
│                  ABI COMPLICATIONS                       │
├──────────────────┬───────────────────────────────────────┤
│  SURGICAL        │  DEVICE-RELATED                       │
├──────────────────┼───────────────────────────────────────┤
│ CSF leak         │ Electrode migration                   │
│ Meningitis       │ Device failure/extrusion              │
│ Facial palsy     │ Nonauditory stimulation               │
│ Lower CN injury  │ No auditory response                  │
│ (IX, X, XI)      │                                       │
│ Cerebellar       │                                       │
│  contusion       │                                       │
│ Hydrocephalus    │                                       │
│ Pseudomeningo-   │                                       │
│  cele            │                                       │
│ Haemorrhage      │                                       │
│ Tinnitus         │                                       │
│ Headache         │                                       │
└──────────────────┴───────────────────────────────────────┘

8A. Nonauditory Stimulation (Most Common)

• Most frequent complication (seen in majority at initial activation)
• Caused by current spread to adjacent cranial nerves and brainstem structures
• Manifestations:
  • Facial twitching (CN VII)
  • Neck/shoulder muscle contraction (CN XI)
  • Dizziness/vertigo (vestibular nucleus)
  • Tingling of face or throat (CN V, IX)
  • Chest or epigastric sensations (CN X)
• Management: Deactivate offending electrodes; adjust stimulation parameters; change ground electrode
• 9% of patients report persistent nonauditory sensations (Cummings, p. 3182)

8B. CSF Leak

• Second most common complication
• Occurs through the electrode entry site at the lateral recess
• Prevention: Abdominal fat graft packing around electrode lead at the dura/lateral recess
• Management: Lumbar drain, re-exploration if persistent

8C. Electrode Migration

• Originally the primary cause of ABI failure (early devices, 1979)
• Prevented by Dacron mesh backing on electrode paddle
• Results in loss of auditory response or change in electrode performance
• May require re-exploration

8D. Cranial Nerve Injury

• CN VII (Facial nerve): Palsy — more common in NF2 due to large tumors distorting anatomy
• CN IX, X, XI: Injury during foramen of Luschka identification → dysphagia, hoarseness, aspiration
• CN X monitoring via laryngeal EMG tube helps identify and preserve vagus nerve

8E. Surgical Complications

(Cummings, p. 3124) — These complications significantly less common in nontumor patients than in NF2 patients

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9. OUTCOMES

9A. NF2 Patients (Cummings, pp. 3125–3126)

• Brainstem trauma during tumour removal
• Venous damage to brainstem
• Larger tumours with more distorted anatomy

9B. Nontumor (Non-NF2) Patients

• Superior outcomes compared to NF2 patients
• Higher rates of open-set speech recognition
• Colletti et al. reported some nontumor patients achieving near-normal speech recognition
• Children implanted for cochlear nerve aplasia show encouraging auditory development

9C. Paediatric Outcomes

• Growing experience from Europe (particularly Italy) and now USA
• Goal: Provide auditory input during critical period of auditory development
• Outcomes variable but encouraging for environmental sound awareness and speech development
• Best results when implanted at young age (<3 years)

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10. SPECIAL CONSIDERATIONS IN PAEDIATRIC ABI

PAEDIATRIC ABI — KEY DIFFERENCES FROM ADULT

  • Only at centres with adult ABI AND paediatric neurosurgery expertise
  • Smaller anatomy → higher surgical risk
  • Anaesthetic considerations for infant craniotomy
  • Developmental assessment essential preoperatively
  • Parental counselling about realistic expectations
  • Early activation (as soon as healed) to exploit plasticity
  • Intensive post-implant auditory-verbal therapy
  • Long-term multidisciplinary follow-up

1. Bilateral cochlear nerve aplasia/hypoplasia
2. Complete cochlear aplasia (Michel deformity)
3. Complete bilateral cochlear ossification (post-meningitis)
4. Failed cochlear implantation
5. NF2 (rare in children)

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11. COMPARISON: ABI vs. COCHLEAR IMPLANT

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12. RECENT ADVANCES

12A. Penetrating ABI (PABI)

• Penetrating microelectrodes rather than surface paddle
• Theoretically better tonotopic specificity (CN is organized deep-to-surface, not surface-to-surface)
• Results: Did not prove superior to surface ABI in speech perception (Cummings, p. 3125)
• Research ongoing

12B. Auditory Midbrain Implant (AMI)

• Targets inferior colliculus (auditory midbrain)
• Used when cochlear nucleus is also damaged
• Currently experimental

12C. Improved Signal Processing

• Current Focus Stimulation (CFS) — reduces current spread, minimizes nonauditory stimulation
• Tonotopic mapping guided by EABR
• Cloud-based remote programming platforms

12D. Paediatric ABI Expansion

• FDA IDE trials at multiple US centres for paediatric nontumor ABI (since 2013)
• European centres reporting positive language development outcomes
• Combining with cochlear implant if any residual cochlear nerve present

12E. Imaging-Guided Placement

• Intraoperative CT and neuronavigation for electrode positioning
• Intraoperative MRI in specialized centres

12F. Newer Electrode Designs

• Flexible polyimide substrate electrodes (better conform to curved CN surface)
• Increased electrode count for improved frequency resolution

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13. FLOWCHART: PATIENT SELECTION AND MANAGEMENT

PATIENT WITH PROFOUND SENSORINEURAL HEARING LOSS
                      │
                      ▼
        Is cochlear nerve intact?
        ┌─────────────┴──────────────┐
       YES                          NO / ABSENT
        │                            │
        ▼                            ▼
  Cochlear Implant               Consider ABI
  (first choice)                     │
                                      ▼
                        Is cochlear nucleus intact?
                        ┌──────────────┴──────────┐
                       YES                        NO
                        │                          │
                        ▼                          ▼
                    ABI indicated            Auditory Midbrain
                        │                    Implant (AMI)
                        ▼                    (experimental)
              Multidisciplinary Evaluation
              MRI / CT / Audiometry / EABR
                        │
                        ▼
             Surgical Candidacy Confirmed?
             ┌──────────┴───────────┐
            YES                    NO
             │                      │
             ▼                      ▼
      ABI Surgery               Hearing Aids /
   (Translabyrinthine         Vibrotactile Aids /
    or Retrosigmoid)           BAHAs / Counselling
             │
             ▼
   Intraoperative EABR
   Confirms Placement
             │
             ▼
   4–6 Weeks: Device Activation
             │
             ▼
   Audiological Programming (Mapping)
   Deactivate nonauditory electrodes
             │
             ▼
   Intensive Aural Rehabilitation
             │
             ▼
   Regular Follow-up & Reprogramming

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14. SUMMARY TABLE

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REFERENCES FROM TEXTBOOKS

1. Cummings Otolaryngology Head and Neck Surgery (6th/7th ed.) — Chapter 163: Auditory Brainstem Implantation. Choudhury B, Deep NL, Wilkinson EP, Roland JT Jr. pp. 3119–3130
2. Scott-Brown's Otorhinolaryngology, Head and Neck Surgery — Chapter on cochlear and brainstem implants (auditory rehabilitation)
3. Stell and Maran's Head and Neck Surgery — Neurotological prosthetic devices
4. Dhingra's Diseases of Ear, Nose and Throat — Chapter on hearing rehabilitation and implantable devices
5. Hazarika's Textbook of Ear, Nose and Throat — Cochlear and auditory brainstem implants
6. Zakir Hussain — ENT principles applied to implantable hearing devices

Key Literature Referenced in Cummings:

• Lenarz M et al.: ABI Part II — subjective assessment (Oticon Medical, 2013)
• Otto SR et al.: Multichannel ABI — update in 61 patients (J Neurosurg, 2002)
• Colletti V et al.: ABI in nontumor patients (OHNS, 2005)
• Behr R et al.: Multi-centre study of optimal ABI performers (2014)
• McSorley A et al.: Subjective benefits of ABI — questionnaire study (2015)

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Examiner's Note (RGUHS): For a 50-mark answer, candidates are expected to cover: historical background (5 marks), anatomy (5 marks), indications (10 marks), technique in detail with diagrams (15 marks), complications (10 marks), outcomes and recent advances (5 marks). The above answer covers all these domains with reference to standard ENT textbooks used in the RGUHS curriculum.