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Auditory Brainstem Implants (ABI)

Auditory Brainstem Implants: A Comprehensive Guide
 

For the vast majority of patients with severe-to-profound sensorineural hearing loss, a standard cochlear implant provides excellent sound restoration by electrically stimulating auditory nerve fibers within the cochlea. However, if the inner ear is profoundly malformed, the cochlea is completely ossified, or the auditory nerve itself is damaged or absent, stimulation at the cochlear level is ineffective.

In these challenging clinical scenarios, an Auditory Brainstem Implant (ABI) provides a direct neural interface by bypassing both the cochlea and the auditory nerve to stimulate the primary auditory centers of the central nervous system.
 

How Auditory Brainstem Implants Work

An ABI bypasses the peripheral auditory structures entirely. Instead of placing an electrode array inside the cochlea, a small, flexible paddle electrode is surgically positioned on the surface of the cochlear nucleus complex within the lateral recess of the fourth ventricle of the brainstem.
 

System Components

The system architecture of an ABI is functionally similar to a traditional cochlear implant:

  1. External Sound Processor: Worn behind or off the ear, containing microphones and digital signal processing hardware that convert environmental sounds into coded radiofrequency signals.

  2. Transmitting Coil: Worn against the scalp, held in alignment over the internal device via a magnetic coupling.

  3. Internal Receiver/Stimulator: Surgically implanted beneath the scalp skin within the temporal region.

  4. Brainstem Paddle Electrode: A flat array containing multiple active contacts (typically 12 to 21 micro-electrodes) designed to conform to the surface of the brainstem.

[Microphone & Processor] ──> [Transmitting Coil] ──(Transcutaneous RF)──> [Internal Receiver] ──> [Paddle Electrode on Brainstem] ──> [Cochlear Nuclei]
 

Complex Tonotopic Mapping

The cochlear nuclei contain complex, three-dimensional tonotopic maps. While pitch perception is encoded by stimulating different micro-electrodes across the paddle array, the spatial distribution of neurons across depths and angles in the brainstem makes frequency selectivity far more complex than the natural linear arrangement found within the cochlea.
 

Clinical Indications

ABIs are indicated for adult and pediatric candidates who cannot benefit from conventional hearing aids or cochlear implants due to structural or neural limitations:

1. Neurofibromatosis Type 2 (NF2-related Schwannomatosis)

Historically, the primary indication for ABI surgery was in patients with NF2 who developed bilateral vestibular schwannomas (acoustic neuromas). Implantation is frequently performed concurrently with tumor resection or sequentially if nerve integrity is compromised.

2. Non-NF2 / Pediatric Indications (Expanded Criteria)

In modern neurotology, candidacy has expanded significantly to non-NF2 conditions where the auditory nerve is compromised:

  • Auditory Nerve Aplasia or Hypoplasia: Children or adults born without functional auditory nerves.

  • Severe Cochlear Agenesis or Aplasia: Complete congenital absence of inner ear structures (Michel aplasia).

  • Complete Cochlear Ossification: Advanced post-meningitic labyrinthitis ossificans where complete bony obliteration of the cochlear lumen precludes electrode insertion.

  • Severe Temporal Bone Trauma: Fractures resulting in bilateral auditory nerve transection.
     

Clinical Outcomes and Expectations

It is essential to set realistic expectations when evaluating candidates for an Auditory Brainstem Implant:

  • Sound Awareness and Lip-Reading: For the majority of ABI recipients, the primary benefit is restoring environmental sound awareness, intensity discrimination, and timing cues. These acoustic cues significantly enhance speech understanding when combined with lip-reading.

  • Open-Set Speech Perception: Unlike cochlear implant users—many of whom achieve high levels of open-set sentence understanding—open-set speech discrimination without visual cues remains uncommon with ABIs, though non-NF2 pediatric patients and select adult recipients increasingly achieve improved auditory-only performance.

  • Rehabilitation: Post-operative programming ("mapping") and auditory rehabilitation require dedicated, specialized multidisciplinary follow-up to optimize electrode selection and minimize non-auditory side effects (such as localized facial nerve or cranial nerve stimulation).
     

Hardware and Technical Considerations

  • Active Channel Configurations: Leading multichannel ABI systems feature paddle electrode arrays with 12 to 21 active stimulation channels (such as systems from Cochlear™ and MED-EL™).

The Auditory Brainstem Implant (ABI) is a sophisticated medical device designed to provide a sense of sound to people who are deaf due to a non-functioning auditory nerve. Unlike a cochlear implant, which stimulates the cochlea, the ABI bypasses the inner ear and the auditory nerve to stimulate the hearing pathways in the brainstem directly. Mr. Shaida works as part of a highly specialized multidisciplinary team at the Royal National ENT Hospital where patients are assessed and if appropriate for a ABI referred to the London ABI service. 

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