Auditory Brainstem Implant (ABI)

Definition

Historical Background

• First successful ABI was performed in 1979 at the House Ear Institute, California, in a 51-year-old woman with NF2 undergoing vestibular schwannoma removal. Initial electrode: platinum pair with 0.5 mm balls.
• The implant was redesigned in 1981 to a Dacron mesh-mounted paddle (two 1.7 x 2.0 mm platinum pads) to prevent electrode migration.
• In 2000, the FDA approved multichannel ABI for patients ≥12 years with NF2.
• In 2013, FDA approved investigational device exemptions (IDE) for use in children born without adequate cochlear nerve or cochlea, or with cochlear ossification.
• Over 1000 ABI procedures have been performed worldwide. (Cummings Otolaryngology, p. 3119)

Device Components (Hardware)

• Cochlear Corporation (USA - FDA approved): ABI541 - paddle of 21 platinum electrodes (8.5 x 3.0 mm), used with Nucleus 6 processor.
• MED-EL (Austria): 12-contact array (5.5 x 3.0 mm), OPUS 2 processor.
• Oticon Medical: Digisonic SP ABI - 15 surface electrodes, Saphyr neo-collection processor.

Anatomy - Target: Cochlear Nucleus

• The cochlear nucleus (CN) lies in the floor of the lateral recess of the fourth ventricle, accessed through the foramen of Luschka.
• Landmarks: choroid plexus emerging from the foramen; glossopharyngeal nerve (IX) guides the surgeon upward; stump of VIIIth nerve guides downward.
• The CN is identified by its white color and a thin vein running over its surface.
• The taeniae of the fourth ventricle may need to be divided; an arterial loop is often encountered and must be displaced.

Indications

Primary Indication (Classical):

• Neurofibromatosis Type 2 (NF2): Bilateral vestibular schwannomas rendering the patient deaf due to tumor involvement or surgical removal of bilateral auditory nerves. ABI can be placed at the time of first or second vestibular schwannoma resection.

Expanded / Non-Tumor Indications:

• Cochlear aplasia / severe cochlear malformation (bilateral)
• Cochlear ossification (post-meningitis, otosclerosis) - complete bilateral
• Cochlear nerve aplasia or severe hypoplasia (bilateral)
• Bilateral temporal bone fracture damaging both auditory nerves
• Cochlear implant failure - patients who fail to benefit from CI despite optimal placement

Pediatric Indications (expanding):

• Children born without adequate cochlear nerve or cochlea
• Children not benefiting from cochlear implantation
• Bilateral cochlear ossification in children

Candidacy Evaluation

1. Some patients do not receive auditory sensations at all.
2. Sound quality is not normal.
3. Most ABI patients do NOT achieve open-set speech recognition.
4. Regular follow-up is mandatory for device optimization.
5. Maximum benefit takes extended time and rehabilitation.

Surgical Approach

• ABI is placed via a translabyrinthine or retrosigmoid (suboccipital) craniotomy.
• Usually performed as a combined procedure concurrent with vestibular schwannoma resection in NF2.
• The paddle electrode is slid into the lateral recess of the fourth ventricle and positioned over the cochlear nucleus.
• Intraoperative Electrically Evoked Auditory Brainstem Responses (eABR) are monitored to confirm correct placement and auditory responses before the wound is closed.
• Facial nerve monitoring is essential throughout.

Outcomes

NF2 Patients:

• Most patients gain environmental sound awareness and enhanced lip-reading ability.
• A minority (roughly 10-20%) achieve open-set speech recognition.
• Sound awareness is present in up to 80% of recipients.
• Performance is significantly lower than cochlear implants due to absence of tonotopic coding precision at the CN level.

Non-Tumor Patients (Nontumor Adults and Children):

• Results are generally better than in NF2 patients, possibly because the brainstem is not distorted by tumor or radiation.
• Children with cochlear nerve aplasia show variable outcomes; some achieve open-set speech.

• NF2 patients fare worse than non-tumor patients.
• Absence of prior radiation improves outcomes.
• Younger age at implantation in non-NF2 patients is associated with better speech perception.

Complications

Surgery-Related (related to posterior fossa craniotomy):

• CSF leak - most common surgical complication
• Meningitis
• Cerebrovascular injury / hemorrhage
• Facial nerve injury
• Lower cranial nerve (IX, X, XI) injury - dysphagia, hoarseness
• Cerebellar injury
• Air embolism (especially in sitting position)
• Death (rare, higher in large NF2 tumors)

ABI-Specific Complications:

• Electrode migration from the lateral recess - loss of auditory perception
• Non-auditory side effects: tingling, pain, dizziness, facial twitching from stimulation of adjacent cranial nerves (most common in NF2 patients with distorted anatomy)
• No auditory sensation despite correct placement (10-30% of recipients)
• Device failure / malfunction

Comparison: ABI vs Cochlear Implant (CI)

Penetrating Electrode ABI (PABI)

• A modified design using penetrating needle microelectrodes inserted into the CN (rather than surface electrodes), developed to achieve more focused stimulation and a wider range of pitch percepts.
• Some patients perform best with a combination of surface and penetrating electrodes working synergistically.
• Limitation: a deviation of even 1 mm from the target can result in no auditory response; harder to monitor with intraoperative EABR.

Auditory Midbrain Implant (AMI) - Next Step

Summary Box for Exam

ABI = electrical stimulation of cochlear nucleus in lateral recess of 4th ventricle via foramen of Luschka. Primary indication = NF2 with bilateral acoustic neuromas. Placed via translabyrinthine/retrosigmoid approach with intraoperative eABR monitoring. Provides environmental sound awareness and lip-reading enhancement; open-set speech recognition in minority. Key complication = non-auditory stimulation side effects and electrode migration.

• Cummings Otolaryngology Head and Neck Surgery, pp. 3119-3130
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol 2, pp. 4735-4738
• Shambaugh Surgery of the Ear, pp. 717-718