Implantable Hearing Devices

Overview & Classification - Hearing Helpers Within

• Surgically implanted electronic devices; manage hearing loss when conventional hearing aids are insufficient or contraindicated.
• Major Categories:
  • Bone Conduction Devices (BCDs/BAHAs): Transmit sound via bone vibration. For conductive/mixed hearing loss (HL), single-sided deafness (SSD).
  • Middle Ear Implants (MEIs): Directly drive ossicles or cochlear windows. For sensorineural/mixed HL.
  • Cochlear Implants (CIs): Electrically stimulate auditory nerve. For severe-profound SNHL.
  • Auditory Brainstem Implants (ABIs): Stimulate cochlear nucleus. For cochlear nerve aplasia/Neurofibromatosis type 2 (NF2).

⭐ Cochlear Implants bypass damaged hair cells to directly stimulate spiral ganglion neurons, offering hearing to those with severe-to-profound sensorineural hearing loss (SNHL).

Cochlear Implants (CI) - Sounding Out Success

• Principle: Bypasses damaged/absent cochlear hair cells; provides direct electrical stimulation to auditory nerve fibers.
• Components:
  • External: Microphone, speech processor, transmitting coil.
  • Internal: Receiver-stimulator package, electrode array (inserted into scala tympani of cochlea).
• Key Candidacy Criteria:
  • Bilateral severe-to-profound SNHL.
  • Limited benefit from appropriately fitted hearing aids (e.g., sentence recognition < 50% in ear to be implanted, < 60% bilaterally).
  • No medical contraindications; realistic expectations.
• Sound Pathway:

⭐ The electrode array of a CI is typically inserted into the scala tympani of the cochlea to stimulate different frequency regions along the basilar membrane.

Bone Conduction Devices - Bone-Deep Beats

• Bypass outer/middle ear, directly stimulate cochlea via bone vibration.
• Indications:
  • Conductive Hearing Loss (CHL): Chronic otitis media, atresia, ossicular chain disruption.
  • Mixed Hearing Loss (MHL).
  • Single-Sided Deafness (SSD): Transcranial routing of signal.
• Types:
  • Percutaneous (e.g., BAHA Connect): Abutment through skin.
  • Transcutaneous (e.g., BAHA Attract, Sophono, Bonebridge): Magnet or passive implant under intact skin.
• Advantages: Good for draining ears, avoids occlusion effect.

⭐ BAHA is FDA-approved for children aged 5 years and older for CHL/MHL, and for SSD.

MEI & ABI - Beyond the Cochlea

• Middle Ear Implants (MEI):

  • Directly stimulate middle ear structures (ossicles) or round window.
  • For moderate-severe sensorineural hearing loss (SNHL) or conductive/mixed hearing loss.
  • Types: Piezoelectric, electromagnetic.
  • Advantages: No occlusion effect, better sound quality for some.
  • E.g., Vibrant Soundbridge, Esteem.

• Auditory Brainstem Implants (ABI):

  • Stimulate cochlear nucleus in brainstem.
  • For patients with non-functional cochlear nerves (e.g., bilateral acoustic neuromas like in NF2, cochlear aplasia/hypoplasia, cochlear nerve aplasia, ossified cochleas not suitable for CI).
  • Provides sound awareness, aids lip-reading.

⭐ ABI is the treatment of choice for hearing rehabilitation in Neurofibromatosis type 2 (NF2) patients post bilateral vestibular schwannoma surgery where cochlear nerves are not preserved.

Complications & Comparison - Weighing the Wires

• General Complications:
  • Infection (wound, device), device extrusion/failure.
  • Nerve injury (e.g., facial nerve, chorda tympani).
  • Pain, vertigo, tinnitus, skin flap necrosis.
• Device Comparison Highlights:
  • CI: For severe-profound SNHL; direct cochlear nerve stimulation.
  • AMEI: SNHL/mixed/conductive; direct ossicular/RW drive.
  • BCD (e.g., BAHA): Conductive/mixed loss, SSD; skull vibration.

⭐ MRI safety is paramount: field strength limits (e.g., 1.5T, 3T), magnet removal/splinting often required. Device failure is a long-term risk.

High‑Yield Points - ⚡ Biggest Takeaways

• Cochlear Implants (CI): For severe-profound SNHL; directly stimulate auditory nerve.
• BAHA/BCHA: For CHL, MHL, SSD; utilize bone conduction.
• Middle Ear Implants (MEI): For moderate-severe SNHL/mixed/conductive loss; vibrate ossicles.
• Auditory Brainstem Implants (ABI): For absent cochlear nerves (e.g., NF2), cochlear ossification; stimulates cochlear nucleus.
• EAS (Electro-Acoustic Stimulation): Combines CI (high-freq) & acoustic amplification (low-freq) for partial deafness.
• CI candidacy: Bilateral profound SNHL, <50% sentence recognition (aided).
• Risks: Infection, device failure, facial nerve stimulation, CSF leak.