Otoacoustic Emissions

OAE Basics - Cochlear Whispers

• Low-intensity sounds from cochlea; "cochlear whispers".
• Indicate healthy Outer Hair Cell (OHC) function.
• Objective, non-invasive cochlear assessment.
• Present if hearing better than 30-40 dB HL.
• Absent/reduced in sensorineural hearing loss affecting OHCs.
• Types: Spontaneous (SOAEs), Evoked (TEOAEs, DPOAEs).

⭐ Otoacoustic emissions (OAEs) are sounds generated within the normal cochlea, reflecting active biomechanical processes, primarily from Outer Hair Cells (OHCs).

OAE Types - Sound Varieties

• Spontaneous OAEs (SOAEs):
  • Sound: Narrow-band tonal emissions.
  • Stimulus: None (spontaneous).
  • Prevalence: ~50% normal hearing.
  • Use: Limited diagnostically.
• Evoked OAEs (EOAEs): Require external acoustic stimulus.
  • Transient Evoked OAEs (TEOAEs):
    • Stimulus: Brief clicks or tone-pips.
    • Sound: Broadband, wide cochlear activity.
    • Use: Newborn screening, cochlear integrity.
  • Distortion Product OAEs (DPOAEs):
    • Stimulus: Two pure tones ($f_1, f_2$; $f_2 > f_1$).
    • Sound: Intermodulation distortion; key is $2f_1-f_2$.

    ⭐ Distortion Product OAEs (DPOAEs), typically measured at the $2f_1-f_2$ frequency, allow for frequency-specific assessment of cochlear function.

  • Stimulus Frequency OAEs (SFOAEs):
    • Stimulus: Single continuous pure tone.
    • Sound: Emission at stimulus frequency.
    • Use: Rare (measurement issues).

OAE Mechanism - OHC Magic

• Outer Hair Cells (OHCs): Primary generators of OAEs.
• Electromotility: OHCs rapidly change length (contract/elongate).
  • Mediated by prestin, a motor protein in OHC lateral wall.
• Cochlear Amplifier:
  • OHC electromotility amplifies basilar membrane motion for low-intensity sounds (up to 50-60 dB).
  • Sharpens frequency tuning.
• OAE Generation: This amplified mechanical energy propagates outwards, creating sound detected as OAEs.

⭐ The electromotility of Outer Hair Cells (OHCs), driven by the motor protein prestin, is the primary mechanism behind the cochlear amplifier and OAE generation.

OAE Clinical Use - Diagnostic Clues

• OAEs Present:
  • Indicates healthy Outer Hair Cell (OHC) function.
  • Suggests patent external & middle ear (or only mild conductive loss).
  • Rules out significant cochlear hearing loss (typically >30-40 dB HL) at test frequencies.
• OAEs Absent:
  • Suggests OHC dysfunction (cochlear pathology).
  • Or, significant conductive hearing loss (e.g., middle ear effusion, air-bone gap ≥30-35 dB).
  • Requires further audiological assessment (e.g., tympanometry, ABR).
• Specific Conditions & Clues:
  • Noise-Induced Hearing Loss (NIHL) / Ototoxicity: Early OHC damage detection (OAEs ↓ or absent, often before audiogram changes).
  • Meniere's Disease: OAEs may be variable, reduced, or absent depending on stage.
  • Vestibular Schwannoma: OAEs often present if cochlea and its blood supply are spared.
  • Non-organic/Functional Hearing Loss: Present OAEs with significant reported hearing loss can be a key indicator.

⭐ In Auditory Neuropathy Spectrum Disorder (ANSD), OAEs are typically present, indicating normal OHC function, while Auditory Brainstem Responses (ABR) are absent or severely abnormal.

OAE Interpretation - Reading Signals

• Present OAEs: Normal outer hair cell (OHC) function.
• Absent OAEs: OHC dysfunction or conductive loss.
• SNR Criteria:
  • DPOAEs: ≥ 6 dB SPL.
  • TEOAEs: ≥ 3-4 dB SPL.
• Reproducibility: >70% for reliable response.
• Influencers: Cerumen, middle ear fluid, noise.

⭐ The presence of middle ear fluid (otitis media with effusion) is a common cause of absent OAEs, even with normal cochlear function, necessitating middle ear assessment.

High‑Yield Points - ⚡ Biggest Takeaways

• Otoacoustic Emissions (OAEs) are low-intensity sounds from healthy outer hair cells (OHCs) of the cochlea.
• Key types: Transient Evoked OAEs (TEOAEs) & Distortion Product OAEs (DPOAEs).
• Objective, non-invasive assessment of cochlear (OHC) integrity.
• Crucial for universal newborn hearing screening programs.
• Requires patent EAC and normal middle ear function for reliable recording.
• Absent OAEs indicate potential OHC dysfunction or significant middle ear pathology.
• Present OAEs typically suggest hearing thresholds better than 30-40 dB HL at tested frequencies_._