Auditory Brainstem Response

Auditory Brainstem Response: Basics - Sound Waves Speak

• Objective electrophysiological test: Assesses auditory pathway function from cochlea to brainstem.
• Records brainwave activity (evoked potentials) in response to sound stimuli (clicks or tone bursts).
• Non-invasive; sedation may be required for infants/young children to ensure they remain still.
• Key Uses:
  • Estimating hearing thresholds (especially in infants, uncooperative patients).
  • Newborn hearing screening programs.
  • Identifying retrocochlear pathology (e.g., vestibular schwannoma).
  • Intraoperative nerve monitoring.

⭐ ABR is crucial for detecting auditory neuropathy spectrum disorder (ANSD), where otoacoustic emissions (OAEs) may be present but ABR is absent or severely abnormal, indicating a neural dys-synchrony rather than cochlear hair cell dysfunction alone.

Auditory Brainstem Response: Waveforms - Peaks & Origins

• ABR Waves & Neural Generators (Approx. Latencies):
  • Wave I: Distal CN VIII (Auditory N.) - ~1.6 ms
  • Wave II: Proximal CN VIII / Cochlear Nuc. - ~2.6 ms
  • Wave III: Superior Olivary Complex (SOC) - ~3.7 ms
  • Wave IV: Lateral Lemniscus (LL) - ~4.8 ms
  • Wave V: Inferior Colliculus (IC) - ~5.6 ms
• 📌 Mnemonic (Generators I-V): "Eight nerve, Cochlear nucleus, Olivary complex, Lateral lemniscus, Inferior colliculus" (E COLI)
• Key Interpeak Latencies (IPLs):
  • I-III: ~2.0 ms
  • III-V: ~2.0 ms
  • I-V: ~4.0 ms (Central Conduction Time)

⭐ Wave V is the most robust ABR component, persisting near auditory threshold; crucial for objective hearing assessment.

Auditory Brainstem Response: Test Setup - Eliciting Echoes

• Stimulus Parameters:
  • Type: Click (broadband, for neuro ABR), Tone-bursts (frequency-specific).
  • Intensity: 70-90 dB nHL (diagnostic).
  • Rate: 11.1-21.1/sec; >50/sec for stress.
  • Polarity: Alternating (cancels artifact), Condensation, Rarefaction.
• Recording Parameters:
  • Electrodes: Active (Cz/Fz), Reference (ipsilateral mastoid M1/earlobe A1), Ground (contralateral mastoid M2/Fpz).
  • Filters: Band-pass 100-3000 Hz (adults), 30-3000 Hz (infants).
  • Averaging: 1000-2000 sweeps (improves SNR).
  • Time Window: 10-15 ms.

⭐ Click stimulus is preferred for neurological ABR as it synchronizes firing of a large number of cochlear nerve fibers.

Auditory Brainstem Response: Clinical Use - ABR in Action

ABR is a versatile objective test with diverse clinical applications:

• Newborn Hearing Screening (NHS):
  • Automated ABR (AABR) for universal screening.

  ⭐ ABR is a cornerstone for early detection of congenital hearing loss.

• Hearing Threshold Estimation:
  • For infants, children, or uncooperative adults.
  • Estimates hearing sensitivity, particularly in the 2-4 kHz range.
• Retrocochlear Pathology (RCP) Diagnosis:
  • Detects retrocochlear lesions (e.g., acoustic neuroma).
  • Key signs: ↑ I-V, III-V IPLs; Wave V ILD > 0.2 ms.
• Intraoperative Monitoring (IOM):
  • Monitors auditory nerve & brainstem integrity during neurosurgery.
• Other Uses: Brainstem lesion localization, confirmation of brain death.

Auditory Brainstem Response: Interpretation - Reading Ripples

• Waves: I (VIII N), III (SOC), V (LL/IC - most robust).
• Key Parameters:
  • Interpeak Latency (IPL): I-V ~4 ms. If >4.4 ms → retrocochlear.
  • Amplitude: V/I ratio >1.
  • Morphology: Clear, well-defined peaks.
• Pathology Clues:
  • Conductive: All latencies ↑, normal IPLs.
  • Cochlear: Wave V latency ↑, early waves may vanish.
  • Retrocochlear: IPLs ↑ (e.g., I-V).

⭐ Wave V: Most robust, used for threshold estimation; last to disappear with ↓ stimulus intensity.

High‑Yield Points - ⚡ Biggest Takeaways

• ABR is an objective test for auditory pathway function up to the brainstem.
• Records electrical potentials (waves I-V) in response to click stimuli.
• Wave V, from inferior colliculus region, is the most robust and clinically used.
• Interpeak latencies (e.g., I-V) are vital for detecting retrocochlear pathology.
• Key for newborn hearing screening and threshold estimation in difficult-to-test individuals.
• Helps identify acoustic neuromas and demyelinating diseases.