Auditory Brainstem Response (ABR) Diagnostic Protocols & Wave Analysis

Auditory Brainstem Response Diagnostic Protocols & Wave Analysis

The Auditory Brainstem Response (ABR), first systematically characterized by Don Jewett and Donald Williston in 1971, is an auditory evoked potential that tracks the synchronous electrophysiological activity of Cranial Nerve VIII and the ascending brainstem auditory pathways. Occurring within the initial $10-12\,\text{ms}$ following acoustic stimulation, the ABR is the preeminent electrophysiological tool for objective threshold estimation in uncooperative or pediatric populations and for neurodiagnostic assessment of retrocochlear lesions, including vestibular schwannomas and auditory neuropathy.

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Far-Field Electrophysiology & Differential Montages

The electrical potentials recorded during an ABR are far-field potentials, meaning the recording electrodes are located at a considerable distance from the anatomical generators (on the scalp rather than directly on the brainstem or nerve). As action potentials travel along fiber tracts and synaptic junctions, they establish time-varying electrical dipoles. These currents propagate through conductive intracranial fluids, brain tissue, skull bone, and pericranial musculature to reach the skin surface.

Signal-to-Noise Ratio & Differential Amplification

Because of tissue resistance and geometric spreading, far-field ABR potentials measured at the scalp are extraordinarily small, typically ranging from $0.1\,\mu\text{V}$ to $0.5\,\mu\text{V}$. In contrast, ongoing physiological noise—such as myogenic potentials from scalp and neck muscles (electromyography, EMG, spanning $10-100\,\mu\text{V}$), background cortical encephalographic activity (EEG, $10-50\,\mu\text{V}$), and ambient $60\,\text{Hz}$ power-line electromagnetic fields—is hundreds of times larger. To extract the microvolt ABR signal, three signal-processing principles are required:

1. Differential Amplification: The instrument measures the voltage difference between two active recording sites: the non-inverting electrode ($V_+$) and the inverting electrode ($V_-$): $V_{\text{out}} = A(V_+ - V_-)$ Signals that arrive at both electrodes with identical amplitude and phase—such as distant electrical power-line hum or generalized muscular tension—are treated as common-mode noise and canceled out. The effectiveness of this cancellation is measured by the Common-Mode Rejection Ratio (CMRR), which must exceed $100-110\,\text{dB}$ in clinical evoked potential systems.
2. Electrode Impedance Matching: To preserve CMRR, skin-electrode contact impedance must be strictly controlled. All individual electrode impedances should be $<5\,\text{k}\Omega$, and the inter-electrode impedance difference between channels must be $\le 1-2\,\text{k}\Omega$. High or unbalanced impedance severely degrades common-mode rejection, contaminating the trace with $60\,\text{Hz}$ artifact.
3. Signal Averaging (Sweeps): The stimulus is presented repeatedly (typically $1000$ to $4000$ times per run). Because the biological ABR is strictly phase-locked (time-locked) to the stimulus onset, it sums constructively with each sweep. In contrast, random background noise has arbitrary phase and sums toward zero. Signal averaging improves the signal-to-noise ratio in proportion to the square root of the number of sweeps ($N$): $\text{SNR Improvement} \propto \sqrt{N}$ Averaging $2000$ sweeps improves SNR by a factor of $\sqrt{2000} \approx 44.7$, converting an unreadable trace into distinct, reproducible waveform peaks.

Clinical Electrode Montages (International 10-20 Standard)

Standard clinical recordings utilize gold-cup or disposable silver/silver-chloride ($Ag/AgCl$) electrodes placed according to the International 10-20 system:

Two-Channel Clinical Utility: In single-channel recordings, Waves IV and V frequently fuse into a single broad complex. Recording simultaneously on a second channel using the contralateral ear electrode ($Cz - M_2$) causes Wave I to vanish and causes Wave IV and Wave V to split into separate, distinct peaks, resolving ambiguities in Wave V identification.

Filter Settings & Bandpass Dynamics

• Neurodiagnostic Adult ABR: High-pass filter at $100\,\text{Hz}$, low-pass filter at $3000\,\text{Hz}$ (with a $12\,\text{dB/octave}$ roll-off slope). The $100\,\text{Hz}$ high-pass cutoff effectively eliminates low-frequency myogenic drift and EEG rhythms without attenuating sharp neural peaks.
• Pediatric & Threshold ABR: High-pass filter widened down to $30\,\text{Hz}$ (or $30-1500\,\text{Hz}$). Infants exhibit broader, slower Wave V components and a prominent slow negative baseline deflection (the SN10 potential). A $100\,\text{Hz}$ filter excessively filters out this slow negative energy, artificially reducing Wave V amplitude near threshold.
• Notch Filter Warning: The $60\,\text{Hz}$ (or $50\,\text{Hz}$) analog notch filter should generally be deactivated during diagnostic recordings because it introduces phase distortion and ringing that can mimic physiological peaks or distort Wave I-V latencies.

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Anatomical Generators of Waves I through V

The ABR waveform consists of five to seven vertex-positive peaks occurring within $1.5$ to $6.0\,\text{ms}$ following high-intensity click stimulation in normal-hearing adults. Intracranial recordings, lesion studies, and magnetoencephalography have delineated the primary neuroanatomical generators:

Normative Interpeak Intervals (IPIs)

Because absolute wave latencies vary with peripheral conductive status, body temperature, and infant age, interpeak intervals (IPIs)—representing central transmission times (central conduction time) between brainstem landmarks—are the gold standard for neurodiagnostic evaluation:

• Interval I–III: $\approx 2.1\,\text{ms}$ (upper limit of normal: $\le 2.3 - 2.4\,\text{ms}$). Measures peripheral-to-lower pontine conduction time.
• Interval III–V: $\approx 1.9\,\text{ms}$ (upper limit of normal: $\le 2.1 - 2.2\,\text{ms}$). Measures lower pontine-to-midbrain conduction time.
• Interval I–V: $\approx 4.0\,\text{ms}$ (upper limit of normal: $\le 4.4\,\text{ms}$). Measures total brainstem auditory transmission time.
• Interaural Wave V Latency Difference ($IT_5$): Comparing Wave V latency between ears at identical intensity ($80\,\text{dB nHL}$), the interaural difference must not exceed $0.3 - 0.4\,\text{ms}$ in normal systems.

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Stimulus Parameters: Clicks, Tone Bursts & Chirps

The acoustic stimulus profoundly shapes the morphology, synchrony, and frequency specificity of the evoked response.

1. Clicks

• Physical Properties: A brief rectangular electrical pulse (typically $100\,\mu\text{s}$ duration) delivered to an acoustic transducer. In the ear canal, this produces an acoustic impulse with a broadband spectrum spanning $100-10,000\,\text{Hz}$, peaking prominently between $2000\,\text{Hz}$ and $4000\,\text{Hz}$.
• Physiological Action: Clicks evoke exquisite, instantaneous neural synchrony across thousands of auditory nerve fibers, yielding sharp, high-amplitude, readily identifiable wave peaks. Clicks are the gold standard for neurodiagnostic evaluations.
• Limitation: Clicks lack frequency specificity below $1500\,\text{Hz}$. A patient with profound low-frequency hearing loss and normal high-frequency hearing will display a completely normal click ABR.

2. Stimulus Polarity

• Condensation: Initial diaphragm displacement moves toward the tympanic membrane, generating a positive pressure wave. Pushes the tympanic membrane and stapes footplate inward. Produces an initial displacement of the basilar membrane toward the scala tympani, slightly delaying hair cell depolarization (Wave I latency is typically $\approx 0.1-0.2\,\text{ms}$ longer than with rarefaction).
• Rarefaction: Initial diaphragm displacement moves away from the tympanic membrane, generating a negative pressure wave. Pulls the stapes outward, driving the basilar membrane upward toward the scala vestibuli. This immediately opens stereocilia tip-link ion channels, eliciting faster depolarization and slightly shorter Wave I latencies. Rarefaction frequently provides superior Wave I-to-V amplitude morphology.
• Alternating Polarity: Alternates between condensation and rarefaction clicks on successive sweeps. When averaged, stimulus artifact and pre-neural receptor potentials (Cochlear Microphonic) invert and cancel out completely ($[+X] + [-X] = 0$). Alternating polarity is essential during threshold estimation to prevent stimulus artifact from obscuring low-amplitude Wave V peaks near threshold.

3. Tone Bursts

• Frequency-Specific Threshold Testing: Brief pure-tone bursts presented at $500, 1000, 2000,$ and $4000\,\text{Hz}$ provide the electrophysiological data necessary to reconstruct a behavioral audiogram in infants.
• Gating Windows & Spectral Splatter: Turning a pure tone on and off instantaneously generates abrupt acoustic transients that splatter energy across the entire frequency spectrum. To ensure frequency specificity, tone bursts must be mathematically windowed:
  • Linear Gating (2-1-2 Cycle Rule): 2 cycles rise time, 1 cycle plateau, 2 cycles fall time. For example, at $1000\,\text{Hz}$ ($1\,\text{cycle} = 1.0\,\text{ms}$), the tone burst has a $2\,\text{ms}$ rise, $1\,\text{ms}$ plateau, and $2\,\text{ms}$ fall ($5\,\text{ms}$ total duration).
  • Blackman Windowing: A specialized bell-shaped cosine envelope that rolls off side lobes by $>50\,\text{dB}$, concentrating $>99\%$ of acoustic energy within the target frequency octave.
• Masking: High-pass or notched noise masking is occasionally mixed with tone bursts to silence adjacent cochlear regions and prevent off-frequency basal listening.

4. Chirp Stimuli (Level-Specific and Octave-Band Chirps)

In the human cochlea, traveling waves propagate along a tonotopic gradient: high frequencies are transduced at the stiff basal turn within microseconds, whereas low frequencies require up to $4-5\,\text{ms}$ to travel to the compliant apical turn. Standard clicks stimulate the base long before the apex, causing phase cancellation between different cochlear regions.

• The Chirp Mechanism: A chirp is an engineered acoustic stimulus that sweeps from low to high frequencies, presenting low frequencies milliseconds before high frequencies. By delaying the high-frequency components, acoustic energy arrives at different cochlear regions simultaneously, synchronizing hair cell discharge across the entire basilar membrane.
• Clinical Benefit: Produces exceptional spatial-temporal neural synchrony, generating Wave V amplitudes that are 1.5 to 2 times larger than click-evoked responses at near-threshold levels. Chirps drastically reduce recording time during universal newborn hearing screening and infant threshold searches.

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Neurodiagnostic Analysis: Rate Series, Interpeak Intervals & Latency Shifts

When evaluating patients for retrocochlear pathology (e.g., vestibular schwannoma, acoustic neuroma, cerebellopontine angle meningioma, or multiple sclerosis), the audiologist applies specific neurodiagnostic stress protocols:

Classical Retrocochlear ABR Indicators

1. Prolonged Absolute Wave V Latency: Exceeding normative limits for age and gender ($>5.8 - 6.0\,\text{ms}$ at $80\,\text{dB nHL}$).
2. Prolonged Interpeak Intervals: Pathological widening of Interval I–V ($>4.4\,\text{ms}$) or Interval I–III ($>2.3\,\text{ms}$), indicating slowed conduction along Cranial Nerve VIII.
3. Interaural Latency Difference ($IT_5$): An interaural difference in Wave V latency between ears exceeding $0.3 - 0.4\,\text{ms}$ (after adjusting for any high-frequency cochlear threshold asymmetry).
4. Wave V/I Amplitude Ratio: In normal subjects, Wave V is larger than Wave I (ratio $>1.0$). A ratio $<0.50$ is considered suspicious for retrocochlear compromise.
5. Absent Waves III and V in the Presence of Wave I: Highly indicative of retrocochlear block at the root entry zone or brainstem.

Repetition Rate Series (Neurological Stress Test)

• Protocol: A click ABR is recorded at a slow, baseline repetition rate (e.g., $11.1$ or $21.1\,\text{clicks/second}$) and compared against a high, stressing repetition rate (e.g., $77.7$ or $89.1\,\text{clicks/second}$).
• Normal Physiological Response: Rapid stimulation stresses neural synapses, depleting synaptic vesicles and prolonging refractory recovery. In normal subjects, increasing the rate from $11.1$ to $77.7\,\text{clicks/s}$ causes a modest, predictable shift: absolute Wave V latency prolongs by $\le 0.6 - 0.8\,\text{ms}$, with Wave V remaining crisp and well-defined.
• Pathological Response: In retrocochlear lesions (demylination from multiple sclerosis or mechanical compression by tumors), neural fibers cannot sustain rapid transmission. The high-rate run produces an excessive Wave V latency shift ($>0.8 - 1.0\,\text{ms}$), severe interpeak interval prolongation, or total morphological breakdown/obliteration of Wave V.

The Stacked ABR Protocol

Standard click ABR has high sensitivity ($>95\%$) for large vestibular schwannomas ($>1.5\,\text{cm}$), but its sensitivity drops precipitously to $<60-70\%$ for small, intracanalicular tumors ($<1.0\,\text{cm}$) because small tumors compress only a fraction of the eighth nerve, leaving high-frequency fibers synchronized enough to generate a normal Wave V.

• Protocol: Don et al. developed the Stacked ABR, which presents clicks alongside high-pass pink masking noise at multiple cutoff frequencies ($8000, 4000, 2000, 1000,$ and $500\,\text{Hz}$). By subtracting sequential runs, derived narrow-band ABR waveforms representing individual octave bands are isolated.
• The isolated Wave V peaks from all five octave bands are then time-aligned (stacked) and summed together. In normal ears, this produces a massive, stacked Wave V amplitude. In ears with small intracanalicular tumors, desynchronization in even a single frequency band significantly reduces the stacked amplitude, raising detection sensitivity for tumors $<1\,\text{cm}$ to $>95\%$.

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Threshold Estimation, Latency-Intensity Functions & ANSD Protocols

The Latency-Intensity Function (LIF)

Tracking absolute Wave V latency as stimulus intensity decreases from suprathreshold levels ($80\,\text{dB nHL}$) down to threshold ($15-20\,\text{dB nHL}$) generates the Latency-Intensity Function (LIF). In normal ears, decreasing intensity causes a progressive, systematic increase in Wave V latency (shifting from $\approx 5.6\,\text{ms}$ at $80\,\text{dB}$ to $\approx 8.5-9.0\,\text{ms}$ near $20\,\text{dB}$), accompanied by a steady decrease in wave amplitude.

Calibration & Correction Factors: dB nHL to eHL

• dB nHL (Normalized Hearing Level): The decibel level referenced to the average behavioral threshold of a group of otologically normal young adults for that specific transient stimulus and transducer. Because brief clicks and tone bursts have much shorter durations than standard pure tones, human temporal integration requires more physical energy to reach threshold; thus, $0\,\text{dB nHL}$ represents higher physical SPL than $0\,\text{dB HL}$.
• Estimated Hearing Level (eHL): To convert electrophysiological thresholds (dB nHL) into audiometric pure-tone equivalents (eHL) for hearing aid fitting, frequency-specific correction offsets must be subtracted: $\text{eHL} = \text{Threshold (dB nHL)} - \text{Correction Factor}$

Clinical Application: If an infant demonstrates a repeatable Wave V threshold at $45\,\text{dB nHL}$ for a $500\,\text{Hz}$ tone burst, subtracting the $15\,\text{dB}$ correction factor establishes an estimated hearing level of $30\,\text{dB eHL}$ for hearing aid programming.

Auditory Neuropathy Spectrum Disorder (ANSD) Protocol

ANSD is characterized by disordered transmission along Cranial Nerve VIII with normal outer hair cell function. The definitive diagnosis requires a rigorous ABR protocol:

1. High-Intensity Stimulus: Present clicks at $80-90\,\text{dB nHL}$.
2. Separate Condensation and Rarefaction Runs: Record at least one full average using condensation clicks and one full average using rarefaction clicks on separate channels.
3. Identify the Cochlear Microphonic (CM): If a response appears in the first $1-3\,\text{ms}$, inspect its phase. A biological Cochlear Microphonic will invert its phase by $180^\circ$ when switching from condensation to rarefaction. In contrast, neural ABR waves maintain their positive polarity regardless of stimulus polarity.
4. Tube Clamping (Control Maneuver): To rule out electrical stimulus artifact (which also inverts with polarity), pinch or clamp the silicone sound delivery tube of the insert earphone while running the test. Clamping blocks acoustic sound delivery to the tympanic membrane while leaving electrical fields identical. If the microphonic vanishes during tube clamping, it is confirmed as a true biological Cochlear Microphonic. If it persists, it is an electrical artifact.
5. Diagnostic Conclusion: The presence of an inverting biological Cochlear Microphonic (and/or intact OAEs) alongside an absent or severely disordered ABR Wave V confirms ANSD.