BERA (Brainstem Evoked Response Audiometry) Waves

BERA is also called the Auditory Brainstem Response (ABR). It records far-field electrical potentials generated by synchronized neural activity along the auditory pathway in response to an acoustic stimulus (usually a 100 µs click delivered at ~75-85 dB nHL). Between 1000-3000 sweeps are averaged to extract the signal from background noise. The normal ABR contains 5-7 peaks occurring within 10 ms of stimulus onset.

Normal two-channel ABR (ipsilateral channel shows Wave I clearly; contralateral channel separates Waves IV and V):

Normal ABR waveform showing Waves I, III, and V labeled with ipsilateral and contralateral channels

Wave Generators - Mnemonic: "EECOL"

Wave	Generator Site	Approx. Latency
I	Distal eighth nerve (cochlear end)	~1.7 ms
II	Proximal eighth nerve (near porus acusticus)	~2.8 ms
III	Cochlear nucleus complex / trapezoid body	~3.9 ms
IV	Superior olivary complex	~5.0 ms
V	Lateral lemniscus (as it enters the inferior colliculus)	~5.7 ms
VI	Inferior colliculus	>6 ms
VII	Inferior colliculus	>7 ms

Note: The shift from Schwann cell (peripheral) to neuroglial cover (central) near the porus acusticus explains the distinct wave I vs wave II latency difference. Wave V is NOT generated by the inferior colliculus (as was previously thought) - it arises from the lateral lemniscus, per Moller and Jannetta's landmark study.

Waves IV and V often fuse into an IV-V complex in clinical recordings.

Normal Interpeak Latencies (IPL)

Interval	Normal Value	Pathway Assessed
I - III	2.3 ms	Cochlear nerve to cochlear nucleus (lower brainstem)
III - V	2.1 ms	Cochlear nucleus to lateral lemniscus (upper brainstem)
I - V	4.4 ms	Total brainstem conduction time

Recording Technique

Electrodes: Non-inverting at vertex/high forehead; inverting at ipsilateral earlobe; ground at mid-forehead
Two-channel montage: Ipsilateral channel emphasizes Wave I; contralateral channel separates Waves IV and V
Filters: High-pass 100 Hz, low-pass 1000-3000 Hz
Stimulus: Clicks (100 µs) or tone pips at constant or alternating polarity

Wave V - The Most Important Wave

Wave V is the most robust component - it persists even with significant hearing loss and is used to:

Estimate hearing thresholds (particularly in infants/neonates)
Detect retrocochlear pathology
Monitor auditory pathway integrity intraoperatively

Wave I is difficult to resolve when hearing loss exceeds 40-45 dB at high frequencies. An absent wave V in the presence of a replicated wave I or III is a definitive indicator of retrocochlear involvement.

Indicators of Retrocochlear Pathology

Interpeak latency I-V > 4.4 ms
Interaural latency difference of wave V > 0.2 ms (comparing left vs right ear)
Wave V-V latency difference > 2.1 ms
Absent or poorly formed wave V with preserved wave I/III

Key Clinical Applications

Newborn hearing screening (UNHS programs) - ABR matures with brain development; waves I, III, V appear at ~28 weeks gestation; full maturity at ~18 months postnatal
Acoustic neuroma/CPA tumor detection - delayed IPL or absent waves
Neural demyelination (e.g., multiple sclerosis) - prolonged IPLs
Intraoperative monitoring during posterior fossa or CPA surgeries
Threshold estimation in uncooperative patients (infants, malingerers)
Brain death evaluation - ABR unaffected by sedatives/anesthetics

Why ABR is Ideal for Objective Testing

The brainstem is relatively impervious to sedatives, anesthetics, and CNS depressants, making ABR an objective, non-volitional test. It does not require patient cooperation.

Sources: K.J. Lee's Essential Otolaryngology, p. 1211; Cummings Otolaryngology Head and Neck Surgery (Block 30); Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 2, p. 689

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