BERA WAVES
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BERA waves generators origin I II III IV V VI VII brainstem auditory evoked response
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BERA Waves - Brainstem Auditory Evoked Response
What is BERA?
BERA (also called ABR - Auditory Brainstem Response, or BAEP - Brainstem Auditory Evoked Potential) is an objective, non-invasive electrophysiological test that records electrical activity along the auditory pathway from the cochlea to the upper brainstem in response to auditory click stimuli. Electrodes are placed on the scalp (vertex positive convention). The response occurs within the first 10-12 milliseconds after the stimulus.
Key feature: BERA is not affected by sleep, sedation, or anesthesia, making it especially useful in infants and uncooperative patients.
The Seven BERA Waves
BERA produces a series of 7 positive vertex waves labeled with Roman numerals I through VII (Jewett-Williston convention). Waves I-V are the most clinically relevant; waves VI and VII have limited clinical utility.
| Wave | Latency (approx.) | Generator / Anatomical Origin |
|---|---|---|
| I | ~1.5 ms | Distal cochlear nerve (CN VIII) - adjacent to cochlea |
| II | ~2.5 ms | Proximal cochlear nerve (CN VIII) / cochlear nucleus |
| III | ~3.5 ms | Cochlear nucleus (caudal pons) |
| IV | ~5.0 ms | Superior olivary complex (pontine 3rd-order neurons); also contributions from cochlear nucleus and nucleus of lateral lemniscus |
| V | ~5.5 ms | Lateral lemniscus / contralateral inferior colliculus |
| VI | ~7 ms | Inferior colliculus |
| VII | ~9 ms | Medial geniculate nucleus (thalamus) |
Note: Waves IV and V often share a peak (written as IV-V complex) and have complex, multi-generator origins. Waves VI and VII are generated by the inferior colliculus and medial geniculate body, respectively, but their precise sites are uncertain.
Classification by Level
- Waves I-III: Generated by CN VIII and lower brainstem
- Waves IV-V: Generated by the upper brainstem
- Waves VI-VII: Thalamic (medial geniculate body) origin
Key Clinical Parameters
1. Absolute Latencies (Normal Adults)
- Wave I: ~1.5 ms
- Wave III: ~3.5 ms
- Wave V: ~5.5 ms
2. Interpeak Latencies (IPL) - Most Diagnostically Important
| IPL | Reflects | Normal Value |
|---|---|---|
| I-III | Conduction through lower brainstem (CN VIII to cochlear nucleus) | ~2.0 ms |
| III-V | Conduction through upper brainstem (cochlear nucleus to inferior colliculus) | ~2.0 ms |
| I-V | Total brainstem conduction time | ~4.0 ms |
3. Wave V Amplitude
- Wave V is the most robust and largest wave; used for threshold estimation
- V/I amplitude ratio should be >0.5 (reduced ratio suggests retrocochlear pathology)
4. Interaural Latency Difference (ILD)
- Difference in Wave V latency between both ears should be <0.3-0.4 ms
Stimulus Types
- Click stimulus - most commonly used; stimulates the basal cochlea (high frequency region 2-4 kHz)
- Tone burst / tone pip - frequency-specific testing
- Presented at 70-90 dB nHL for diagnostic testing; threshold sweeps done at lower intensities for audiometric estimation
Response Types (by Latency)
| Type | Latency | Source |
|---|---|---|
| Early response (BERA) | 0-10 ms | Brainstem |
| Middle latency response (MLR) | 10-80 ms | Thalamus/primary auditory cortex |
| Late cortical response (CERA) | >80 ms | Auditory cortex |
Clinical Applications
- Hearing screening in neonates and infants (most common use; part of Universal Newborn Hearing Screening)
- Estimation of hearing threshold in uncooperative patients, malingerers, medico-legal cases
- Differentiation of cochlear vs. retrocochlear lesions (e.g., acoustic neuroma/vestibular schwannoma)
- Diagnosis and localization of brainstem lesions (MS, posterior fossa tumors, infarcts)
- Intraoperative monitoring during posterior fossa and cerebellopontine angle surgeries
- Brain death confirmation
Interpretation Summary
| Finding | Interpretation |
|---|---|
| Prolonged absolute latency of all waves | Conductive hearing loss (all shifted equally) |
| Normal Wave I, prolonged I-III IPL | Lower brainstem / CN VIII lesion |
| Normal Wave I-III, prolonged III-V IPL | Upper brainstem lesion |
| Prolonged I-V IPL | Retrocochlear / brainstem pathology |
| Absent Wave I, present Wave V | Cochlear nerve involvement near cochlea |
| All waves absent | Severe sensorineural hearing loss or profound deafness |
| Prolonged I-V with asymmetric ILD >0.4 ms | Acoustic neuroma |
Clinical Pearl - Wave V
Wave V is the most consistent and reliable wave for clinical interpretation. It is:
- The largest amplitude wave
- Last to disappear as stimulus intensity decreases (used for threshold estimation)
- The wave used for neonatal screening pass/fail criteria
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