BERA (Brainstem Evoked Response Audiometry) - Detailed Notes for ENT

1. Definition

• Cummings Otolaryngology, p. 2577

2. Historical Background

• First described by Jewett and Williston who identified a complex occurring under 10 ms post-onset of an auditory stimulus, comprising five distinct potentials (waves I-V).
• Scott-Brown's Otorhinolaryngology, p. 694

3. Basis / Principle

4. Wave Generators ("EECOL" Mnemonic)

• KJ Lee's Essential Otolaryngology, p. 1211
• Cummings Otolaryngology, p. 2578
• Scott-Brown's Otorhinolaryngology, p. 694

Note: Wave V is the most robust and reliable wave. Waves IV and V often fuse into a complex (IV/V complex).

5. Technique / Recording Parameters

Electrodes

• Non-inverting (active): High forehead below the hairline or vertex (Cz)
• Inverting (reference): Medial surface of the ipsilateral earlobe (Ai) - emphasises Wave I
• Contralateral inverting: Medial surface of the contralateral earlobe - emphasises separation of waves IV and V
• Ground: Centre of the forehead

Stimulus

• Click stimulus: 100 microsecond rectangular pulse - most common for retrocochlear testing
• Tone burst/pip: Frequency-specific (used for threshold estimation, e.g. 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz)
• Stimulus rate: 20-30 clicks/second (non-integer rate such as 17.7/sec to avoid periodic noise)
• Fast-rate ABR uses 77.7 clicks/second
• Polarity: Constant or alternating

Signal Averaging

• 1000-3000 sweeps are averaged to extract the evoked potential from background noise
• Each condition is replicated (acquired twice or three times) to confirm reliability

Filtering

• High-pass filter: 100 Hz
• Low-pass filter: 1000-3000 Hz
• For 500 Hz tone burst: lower high-pass to ~30 Hz
• Notch filter (60 Hz) only used sparingly when electrical artefact cannot be eliminated

Recording Window

• 10-15 ms post-stimulus onset - to capture all five waves
• Scott-Brown's Otorhinolaryngology, p. 694-695
• Cummings Otolaryngology, p. 2577-2578

6. Normal Interpeak Latency Values

• KJ Lee's Essential Otolaryngology, p. 1211
• Scott-Brown's Otorhinolaryngology, Table 52.1, p. 695

7. Clinical Indications

A. Neonatal / Paediatric Hearing Screening

• Brainstem function is identifiable on ABR at approximately 28 weeks gestational age - waves I, III and V appear.
• ABR "maturity" (adult-like latencies) is not reached until approximately 18 months after birth - neonatal latencies are prolonged compared to adults.
• BERA is the gold standard test for Universal Newborn Hearing Screening (UNHS) when OAE refer.
• It is the first-choice test for infants in the at-risk registry: birth asphyxia, ototoxic drugs, NICU, family history, congenital infections, craniofacial anomalies, hyperbilirubinaemia.
• KJ Lee's Essential Otolaryngology, p. 1210-1211

B. Threshold Estimation

• Used in children, neonates, and malingerers who cannot or will not cooperate with behavioural audiometry.
• Tone-burst BERA gives frequency-specific thresholds at 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz.
• ABR threshold correlates well with behavioural auditory threshold.

C. Retrocochlear Lesion Detection - Acoustic Neuroma (Vestibular Schwannoma)

• The primary neurodiagnostic indication.
• An acoustic neuroma delays conduction time along the cochlear nerve, prolonging absolute and interwave latencies.
• Diagnostic sensitivity of ABR for acoustic neuroma:
  • I-III interval: ~90% sensitivity, 100% specificity
  • I-V interval: ~75% sensitivity
  • III-V interval: ~45% sensitivity
  • Inter-ear latency difference (ILD): >90% sensitivity and specificity
• Scott-Brown's Otorhinolaryngology, p. 695

D. Neurological / Brainstem Disease

• Multiple sclerosis (demyelination prolongs interwave latencies)
• Brainstem tumours, haemorrhage, infarction
• Coma and brain death evaluation

E. Intraoperative Monitoring

• Used during posterior fossa surgery (e.g. acoustic neuroma resection, microvascular decompression) to monitor auditory nerve integrity in real time.
• Decline or loss of wave V amplitude or marked latency prolongation warns the surgeon of potential cochlear nerve injury.
• Cummings Otolaryngology, p. 2578-2579

8. Interpretation - Patterns of Abnormality

Retrocochlear Lesion (e.g. Acoustic Neuroma)

1. Interpeak latency I-V > 4.4 ms
2. Interaural wave V latency difference > 0.2 ms (KJ Lee) or > 0.5 ms (Scott-Brown normative table)
3. III-V interval > 2.4 ms
4. Absent wave I with present wave V (suggests peripheral cochlear nerve involvement)
5. Absent or poorly formed ABR waveform
6. Fast-rate latency shift > 1.0 ms

Cochlear (Sensory) Hearing Loss

• All absolute latencies are prolonged equally (latency-intensity shift)
• Interwave intervals remain normal
• ABR threshold elevated corresponding to degree of hearing loss

Auditory Neuropathy / Dyssynchrony (AN/AD)

• Normal OAEs (outer hair cells intact)
• Absent or severely abnormal ABR (lack of neural synchrony)
• This dissociation is the hallmark finding.

Conductive Hearing Loss

• All wave latencies shifted to the right (prolonged) by the amount of conductive loss
• Interwave intervals remain normal
• ABR improves with bone conduction stimulation

Normal ABR

• All five waves (I-V) present with normal absolute and interwave latencies
• Symmetrical bilaterally

9. BERA vs. OAE - Comparison

10. Advantages of BERA

1. Objective - does not require patient cooperation or response
2. Unaffected by sedation/anaesthesia - brainstem is impervious to CNS depressants
3. Frequency-specific thresholds possible with tone bursts
4. Neurodiagnostic - detects retrocochlear and brainstem pathology
5. Intraoperative monitoring capability
6. Useful from neonatal period onwards

11. Limitations

• Time-consuming compared to OAE
• Requires quiet, often sedated patient in infants
• A good response requires synchronous neural firing - absent in auditory neuropathy, but this is also diagnostically useful
• Does not test cortical/central auditory processing (needs AMLR or P300 for that)
• Cannot assess hearing above the brainstem level

12. Aural Atresia and BERA

• KJ Lee's Essential Otolaryngology

Summary Table

• Cummings Otolaryngology Head and Neck Surgery, pp. 2577-2579
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol 2, pp. 694-695
• KJ Lee's Essential Otolaryngology, p. 1211