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BERA

Brainstem Auditory Evoked Response (BAER or BERA / Auditory Brainstem Response – ABR)

ABR (Auditory Brainstem Response), also widely known as BAER or BERA, is an objective, non-invasive electrophysiological test that assesses the structural integrity and functional status of the auditory pathway from the spiral ganglia (distal CN VIII) to the level of the lateral lemniscus in the midbrain. Jewett and Williston introduced this test.

Principle: A series of very small electrical potentials are generated by the activation of different parts of the auditory system in response to brief auditory stimuli (clicks or tone bursts). These potentials are recorded by surface electrodes placed on the scalp. Since these neurogenic potentials are extremely small and “buried” in the background spontaneous electrical activity of the brain (EEG waves) and muscle activity (myogenic potentials), an averaging computer is required to summate the responses to hundreds or thousands of stimuli, effectively filtering out noise and revealing the consistent AEPs.

Procedure:

  • Environment: Conduct the test in a quiet, sound-attenuating room.
  • Patient State: The patient lies in a supine position, relaxed, with eyes closed, and preferably asleep (especially for children or uncooperative adults) to reduce myogenic potentials. Accurate assessment in children often requires mild sedation.
  • Testing Protocol: Test one ear at a time.
  • Electrode Placement: Three surface electrodes are typically used:
    • Active Electrode: Placed on the vertex of the scalp (the best location) or, if not feasible, on the top of the forehead just below the hairline.
    • Reference Electrode: Placed on the earlobe or mastoid of the tested ear.
    • Ground Electrode: Placed on the earlobe or mastoid of the opposite ear.
  • Stimulus: A series of 1000-2000 clicks (broadband stimuli, effective for higher frequencies) are delivered at a rapid rate (e.g., 11.1 clicks/second) at an intensity of typically 60-80 dB nHL (normalized hearing level, which correlates to average normal hearing). Tone bursts can be used for more frequency-specific information.
  • Recording Window: The neurogenic potentials elicited are recorded for the first 10-15 milliseconds after stimulus onset. This specific time window captures the electrical responses as they travel through the brainstem alone.
  • Waveforms: In a normal-hearing person, ABR typically produces seven distinct waves (I-VII) in the first 10-15 milliseconds. Waves I, III, and V are the most stable and clinically significant for interpretation.

Characteristics of ABR Waves: The interpretation of ABR relies on analyzing three main characteristics:

  • Latency:
    • Absolute Latency: The time interval (in milliseconds) between the onset of the stimulus and the peak of a specific wave.
    • Inter-wave Latency (Interpeak Latency): The time interval between two different waves in the same ear and the same ABR tracing (e.g., I-III interval, III-V interval, I-V interval). This reflects conduction time between specific brainstem nuclei.
    • Interaural Latency Difference: The difference in the absolute latency of the same wave (most commonly Wave V) between the two ears. A significant interaural difference (e.g., > 0.2-0.3 ms for Wave V) can indicate unilateral pathology.
  • Amplitude: The peak-to-trough voltage of each wave. (Less stable and less clinically used than latency).
  • Morphology: The shape and clarity of the waves.

Upper Limits of Normal ABR Values (Approximate):

  • Latency of Wave V: ~5.5 – 5.9 ms
  • I–V Inter-wave Interval: ~4.0 – 4.4 ms
  • I–V Interaural Latency Difference: < 0.3 ms

Anatomical Generators of ABR Waves (EE COLI Mnemonic for NEET PG):

  • Wave I: Distal portion of the VIIIth Cranial Nerve (Auditory Nerve), near the cochlea.
  • Wave II: Proximal portion of the VIIIth Cranial Nerve, near the brainstem.
  • Wave III: Cochlear Nucleus (in the pons).
  • Wave IV: Superior Olivary Complex (in the pons).
  • Wave V: Lateral Lemniscus (at the level of the inferior colliculus in the midbrain). This is the most robust and clinically important wave, often used for threshold estimation.
  • Waves VI and VII: Inferior Colliculus and higher brainstem pathways.
  • Mnemonic (EE COLI): Eighth nerve (distal), Eighth nerve (proximal), Cochlear nucleus, Olivary complex, Lateral Iemniscus, Inferior colliculus. (Compare with E COLI-MA in pathways of hearing, indicating how the mnemonic maps to the pathway).
Wave I Distal part of CN VIII
Wave II Proximal part of CN VIII near the brainstem
Wave III Cochlear nucleus
Wave IV Superior olivary complex
Wave V Lateral lemniscus
Waves VI and VII Inferior colliculus

As per latest studies these are anatomic site of neural generators for various waves.

Uses of ABR: ABR is a versatile test with numerous applications:

  • Objective Hearing Assessment: It is the gold standard objective test for estimating hearing thresholds in infants, young children, non-cooperative adults, malingerers, and comatose/unconscious patients. It requires minimal patient cooperation.
  • Diagnosing Retrocochlear Pathologies: ABR is highly sensitive (around 90-95%) for detecting lesions of the auditory nerve or brainstem, particularly acoustic neuromas (vestibular schwannomas). Prolongation of absolute latencies (especially Wave V), prolongation of inter-wave latencies (e.g., I-V interval), or a significant interaural latency difference strongly suggests a retrocochlear lesion.
  • Diagnosing Brainstem Pathology: It can help identify the site of lesions within the brainstem, such as those caused by multiple sclerosis or pontine tumors, which affect neural conduction.
  • Intraoperative Monitoring: Surgeons use ABR to monitor and preserve the auditory nerve (CN VIII) during complex neurotological surgeries, such as the removal of acoustic neuromas or other cerebellopontine angle tumors. This helps prevent iatrogenic hearing loss.

Disadvantages and Limitations of ABR:

  • Not Frequency-Specific: ABR, especially when using clicks, primarily reflects high-frequency hearing (2000-4000 Hz) due to the transient nature of the click stimulus. It does not give precise frequency-specific information across the entire audiometric range, and low-frequency hearing losses are often undetectable. Therefore, additional tests like ASSR are needed for a comprehensive frequency-specific audiogram in objective assessments.
  • Sensitivity for Small Tumors: While highly sensitive for detecting large tumors, ABR may miss small acoustic neuromas (<1 cm). Therefore, MRI remains a more sensitive and specific test for definitive diagnosis of acoustic neuroma.
  • Effect of Hearing Loss: ABR waves are typically absent or severely distorted in patients with severe or profound hearing loss (>75 dB HL). Conductive hearing loss also attenuates cochlear stimulation and prolongs ABR wave latencies, necessitating careful interpretation and often masking.
  • No “Gold Standard” Standardization: While widely used, there isn’t a single universal standardization for ABR interpretation across all clinics.
  • Age and Consciousness: While valuable for non-cooperative patients, ABR testing itself requires the patient to be relatively still and quiet. Although sedation does not affect the ABR waves, it may be needed for accurate assessment in infants and young children, adding complexity.
  • Wave I Identification: Wave I is often not easily identifiable, particularly at lower stimulus intensities, which can make I-III and I-V interval calculations challenging.

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    Download full PDF Link:

    BERA Best Lecture Notes Dr Rahul Bagla ENT Textbook


    Reference Textbooks.

    • Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
    • Glasscock-Shambaugh, Textbook of Surgery of the Ear.
    • P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
    • Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
    • Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
    • Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
    • Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
    • Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
    • Rob and smith, Textbook of Operative surgery.
    • Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
    • Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.

    Author:

    Dr. Rahul Bagla ENT Textbook

    Dr. Rahul Bagla
    MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
    Fellow Rhinoplasty & Facial Plastic Surgery.
    Renowned Teaching Faculty
    Mail: msrahulbagla@gmail.com
    India

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