Auditory Brainstem Response

What Is the Auditory Brainstem Response Test?

The Auditory Brainstem Response (ABR) test — also known as Brainstem Evoked Response Audiometry (BERA) — is a sophisticated electrophysiological hearing assessment that measures the electrical activity generated by the auditory nerve and brainstem in response to sound stimulation. Unlike a standard hearing test, which requires the patient to listen for tones and press a button, the ABR is completely objective — it records the brain`s neural response to sound directly, without any need for the patient to cooperate, respond, or even be awake. This makes it one of the most valuable tools in audiology, particularly for testing patients who cannot participate in conventional behavioural audiometry.

ABR testing is the gold standard for objectively estimating hearing thresholds in newborns, infants, and young children, and it plays a critical role in the NHS Newborn Hearing Screening Programme (NHSP). It is also used to investigate suspected retrocochlear pathology — conditions affecting the auditory nerve or brainstem, most notably acoustic neuroma (vestibular schwannoma) — and to diagnose auditory neuropathy spectrum disorder (ANSD), a condition in which the cochlea functions normally but the auditory nerve does not transmit signals reliably.

In the UK, ABR testing is available at NHS audiology departments — particularly paediatric audiology centres, neuro-otology clinics, and audiovestibular medicine departments — by referral from a GP, ENT consultant, or paediatrician. Some private audiology clinics also offer ABR testing, typically at a cost of £200 to £400. The test is painless, non-invasive, and provides clinicians with objective, quantifiable data about hearing sensitivity and neural conduction that no other test can match.

How ABR Testing Works — The Science

The ABR test works by detecting the tiny electrical potentials generated by the auditory nerve and brainstem nuclei as they process sound signals travelling from the ear to the brain. Understanding the underlying mechanism explains why the test is so clinically powerful.

Electrode placement

Small, self-adhesive sensor electrodes are placed on the patient`s skin — typically one on the forehead (or vertex of the head) and one behind each ear (on or near the mastoid bone). A ground electrode is placed on the cheek or lower forehead. The electrodes detect electrical activity from the scalp surface — in the same way that an ECG detects the heart`s electrical activity from chest electrodes. The electrode placement is quick, painless, and non-invasive — the adhesive pads simply stick to the skin after it has been gently cleaned with a mild abrasive prep gel to ensure good contact.

Sound stimulation

Sounds are delivered to the ear through insert earphones (small foam-tipped earpieces placed in the ear canal) or, for newborns, through ear couplers. The stimulus types include:

• Clicks: Very brief, broadband sounds that stimulate a wide region of the cochlea simultaneously. Click-evoked ABR is the fastest method and is used for screening (AABR) and for assessing neural conduction times
• Tone bursts (tone pips): Short bursts of sound at specific frequencies — typically 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. Tone burst ABR provides frequency-specific threshold information, enabling the audiologist to construct an estimated audiogram — essential for fitting hearing aids accurately in babies

Signal averaging

The neural response to a single click or tone burst is extraordinarily small — typically only 0.1 to 0.5 microvolts — buried within the much larger background electrical activity of the brain (the EEG). The ABR equipment overcomes this by presenting thousands of stimuli (typically 1,000 to 4,000 repetitions) and averaging the recorded responses. Because the ABR signal occurs at a fixed time after each stimulus while the background EEG is random, the averaging process progressively cancels out the noise and reveals the characteristic ABR waveform.

Understanding ABR Waveforms — Waves I to V

The ABR waveform consists of a series of five to seven peaks, labelled with Roman numerals, that occur within the first 10 milliseconds after the sound stimulus. Each peak is believed to correspond to electrical activity at a specific point along the auditory pathway:

• Wave I: Generated by the distal portion of the auditory nerve (near the cochlea). Wave I reflects the initial neural firing in response to sound and is the most peripheral component of the ABR. It is often the first wave to be affected in noise-induced hearing loss and cochlear synaptopathy
• Wave II: Generated by the proximal portion of the auditory nerve (near the brainstem). Wave II is often difficult to identify reliably and is of less clinical significance than other peaks
• Wave III: Generated by the cochlear nucleus, the first relay station in the brainstem where auditory nerve fibres synapse
• Wave IV: Generated by the superior olivary complex in the pons — an important brainstem structure involved in sound localisation and binaural processing
• Wave V: Generated by the lateral lemniscus and inferior colliculus in the midbrain. Wave V is the most robust and clinically important peak — it is the last wave to disappear as the stimulus level is reduced, and its presence or absence at different intensities is used to estimate hearing thresholds

The audiologist analyses the waveform for three key features: the presence or absence of each wave, the absolute latency (the time between the stimulus and each peak), and the inter-peak latencies (the time intervals between waves, particularly the I-III, III-V, and I-V intervals). Abnormalities in these measurements — such as prolonged inter-peak latencies or absent later waves — can indicate neural conduction problems, demyelination, or space-occupying lesions along the auditory pathway.

Clinical Applications of ABR Testing

ABR testing serves several distinct clinical purposes, each exploiting different aspects of the waveform analysis:

Threshold Estimation in Newborns and Infants

This is the most common application in UK audiology. When a baby does not pass the newborn hearing screening, diagnostic ABR testing is used to estimate hearing thresholds with frequency-specific accuracy to within 10–15 dB of behavioural thresholds. The audiologist records tone burst ABRs at multiple frequencies (typically 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz) and identifies the lowest stimulus level at which a clear Wave V can be detected at each frequency. The resulting estimated audiogram provides the information needed to fit hearing aids accurately in babies who may be only weeks old.

The importance of this cannot be overstated. Research consistently shows that children whose hearing loss is identified and fitted with amplification by six months of age achieve speech and language outcomes comparable to their hearing peers. The Joint Committee on Infant Hearing (JCIH) recommends screening by one month, diagnosis by three months, and intervention by six months — the 1-3-6 guideline. ABR testing is the diagnostic tool that makes this timeline achievable.

Acoustic Neuroma (Vestibular Schwannoma) Investigation

ABR testing has long been used to investigate suspected acoustic neuroma — a benign tumour arising from the Schwann cells of the vestibular nerve, which can compress the adjacent auditory nerve and cause unilateral hearing loss, tinnitus, and balance disturbance. In acoustic neuroma, the ABR typically shows prolonged inter-peak latencies (particularly the I-V interval) on the affected side, or absent later waves, reflecting the impaired neural conduction caused by the tumour. While MRI has largely replaced ABR as the primary diagnostic tool for acoustic neuroma, ABR remains valuable as a screening test in patients with asymmetric hearing loss, and it provides complementary information about neural function that MRI cannot.

Auditory Neuropathy Spectrum Disorder (ANSD)

ANSD is a condition in which the cochlear outer hair cells function normally (evidenced by present otoacoustic emissions) but the auditory nerve does not transmit signals reliably. The ABR in ANSD is characteristically absent or severely abnormal — often showing no reproducible waveform despite normal OAEs. This dissociation between OAEs and ABR is the diagnostic hallmark of ANSD. The condition is particularly important to identify in newborns who have spent time in neonatal intensive care, as it is more prevalent in this population and would be missed by OAE screening alone — which is why the NHSP uses AABR (not just AOAE) for all NICU babies.

ABR in the NHS Newborn Hearing Screening Programme

Within the NHS Newborn Hearing Screening Programme (NHSP), the Automated Auditory Brainstem Response (AABR) serves as the second-tier screen for babies who do not pass the initial Automated Otoacoustic Emissions (AOAE) test. The AABR is also the primary screening method for babies who have spent more than 48 hours in a neonatal intensive care unit (NICU), because these babies are at higher risk of auditory neuropathy spectrum disorder, which OAE screening alone cannot detect.

The AABR procedure is similar to diagnostic ABR but uses an automated algorithm to determine whether the response is present (pass) or absent (refer). Three small sensor pads are placed on the baby`s forehead, nape of the neck, and shoulder, and soft ear couplers deliver clicking sounds. The device analyses the responses and provides a clear pass or refer result, typically within 5 to 15 minutes per ear. The test is painless and is best performed while the baby is settled or asleep.

Babies who receive a refer result on the AABR are referred to a paediatric audiology centre for comprehensive diagnostic ABR testing, which provides frequency-specific threshold estimation and a definitive diagnosis. The NHSP pathway from screening to diagnosis is designed to be completed by three months of age, with intervention (hearing aids or other support) beginning by six months.

The ABR Test Procedure — What to Expect

If you or your child has been referred for diagnostic ABR testing, here is what the appointment involves:

Before the test

For babies and young children, the test is ideally performed during natural sleep. Parents are usually asked to keep the baby awake for a period before the appointment and to bring a feed (breast or bottle) to settle the baby to sleep just before the test begins. The audiologist will explain the procedure and attach the electrodes while the baby is being fed or settled. Some paediatric audiology centres schedule ABR appointments in the early morning or late afternoon to coincide with the baby`s natural sleep pattern.

For older children (typically 3 months to 4 years) who cannot stay still during natural sleep, sedation may be required. In the UK, this is usually melatonin (to promote sleep) or, in some cases, chloral hydrate. Sedated ABR testing is performed in a clinical setting with appropriate monitoring. Some centres now offer ABR under general anaesthetic for children who cannot be adequately tested by other means, though this is less common and reserved for complex cases.

For adults, the test is performed while the patient lies in a reclined chair or on a couch in a quiet, dimly lit room. You are asked to relax and remain as still as possible — ideally dozing — as muscle activity generates electrical interference that can contaminate the recording. Reading, watching a screen, or engaging in conversation during the test is discouraged.

During the test

The electrodes record the neural responses automatically while sounds are delivered through insert earphones. You will hear a series of rapid clicks or brief tones at varying loudness levels. The equipment collects and averages thousands of responses at each intensity level and frequency. The audiologist monitors the recording in real time, adjusting parameters as needed to obtain clear, interpretable waveforms.

Duration

A full diagnostic ABR assessment typically takes 45 to 90 minutes, depending on the complexity of the case — how many frequencies and intensity levels need to be assessed, whether both ears are tested, and how cooperative the patient is (a sleeping baby produces cleaner recordings than a restless one). The actual recording time may be shorter, but set-up, electrode placement, settling, and waveform analysis add to the overall appointment length.

ABR vs Standard Audiometry — Key Differences

Understanding the differences between ABR and conventional behavioural audiometry helps explain when each test is most appropriate:

• Patient cooperation: Standard audiometry requires the patient to listen, concentrate, and respond consistently. ABR requires only that the patient be still and relaxed — no active participation is needed
• Objectivity: Standard audiometry is a subjective test — it depends on the patient`s honesty, concentration, and ability to respond. ABR is entirely objective — the neural response either occurs or it does not
• Threshold accuracy: Standard audiometry measures behavioural thresholds directly (the quietest sound a person can hear). ABR estimates thresholds based on the lowest stimulus level that produces a detectable Wave V — typically within 10–15 dB of behavioural thresholds. For clinical purposes, this accuracy is sufficient to guide hearing aid fitting and treatment decisions
• Neural information: Standard audiometry tells you how well a person hears but provides no direct information about neural conduction. ABR provides detailed information about the integrity of the auditory nerve and brainstem pathways — making it essential for investigating retrocochlear pathology
• Frequency specificity: Both tests can provide frequency-specific information, though standard audiometry is more precise in this regard. Tone burst ABR covers the key audiometric frequencies adequately for clinical decision-making
• Time and complexity: Standard audiometry takes 15 to 30 minutes and can be performed by any trained audiologist. Diagnostic ABR takes 45 to 90 minutes and requires specialist equipment, training, and expertise in waveform interpretation

In practice, the two tests are complementary rather than competing. Standard audiometry is the first-line assessment for cooperative older children and adults. ABR is reserved for situations where behavioural testing is not possible, not reliable, or where objective neural assessment is needed.

Medico-Legal and Non-Organic Hearing Loss Applications

ABR testing is frequently requested in medico-legal contexts where objective evidence of hearing thresholds is required — for example, in personal injury claims related to noise-induced hearing loss, in industrial deafness compensation cases, and in medical negligence proceedings. Because ABR is objective, it cannot be influenced by the patient`s conscious efforts to exaggerate or minimise their hearing loss. This makes it the definitive tool for confirming or excluding non-organic hearing loss (NOHL) — also known as functional, psychogenic, or feigned hearing loss — where a patient`s behavioural audiogram does not accurately reflect their true hearing ability.

The Health and Safety Executive (HSE) and UK courts accept ABR results as reliable objective evidence of hearing thresholds. For individuals pursuing noise-induced hearing loss claims against current or former employers, ABR testing can provide the independent verification that supports or challenges the behavioural audiometric findings.

Where to Get ABR Testing in the UK

ABR testing is available through several pathways in the UK:

• NHS paediatric audiology: All regional paediatric audiology centres perform diagnostic ABR testing for babies and children referred through the NHSP or by paediatricians and ENT consultants. This is free of charge
• NHS audiovestibular medicine: Adult ABR testing for suspected acoustic neuroma, ANSD, or other retrocochlear conditions is available through NHS audiology and audiovestibular medicine departments, accessed by GP or ENT referral
• Private audiology clinics: Some private clinics offer diagnostic ABR testing, typically at a cost of £200 to £400. This option provides faster access for patients who cannot wait for NHS appointments. Providers such as Boots Hearingcare and Hidden Hearing may be able to arrange referral to specialist ABR clinics
• Medico-legal providers: Specialist medico-legal audiology practices offer ABR as part of independent hearing assessments for legal proceedings

If you have been referred for ABR testing — whether for your baby, your child, or yourself — understanding the procedure can help reduce anxiety and ensure the best possible recording conditions. The test is painless, non-invasive, and provides clinicians with objective information about hearing and neural function that no other assessment can match. For babies, it is the test that transforms an uncertain screening result into a clear diagnosis and a pathway to early intervention. For adults, it is the tool that investigates the auditory nerve when conventional tests cannot provide the full picture. Use our search tool to find audiologists and specialist hearing centres near you.