The Auditory Brainstem Response (ABR) test is an objective, non-invasive method used to assess the function of the hearing pathway, from the inner ear to the brainstem. It estimates hearing sensitivity and checks the neurological integrity of the auditory nerve and brainstem structures. A key advantage of the ABR test is that it does not require active patient participation, making it an excellent tool for testing infants, young children, or individuals who cannot reliably respond to conventional hearing evaluations.
The Physiological Basis of ABR
The ABR test works by tracking the sound signal’s transformation into an electrical impulse as it moves through the auditory pathway toward the brain. Sound waves enter the ear and cause vibrations in the cochlea. These mechanical vibrations are then converted into electrical signals by specialized hair cells within the cochlea.
The electrical signals travel along the eighth cranial nerve (the auditory nerve), the main conduit to the brainstem. As the signal ascends, it passes through several relay stations in the brainstem, including the cochlear nucleus, the superior olivary complex, and the lateral lemniscus. Each of these distinct neural centers generates a measurable burst of electrical activity.
Electrodes placed on the patient’s head capture these synchronized bursts of neural activity, which are very small (typically 0.1 to 1 microvolt). The ABR recording system averages the responses to thousands of repeated clicks or tones to filter out background electrical noise from muscle movement or the brain’s general activity. This averaging process yields the characteristic waveform that represents the auditory signal’s journey from the ear to the brainstem, occurring within the first 10 milliseconds after the sound stimulus.
How the Auditory Brainstem Response Test is Performed
The ABR procedure is conducted in a quiet room or a sound-attenuating booth and is painless. The audiologist prepares the patient by placing small, adhesive electrodes on the scalp, usually on the forehead and the mastoid bone or earlobes behind the ears. These electrodes are wired to a specialized computer that records the electrical responses.
Sound stimuli, which are rapid clicking sounds or frequency-specific tone bursts, are delivered to the ear through insert earphones or headphones. The computer repeatedly presents these sounds and captures the resulting electrical activity picked up by the electrodes. The use of tone bursts allows for the estimation of hearing sensitivity across specific pitches:
- 500 Hz
- 1000 Hz
- 2000 Hz
- 4000 Hz
For the test to produce accurate and reliable data, the patient must remain completely still and relaxed throughout the approximately one to two-hour duration. Any muscle activity, movement, or crying can introduce electrical interference (artifacts), contaminating the small neural signals being measured. Because of this requirement, infants and very young children are often tested while naturally sleeping, or they may require light sedation or anesthesia to ensure stillness.
Primary Clinical Uses of ABR Testing
The ABR test serves two primary clinical functions: estimating hearing sensitivity and assessing the integrity of the auditory neural pathway. It is considered the standard for diagnosing hearing loss in populations that cannot participate in behavioral hearing tests, such as newborn infants. Universal newborn hearing screening programs frequently use a form of ABR to identify potential hearing issues shortly after birth.
For infants who fail the initial screening, a more detailed diagnostic ABR is performed to determine the type and degree of hearing loss in each ear. Early and accurate diagnosis allows for prompt intervention, such as hearing aids or cochlear implants, maximizing language and speech development. The test is also performed on older children or adults who are difficult to test due to developmental delays, cognitive impairment, or other neurological conditions.
Beyond estimating hearing loss, the ABR is used to identify retrocochlear pathologies, which are problems located along the auditory nerve or within the brainstem. These conditions include acoustic neuromas (benign tumors on the auditory nerve) or lesions affecting the brainstem. An ABR can also help diagnose Auditory Neuropathy Spectrum Disorder, a condition where the inner ear is functioning normally but the transmission of sound signals to the brain is disorganized or impaired.
Understanding the ABR Waveforms
The output of the ABR test is a graph displaying up to seven positive peaks, or waves, labeled I through VII, occurring within a 10-millisecond window. The first five waves represent electrical activity from different regions of the auditory pathway. Waves I, III, and V are the most consistently identified and clinically analyzed.
Wave I is generated by the auditory nerve as it leaves the cochlea. Wave III originates from the cochlear nucleus and superior olivary complex. Wave V arises from the lateral lemniscus near the midbrain. The time it takes for a wave to appear after the sound stimulus is presented is called its absolute latency. Changes in latency or the time interval between waves can indicate different types of hearing or neurological issues.
The ABR threshold, the lowest intensity level at which a clear and repeatable Wave V can be detected, provides the estimation of a person’s hearing sensitivity. A prolonged Wave I-V interpeak interval (the time between the first and fifth peaks) suggests a problem with signal transmission through the brainstem, potentially indicating a tumor or demyelinating disease. Analyzing the absolute latency, the interval between waves, and the amplitude allows clinicians to pinpoint where the auditory signal is being delayed or disrupted.