The Auditory Brainstem Response (ABR) test is an objective, non-invasive method used to evaluate how the auditory system responds to sound. It measures the electrical activity generated by the ear and brain in the first milliseconds after sound presentation. The ABR specifically tracks the integrity of the hearing nerve and the lower brain centers, collectively known as the auditory pathway. By recording these minute electrical signals, the test accurately assesses hearing function without requiring a voluntary response from the patient.
Why the ABR Test is Necessary
The ABR test’s primary strength is its ability to measure hearing function objectively, distinguishing it from behavioral tests requiring active participation. This makes it the preferred diagnostic tool for infants, especially those who fail initial newborn hearing screenings. Since a baby cannot verbally respond to sounds, the ABR provides a reliable way to estimate hearing sensitivity early in life.
The test is also frequently used for older children or adults unable to cooperate during a traditional hearing evaluation due to developmental delays or cognitive impairment. Beyond estimating hearing thresholds, the ABR assesses the neurological health of the auditory pathway. An audiologist uses the results to evaluate the function of the auditory nerve and brainstem, helping differentiate between sensory hearing loss and a neural problem affecting sound transmission.
The ABR Testing Process
The ABR test begins with placing small, adhesive electrodes on the patient’s head. Typically, three to four electrodes are positioned on the scalp (such as the vertex and forehead) and on the mastoid bone or earlobe. These painless sensors detect the faint electrical signals generated by the brain in response to auditory stimulation.
Sounds are delivered through small ear inserts or headphones, presenting a series of brief, rapid acoustic stimuli, often called “clicks” or “tone bursts.” Clicks excite a broad range of frequencies, while tone bursts are used to gather frequency-specific information, typically at 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz. As sound travels from the inner ear along the auditory nerve to the brainstem, it generates a sequence of electrical signals.
The electrodes capture these minuscule voltage changes, which a computer system then amplifies and averages over several thousand repetitions. This averaging filters out random electrical noise and isolates the consistent, synchronized responses of the auditory pathway. The resulting output is a waveform tracing that visually represents the brain’s response, with the timing of the response being the primary measurement.
Ensuring Accurate Results
Obtaining a clean, reliable ABR recording depends on the patient remaining completely still throughout the testing period. Muscle movement, crying, or restlessness creates electrical interference, known as artifact, which is significantly larger than the delicate brainstem response being measured. Since the ABR signal is extremely small (often less than one microvolt), it is highly susceptible to this noise.
For infants under six months, the test is ideally performed during natural, deep sleep, as this provides the stillness necessary for effective computer averaging. Parents are often asked to keep the baby awake before the appointment and feed them just before the test to encourage sleep. For older children or patients who cannot maintain stillness, mild sedation or anesthesia may be required to ensure high-quality data for an accurate diagnosis.
Prior to electrode placement, the skin where the sensors are affixed is gently cleaned to reduce electrical resistance. This preparation ensures a strong connection for the electrodes to accurately pick up the brain’s signals. Minimizing external noise and light in the testing room also helps maintain a quiet environment conducive to reliable measurement.
Understanding the Test Results
The computer output of the ABR test is a graph displaying a series of peaks and valleys, known as waveforms, that occur within the first 10 milliseconds after the sound is presented. Audiologists focus on five main positive peaks, labeled I through V, with each wave corresponding to a different anatomical structure along the auditory pathway. For example, Wave I originates from the auditory nerve, and Wave V is generated higher up in the brainstem.
Interpretation involves analyzing two main characteristics: latency and amplitude. Latency refers to the timing, or the amount of time elapsed between the sound stimulus and the appearance of a specific wave peak, indicating the speed of neural transmission. Prolonged latency, particularly in the interwave intervals (the time between peaks I and V), can suggest a problem with neural conduction within the brainstem.
Amplitude measures the size of the wave, reflecting the number of nerve fibers firing synchronously. The audiologist systematically reduces the intensity of the sound stimulus until Wave V, the most robust wave, is no longer reliably detected. The lowest intensity level at which a clear, reproducible Wave V is present estimates the patient’s hearing threshold. This threshold information guides recommendations for follow-up care, such as fitting hearing aids or further diagnostic imaging.