Brainstem Auditory Evoked Response Test

Issues of Concern

Anatomy and Physiology

The brainstem anatomy is particularly important to understand as the BAERs test primarily assesses the integrity of the pathway from the cochlea to the brainstem nuclei.

The auditory pathway begins with the cochlea within the inner ear. The cochlear hair cells convert mechanical sound vibrations into electrical action potentials. These action potentials are transmitted through the auditory nerve, one of the two branches of the vestibulocochlear nerve (cranial nerve VIII).[6]

Cranial nerve VIII then carries these electrical signals to the cochlear nucleus within the brainstem. Each cranial nerve VIII (left and right) sends projections to the ispilateral cochlear nuclei (left and right). At the cochlear nucleus, which is the first major processing center in the brainstem, the auditory information from each ears is integrated and further processed.[7]

From the cochlear nucleus, most axons cross the midline and travel to the contralateral superior olivary nucleus (SON) while others continue onto the ipsilateral SON. The SON is crucial in sound localization and intensity processing by comparing inputs from both ears.[8][9]

Auditory information is then transmitted and refined at the lateral lemniscus (LL). The LL carries signals to the inferior colliculus (IC) within the midbrain. The IC is responsible for integrating and coordinating auditory information from both ears in addition to pitch discrimination and frequency recognition.[10]

The last stage for audiologic processing before reaching high-order processing centers is the medial geniculate nucleus (MGN) of the thalamus. The MGN acts as a relay station, sending processed auditory signals from the IC to the auditory cortex within the temporal lobe.[11]

Basic Concepts

BAERs measure the electrical activity generated in the auditory system in response to auditory stimuli. The stimuli consist of clicks or tone bursts presented to the individual's ears and the neural responses are recorded using electrodes placed on the scalp. These electrical responses are amplified, filtered, and averaged over thousands of trials to produce the BAER waveform. To properly interpret the BAER waveform, it is crucial to understand the basic terminology and concepts behind the BAER test. 

Morphology: The shape of the waveform. This can have significant variation from subject to subject and differences in the same subject due to differences in stimulus type, frequency, and intensity.

Amplitude: The strength or magnitude of the waveform. It reflects the number of neurons firing synchronously in response to the stimulus. Larger amplitudes generally indicate a healthier and more robust auditory system. The amplitude of BAERs is measured from the peak of wave V to the trough that immediately follows and is present at a latency of about 10 ms. The amplitude generally ranges between 0.1 microvolts (µV) and 1 µV. A threshold of 0.04 mV is commonly used as the minimal cutoff to determine the presence or absence of waveform V. The amplitude should correlate directly with stimulus intensity (ie, louder stimulus intensity leading to larger wave amplitudes and vice versa).[12]

Latency: The time it takes for each wave to occur after the presentation of the auditory stimulus. Latency values provide insight into the conduction speed along the auditory pathway. For example, longer latencies may indicate abnormalities in neural conduction or delays in transmitting auditory information, such as in conductive hearing loss. The latency of wave V is generally used as a marker for the robustness of the auditory response and should be approximately 7 milliseconds (ms) to 9 ms. Shifts in latency and amplitude are utilized to determine if a legitimate response is present. A louder intensity stimulus should correlate with a larger amplitude and shorter latency, while a quieter intensity stimulus should correspond to a smaller amplitude and longer latency.[13]

Interpeak Intervals (IPIs): The time period between successive waves. They represent the time taken for neural signals to travel between specific auditory structures. They are calculated by subtracting the latency of one peak from the latency of the preceding peak. These IPIs are crucial for understanding the timing and coordination of neural activity along the auditory pathway. The approximate IPI for waves I-II are 1.5 ms to 2.5 ms, waves III-V are 1.5 ms to 2.5 ms, and waves I-V are 3.0 ms to 4.0 ms. It should be noted that IPIs can vary depending on stimuli and subject-specific factors.[13]

Waveforms

The BAER waveform consists of several distinct components, typically labeled as waves I, II, III, IV, and V. Waves VI and VII have little clinical utility, and therefore, the primary focus is on waves I through V. Each wave represents the synchronized firing of specific groups of neurons along the auditory pathway, from the cochlea to the auditory cortex. These waves provide valuable information about the integrity and functioning of the different structures involved in hearing.

Wave I: Wave I is the first positive deflection observed in the BAER waveform. It represents the initial activation of the auditory nerve fibers in the cochlea. Wave I is primarily generated by the synchronous firing of the distal portion of the auditory nerve fibers and is typically observed within 1 to 2 milliseconds after the auditory stimulus. However, it is important to note that the precise origin of Wave I is still debated among researchers. Factors such as stimulus intensity, electrode placement, and individual variations can influence the amplitude and latency of Wave I. Due to its variability, Wave I may be less reliable for clinical interpretation compared to other components.[14]

Wave II: Following Wave I, Wave II is the first negative deflection observed in the BAER waveform. It reflects the activation of the cochlear nucleus, which is the first site of synaptic processing in the auditory pathway. Wave II is generated by the excitatory postsynaptic potentials of the cochlear nucleus neurons in response to the auditory stimulus. It typically appears around 2 to 4 milliseconds after the stimulus onset. The amplitude of Wave II is relatively stable and less variable compared to Wave I, making it a reliable component for waveform analysis. However, it is essential to consider that abnormalities in the cochlear nucleus or the ascending auditory pathway can affect the characteristics of Wave II.[15]

Wave III: Wave III is the subsequent positive deflection in the BAER waveform. It corresponds to the activation of the SON, a brainstem structure involved in the binaural processing of sound. Wave III is generated by the summation of excitatory and inhibitory postsynaptic potentials of the neurons within the SON. It typically appears around 3 to 5 milliseconds after the stimulus onset. Wave III exhibits a relatively consistent morphology and amplitude across recordings, making it a reliable marker in BAER interpretation. However, it is influenced by factors such as the intensity and frequency of the stimulus, as well as the individual's attention and state of arousal.[8]

Wave IV: Following Wave III, Wave IV is a negative deflection representing the activation of the LL, a pathway involved in relaying auditory information from the brainstem to higher auditory centers. The summation of postsynaptic potentials in the LL neurons generates wave IV. It typically appears around 4 to 6 milliseconds after the stimulus onset. Wave IV is known for its robust and reliable waveform, showing consistent morphology and amplitude across recordings. However, it can be affected by factors such as the stimulus parameters, electrode placement, and the individual's state of consciousness.[16]

Wave V: Wave V is the final positive deflection in the BAER waveform. It corresponds to the activation of the IC, a key structure involved in processing sound localization and integration. Wave V is generated by the summation of postsynaptic potentials in the IC neurons. It is observed approximately 5 to 7 milliseconds after the stimulus onset. Wave V is the most prominent and consistent component of the BAER waveform, exhibiting a distinct morphology and amplitude. It is relatively less influenced by individual variations and factors such as stimulus intensity, making it a reliable marker for clinical interpretation. As wave V is the easiest to identify within the BAERs waveform, it is routinely used as a marker to test a subject's hearing sensitivity threshold.[16]

These waveforms represent the sequential activation of different anatomical structures along the auditory pathway, providing valuable insights into the conduction and processing of auditory information. Analyzing the latencies and amplitudes of these waveforms allows for the assessment of the integrity and function of the auditory system and aids in the diagnosis and management of various auditory disorders. However, it is essential to consider that waveform interpretation should be performed by skilled professionals who consider the variability and limitations associated with BAER testing.

Interpretation

The morphology of BAER waveform can vary and is affected by several factors, such as the individual's hearing status, age, and the presence of background noise or artifacts. Additionally, certain medical conditions or medications can influence BAER recordings. False-positive or false-negative results can occur, emphasizing the need for skilled interpretation and consideration of the individual's clinical history.

Stimulus Types: BAERs can be elicited using various types of auditory stimuli, such as clicks, tone bursts, or speech stimuli. Understanding the characteristics of the stimulus used in the test is crucial because it can affect the waveform morphology and latency values. The choice of stimulus should be appropriate for the specific clinical or research objective.[17]

1. Click Stimuli: Broadband sounds consisting of a wide range of frequencies. They are commonly used as the standard stimulus in BAER testing. Click stimuli tend to produce complex waveforms with multiple peaks, and typically result in shorter latencies compared to other stimulus types due to their broadband nature.
2. Tone bursts: Narrowband stimuli with a specific frequency and duration. They are often used in BAERs testing to assess frequency-specific information and to evaluate specific components of the auditory pathway. Tone burst stimuli can produce waveforms with distinct peaks similar to click stimuli. However, the morphology of the waveform may vary depending on the specific frequency and intensity of the tone burst used. The latency values of tone burst-evoked waves may differ from those evoked by clicks due to the specific frequency of the tone burst.
3. Speech stimuli: In some cases, speech stimuli may be used in BAER testing, particularly when evaluating the higher-level processing of the auditory system, such as cortical responses. Speech stimuli evoke more complex and varied waveforms compared to simple click or tone burst stimuli. The waveforms can exhibit additional components reflecting higher-level neural processing related to speech perception. Speech stimuli involve more elaborate neural processing, resulting in longer latencies compared to simple stimuli.

Electrode Placement: Electrodes are placed on the scalp to record the electrical responses generated by the auditory pathway. The placement of electrodes follows the international 10-20 system, which ensures consistency across different recordings. The specific electrode configuration used for BAERs typically includes an active electrode over the vertex (Cz) and reference and ground electrodes at specific positions. Proper electrode placement is vital for obtaining accurate and reliable results.[18]

Recording Parameters: Several recording parameters need to be considered when conducting a BAERs exam. These include the amplification, filter settings, and sampling rate. The amplification determines the sensitivity of the recording, while the filter settings help remove unwanted electrical noise. The sampling rate determines the temporal resolution of the recording. Setting appropriate recording parameters is crucial to capture the desired frequency range and minimize artifacts.

Waveform Analysis: The analysis of BAER waveforms involves assessing various components, such as Wave I, Wave II, Wave III, Wave IV, and Wave V. Key aspects to consider during waveform analysis include the latency (the time taken for each waveform to appear) and the amplitude (the magnitude of each waveform). Comparison of these values to normative data or baseline measurements is important to identify any deviations from the expected values.

Normative Data: Normative data provides reference values for BAER waveform characteristics in the general population. It serves as a basis for comparison when interpreting individual test results. Normative data can vary depending on factors such as age, gender, and stimulation parameters. It is crucial to consider the appropriate normative data for the specific population being assessed.

Clinical Correlation: BAERs should not be interpreted in isolation but rather in the context of the patient's clinical history, symptoms, and other test results. Correlating the BAER findings with the patient's overall presentation helps to formulate a comprehensive understanding of the auditory system's functioning and potential underlying pathologies.

Residual Noise: Refers to the unwanted electrical activity that can be present in the recorded signal during a BAER test. This noise can originate from various sources and can affect the clarity and quality of the recorded waveforms. This provides an indication about the quality of the waveform, the lower the residual noise, the higher the quality of the waveform. This is especially useful to consider in cases where no waveform is detected and it is unclear if there is damage to the auditory pathway or poor testing conditions are to blame. 

Waveform ratios: Involves calculating the ratio of the amplitude of a specific wave on one side to the amplitude of the same wave on the other side. By comparing these ratios, it is possible to assess the symmetry of the auditory pathway and identify any interaural differences in the wave amplitudes. Significant differences in waveform ratios between the two sides can indicate abnormalities or imbalances in the auditory system, helping to localize potential pathology or dysfunction. Comparing same-wave ratios across each side provides valuable information for diagnostic evaluation and treatment planning in BAERs testing.[19]

Frequency Following Response Magnitude Pattern (FMP): A statistical calculation that looks at the likelihood of a BAER waveform being present, for newborn babies a minimal value of 7 is recommended, and a minimum of 800 "sweeps," or individual presentations of an auditory stimulus, must be collected before an FMP can be read.[20]

Optimal testing parameters: The amplitude range of the BAERs test can vary from 0.1 µV in wave I to 3.0 µV or higher in wave V but is still considerably smaller than the amplitude of brainwaves generated by the cortical regions of the brain (generally about 10-50 µV). To optimize the signal-to-noise ratio of the BAERs test, it is recommended to perform testing when the patient is asleep or sedated. This lowers the residual noise and allows for quality recordings to be obtained.

By considering these basic concepts when analyzing a BAER exam, clinicians and researchers can ensure a more accurate and meaningful interpretation of the results. This holistic approach enhances the diagnostic value of BAERs and facilitates appropriate clinical decision-making.

Indications and Limitations

The BAERs exam is routinely used to test the auditory capabilities of newborns and those unable to participate in traditional hearing tests (e.g., under anesthesia during surgery). Its uses in the clinical setting have grown in recent years as well. Patients with sudden unexpected findings on routine hearing tests or cranial nerve examinations may be referred for a BAER exam to diagnose and localize lesions more accurately.[21] Additionally, BAER testing is now commonly employed in neurosurgical procedures such as resection of vestibular schwannomas, medial acoustic tumors, and other lesions near the cerebellopontine angle.

While the BAER exam is a valuable diagnostic tool, it is important to be aware of its drawbacks and limitations. These factors can affect the interpretation and reliability of the test results, and understanding them is crucial for proper clinical assessment. Here are some of the key limitations of the BAERs exam:

Subject Variability: Individual factors can influence BAER recordings. Age, attention, arousal, and state of consciousness can impact the waveform characteristics. Additionally, preexisting conditions such as hearing loss, neurological disorders, or medications can alter BAER responses, making interpretation more challenging.

False-Negative Results: BAERs are most sensitive to lesions affecting the brainstem and auditory pathways. However, small or localized lesions may not produce detectable changes in the BAER waveform. Thus, a normal BAER result does not completely rule out the presence of pathology, especially in cases of focal lesions or early-stage disorders.

False-Positive Results: BAERs can sometimes yield abnormal findings without any clinical symptoms or significant pathology. This can occur due to technical factors, such as noise contamination, improper electrode placement, or movement artifacts. It highlights the importance of ensuring a controlled testing environment and skilled administration.

Limited Spatial Resolution: While BAERs provide valuable information about the integrity and function of the auditory pathways, they have limited spatial resolution. The test does not precisely localize the site of pathology along the auditory pathway or provide detailed information about the specific etiology. Additional imaging studies, clinical assessment, and further diagnostic tests may be required for accurate localization and diagnosis.

Non-specificity: BAERs are influenced by various factors, and the waveforms themselves are not specific to a particular etiology. Various conditions, including peripheral hearing loss, brainstem lesions, demyelinating diseases, or systemic disorders, can lead to similar BAER abnormalities. Therefore, BAERs serve as a screening and diagnostic tool but should be interpreted in conjunction with other clinical findings and imaging studies.[22]

Interpretation Expertise: The interpretation of BAERs requires expertise and familiarity with the test principles, waveform characteristics, and potential confounding factors. Skilled interpretation is crucial for distinguishing true abnormalities from artifacts and for identifying subtle changes in the waveform that may indicate pathology.

Clinical Significance

The clinical significance of the BAERs test lies in its ability to provide critical information about the integrity and functionality of the auditory pathways, particularly in individuals who cannot reliably communicate their auditory experiences. This non-invasive test assists in diagnosing various hearing disorders, such as sensorineural hearing loss, auditory neuropathy, and brainstem lesions, by pinpointing the location and nature of the impairment along the auditory pathway. Additionally, BAERs play a crucial role in newborn hearing screenings, enabling early detection and intervention for infants at risk of hearing deficits, thereby contributing to improved developmental outcomes. Furthermore, BAERs are invaluable in guiding surgical procedures near the auditory pathway, ensuring real-time monitoring of nerve function and minimizing potential damage.

Enhancing Healthcare Team Outcomes

The incorporation of BAERs into healthcare protocols enhances interdisciplinary outcomes by allowing for more accurate diagnoses and informed treatment decisions. By providing objective and quantifiable data on auditory pathway function, BAERs enable healthcare teams to collaborate effectively in tailoring interventions for patients with hearing disorders, ultimately optimizing patient care and management. Moreover, the early identification of hearing impairments through BAER-based newborn screenings empowers healthcare teams to initiate timely interventions, thereby preventing developmental delays and improving long-term outcomes. Additionally, during surgical procedures involving the auditory system, real-time monitoring of nerve function using BAERs ensures a safer and more precise operative environment, promoting successful surgical outcomes and minimizing postoperative complications.