What causes abnormal ABR?

An abnormal Auditory Brainstem Response (ABR) typically indicates issues along the auditory pathway, ranging from the inner ear to the brainstem, or can be influenced by various patient and testing factors.

The Auditory Brainstem Response (ABR) is an electrophysiological test that measures the electrical activity in the auditory nerve and brainstem in response to sound. It provides valuable insights into the integrity of the auditory pathway from the inner ear up to the brainstem, helping to identify neurological conditions or estimate hearing thresholds. An abnormal ABR result signifies a deviation from expected wave patterns, latencies, or amplitudes, prompting further investigation.

Several key factors can lead to an abnormal ABR:

Retrocochlear Pathologies

These are conditions affecting the auditory nerve (beyond the cochlea) and the brainstem itself. They are often the most concerning causes of ABR abnormalities as they can indicate neurological issues.

• Acoustic Neuroma (Vestibular Schwannoma): This is a benign, slow-growing tumor on the vestibular nerve, which is closely associated with the auditory nerve. Even small tumors can compress the auditory nerve, disrupting signal transmission and leading to delayed or absent ABR waves.
• Multiple Sclerosis (MS): A chronic disease affecting the central nervous system, MS causes demyelination (damage to the protective sheath of nerve fibers). When demyelination occurs in the brainstem, it can impair the speed and integrity of neural impulses, resulting in prolonged ABR latencies and poorly formed waves.
• Brainstem Lesions/Stroke: Any damage to the brainstem due to stroke, hemorrhage, trauma, or other lesions can directly disrupt the auditory pathways, leading to severe ABR abnormalities or absence of responses.
• Other Central Nervous System Disorders: Conditions like hydrocephalus, certain neurodegenerative diseases, or other tumors affecting the auditory brainstem pathways can also manifest as abnormal ABRs.

Hearing Loss Characteristics

The nature and degree of a person's hearing loss significantly influence ABR results.

• Sensorineural Hearing Loss (SNHL): A high degree of sensorineural hearing loss, particularly severe to profound loss originating in the cochlea, can affect the generation of ABR waves, leading to elevated thresholds, reduced wave amplitudes, or absent responses. The ABR reflects the activity of the auditory nerve fibers, and if these fibers are not adequately stimulated due to significant cochlear damage, the response will be affected.
• Asymmetry of Hearing Loss: A significant difference in hearing sensitivity between the two ears can raise suspicion for retrocochlear issues and must be carefully considered during ABR interpretation. The ABR can help differentiate between cochlear and retrocochlear causes of asymmetrical hearing loss.
• Conductive Hearing Loss: While not directly affecting the neural structures, conductive hearing loss (e.g., fluid in the middle ear, eardrum perforation, otosclerosis) reduces the intensity of sound reaching the inner ear. This can cause a uniform delay in all ABR waves due to reduced sound transmission, making the ABR appear abnormal in terms of latency, even if the brainstem function is normal.

Patient-Related Factors

Various individual patient characteristics and conditions can influence ABR outcomes, and these must be taken into account during testing and analysis.

• Age: In infants, particularly premature babies, the auditory system and brainstem are still maturing, which can result in prolonged latencies and less robust ABR waveforms compared to adults.
• Neurological Conditions: General neurological impairment or severe systemic illnesses can affect the overall health and functioning of the central nervous system, potentially impacting ABR results.
• Body Temperature: Hypothermia (low body temperature) can slow nerve conduction velocity, leading to prolonged ABR latencies.
• Medications and Sedation: Certain medications, especially those affecting the central nervous system, and general anesthesia or heavy sedation, can influence ABR waveform morphology and latencies.
• State of Consciousness: While the ABR is relatively robust to changes in consciousness (e.g., sleep), extreme states due to severe illness or coma can sometimes affect responses indirectly.

Test Parameters

The way an ABR test is conducted can also lead to seemingly abnormal results if not properly controlled. These are technical influences that must be factored in.

• Stimulus Type and Rate: The type of sound stimulus (e.g., clicks, tone bursts) and the rate at which they are presented can affect ABR waveform characteristics. An overly fast stimulation rate can reduce wave amplitudes or prolong latencies in some individuals.
• Intensity Level: The loudness of the sound presented directly affects the amplitude and latency of ABR waves. If the intensity is too low for the patient's hearing threshold, responses may appear absent or abnormal.
• Filter Settings: The electrical filtering used during recording can inadvertently filter out important components of the ABR waveform if not set appropriately.
• Electrode Placement: Incorrect placement of electrodes on the scalp can lead to poor signal acquisition and a distorted or absent ABR response.
• Noise Levels: Excessive electrical noise from equipment or physiological noise (e.g., muscle artifact) from the patient can obscure the subtle ABR response, making it appear absent or abnormal.

Summary of Influencing Factors

To provide a clearer overview, here's a summary of the factors influencing ABR results:

It is crucial that all these influences are thoroughly factored in when performing and analyzing an ABR result. An abnormal ABR is not a diagnosis in itself but rather a significant finding that requires comprehensive clinical correlation with other audiological and medical information to determine the underlying cause and guide appropriate management.