What does the Auditory Brainstem Response (ABR) test show in patients with suspected auditory pathway abnormalities?

What ABR Shows

ABR (Auditory Brainstem Response) measures the electrical activity of the auditory pathway from the cochlea through the auditory nerve to the brainstem in response to sound stimulation—it does NOT assess cortical processing of sound or true hearing perception. 1

Anatomical Structures Assessed

ABR evaluates the integrity of specific anatomical structures along the ascending auditory pathway:

• Wave I: Represents the summated response from the spiral ganglion and auditory nerve 2
• Waves II-V: Represent responses from progressively higher nuclei in the ascending auditory pathway through the brainstem 2, 3
• The test evaluates from cochlea to mesencephalon (midbrain level), but stops before reaching cortical auditory processing centers 3

Clinical Information Provided

Hearing Threshold Estimation

• Frequency-specific ABR using air-conducted tone bursts creates a map of hearing thresholds across multiple frequencies, similar to an audiogram 1, 4
• ABR thresholds indicate the lowest sound pressure level that generates identifiable electrical response waves 2
• In automated screening, a "fail" report implies hearing level worse than 40 dB 1

Type of Hearing Loss Differentiation

• Air-conduction ABR assesses the entire auditory pathway including outer, middle, and inner ear 1
• Bone-conduction ABR bypasses the outer and middle ear, allowing differentiation between conductive and sensorineural hearing loss 1, 5

Detection of Auditory Neuropathy/Dyssynchrony

• Click-evoked ABR using both condensation and rarefaction stimuli can detect the presence of cochlear microphonics 1, 4
• This is critical for identifying neural hearing loss (auditory neuropathy), which OAE testing would miss since OAEs only assess cochlear function 6, 4

Critical Limitations

ABR is NOT a true test of hearing because it only measures structural integrity of the auditory pathway up to the brainstem—it cannot assess cortical processing of sound, which is essential for actual hearing perception 1, 4

Other Important Limitations:

• Motion artifacts interfere with results, requiring the patient to remain quiet (sedation often needed in children) 1
• Middle ear dysfunction affects ABR accuracy because sound must transmit through the middle ear to reach the cochlea 6
• ABR provides ear-specific results when performed with insert earphones, testing one ear at a time 1

Clinical Applications by Population

NICU Infants

• Automated ABR is mandatory for all NICU infants hospitalized >5 days because they are at higher risk for auditory neuropathy that OAE screening would miss 4
• Failed NICU ABR screening requires direct referral to audiologist, bypassing rescreening 4

Well Infants (Birth to 6 Months)

• Either automated ABR or OAE can be used for initial screening 4
• Diagnostic ABR must include frequency-specific tone bursts when permanent hearing loss is suspected 1, 4

Neurological Applications

• ABR is the most sensitive test for detecting disorders affecting the brainstem auditory pathway 7
• Can detect intra- and extra-axial tumors, demyelinating lesions, and polyneuropathies affecting brainstem structures 7
• Useful for monitoring neural maturation, tumor growth, coma, and effects of neurologic therapies 7

Key Interpretation Parameters

The test analyzes waves I-V based on:

• Latency (timing) of individual wave peaks 3, 8
• Interpeak latencies (intervals between waves) 3, 8
• Amplitude of responses 3
• Wave V is most prominent for threshold determination due to its consistency and stability 8

Common Pitfalls to Avoid

• Never interpret ABR without knowing the audiogram, as peripheral hearing loss severely compromises central testing applications 7
• Do not assume ABR alone defines the precise lesion location—it indicates level and extent of involvement but cannot determine the specific pathology 7
• Remember that ABR abnormalities may reflect pressure/distortion effects on the brainstem rather than direct involvement by disease 7
• Account for variables affecting interpretation: age, gender, body temperature, drug treatments, stimulus parameters, and acquisition settings all influence results 3, 8