What is the clinical use and protocol for auditory-evoked potentials (AEPs) in assessing auditory function and neurological integrity?

Auditory-Evoked Potentials: Clinical Applications and Protocols

Auditory-evoked potentials (AEPs) are essential neurophysiological tests that measure electrical responses generated at various levels of the auditory pathway following acoustic stimulation, providing critical information about auditory function and neurological integrity for both diagnostic and monitoring purposes. These tests are particularly valuable when behavioral audiometric testing is unreliable or impossible.

Types of Auditory-Evoked Potentials

1. Early Auditory-Evoked Potentials (0-10 ms)

• Electrocochleography (ECochG)

  • Measures electrical activity from the inner ear and auditory nerve
  • Recorded using electrodes placed on the promontory, tympanic membrane, or ear canal
  • Clinical applications: Diagnosis of Ménière's disease and auditory neuropathy spectrum disorder (ANSD) 1

• Auditory Brainstem Response (ABR)

  • Records activity from the auditory nerve and brainstem pathways (waves I-VII) within 10 ms of stimulus
  • Parameters evaluated: Latencies of waves I, III, and V; interpeak latencies I-III, III-V, and I-V; amplitude ratio of wave V to wave I 2
  • Recording method: Two-channel recordings (vertex to ipsilateral ear and vertex to contralateral earlobe) 2

2. Middle Latency Responses (10-50 ms)

• Generated by thalamo-cortical pathways and primary auditory cortex

3. Late/Cortical Responses (50+ ms)

• Generated by higher auditory cortical areas
• Includes cognitive evoked potentials (P300 paradigm) 3

Clinical Applications

1. Hearing Assessment

• Neonatal and infant hearing screening

  • Automated ABR can test each ear individually at any age 3
  • Provides ear-specific results without requiring patient cooperation 3
  • Critical for early detection of hearing loss in newborns 4
  • More comprehensive than otoacoustic emissions (OAEs) for detecting ANSD 1

• Threshold estimation in difficult-to-test populations

  • Particularly valuable for patients who lack verbal communication or behavioral responses 5
  • Essential for patients with multiple disabilities 5

2. Neurological Assessment

• Brainstem function evaluation

  • Assessment of comatose patients 2
  • Detection of demyelinating disorders 2
  • Evaluation of posterior fossa tumors 2

• Hepatic Encephalopathy

  • Provides quantitative assessment of neurological function 3
  • Helps differentiate hepatic encephalopathy from other causes of altered consciousness 3

• Wilson's Disease

  • Auditory-evoked brainstem potentials help document functional impairment and improvement with treatment 3

• Multiple Sclerosis

  • Complements visual evoked potentials in detecting subclinical lesions 6

3. Intraoperative Monitoring

• Monitoring of eighth nerve and brainstem function during posterior fossa surgery 2
• Provides real-time feedback on neural integrity during procedures

Testing Protocol

Standard ABR Protocol

1. Patient preparation

   • Patient should be relaxed or sedated (especially infants) to minimize motion artifacts 3
   • Testing environment should be quiet

2. Electrode placement

   • Vertex (Cz)
   • Ipsilateral and contralateral earlobes or mastoids
   • Ground electrode on forehead

3. Stimulus parameters

   • Click stimuli (for general assessment) or tone bursts (for frequency-specific assessment)
   • Stimulus intensity: Typically starts at high intensity (70-90 dB) and decreases as needed
   • Stimulus rate: 10-30 clicks per second

4. Recording parameters

   • Analysis time: 10-15 ms post-stimulus
   • Filter settings: 100-3000 Hz
   • Minimum 1000-2000 averages per trial

Pediatric Considerations

• For infants (0-6 months)

  • ABR and OAEs are primary assessment tools 3
  • Testing during natural sleep or after feeding is optimal
  • May require sedation if motion artifacts interfere with testing 3

• For children (6-36 months)

  • Comprehensive assessment includes:
    • Child and family history
    • Parental report of auditory behaviors
    • Behavioral audiometry (visual reinforcement or conditioned-play)
    • OAE testing
    • Acoustic immittance measures
    • ABR if behavioral responses are unreliable 3

Interpretation Guidelines

Normal Findings

• Wave I: 1.5-1.9 ms (represents auditory nerve)
• Wave III: 3.5-4.0 ms (represents cochlear nucleus/superior olivary complex)
• Wave V: 5.5-6.0 ms (represents lateral lemniscus/inferior colliculus)
• Interpeak latencies:
  • I-III: 2.0-2.4 ms
  • III-V: 1.8-2.2 ms
  • I-V: 3.8-4.4 ms

Abnormal Findings

• Prolonged absolute latencies: Suggests conductive hearing loss
• Prolonged interpeak latencies: Suggests retrocochlear pathology or demyelinating disorders
• Absence of waves: May indicate significant hearing loss or neural dysfunction
• Asymmetry between ears: May indicate unilateral pathology

Limitations and Considerations

• Not a comprehensive test of hearing as it does not assess cortical processing of sound 3
• Sensitive to middle-ear effusions, cerumen, and vernix in the ear canal 3
• Requires patient to remain relatively inactive during testing 3
• May require sedation in young children 3
• Should be interpreted in conjunction with other audiological and neurological findings

Clinical Decision Making

• When to use ABR vs. OAE:

  • Use ABR when assessing neural integrity is crucial
  • Use OAE for quick cochlear function screening
  • Use both when comprehensive assessment is needed, especially in suspected ANSD

• When to use AEPs vs. behavioral testing:

  • Use AEPs when behavioral responses are unreliable (infants, cognitively impaired)
  • Use behavioral testing when possible for direct assessment of auditory perception
  • Consider both for comprehensive evaluation

By following these protocols and understanding the clinical applications, AEPs can provide valuable information about auditory function and neurological integrity, guiding diagnosis and management decisions in various clinical scenarios.