Essentials of ABR and OAE testing

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Essentials of ABR and OAE testing

• Vivien Thorpe
• Clinical Scientist

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Introduction

• Auditory Brainstem Response
• What is it?
• Stimuli
• Recording
• Uses
• Otoacoustic Emissions
• What are they?
• Stimuli
• Recording
• Uses

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ABR What is it?

• Auditory Brainstem Response
• Neural signal generated in the VIIIth cranial
  nerve and brainstem in response to an auditory
  stimulus
• Consists of a series of peaks and troughs
  occurring in the 10ms after stimulus presentation
• Recorded using surface electrodes on the scalp

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ABR What is it?
70dBnHL
0.16µV/div, 1.5ms/div
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ABR What is it?

• Wave I is the action potential in the distal
  portion of the VIIIth nerve
• Generators of other waves are less clear and
  possibly a combination of different nuclei in the
  brainstem
• Relies on the integrity of the auditory system up
  to the level of the brainstem
• Not a test of hearing but has good correlation
  with behavioural thresholds

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ABR - Stimuli

• Most common stimulus is the broadband click
• Contains frequencies from 1 8kHz
• Can produce clear responses with large waveforms
• Lack of frequency specificity
• Better frequency specificity obtained with
  tone-pip stimuli

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ABR - Recording

• ABR recorded using surface electrodes
• Active High forehead or vertex
• Reference Ipsilateral mastoid
• Guard temple
• Stimulus delivered using headphones or insert
  earphones for air conduction testing
• Bone vibrator also used for bone conduction
  testing

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ABR - Recording

• ABR has a very small amplitude compared to
  background EEG
• Signal averaging used
• Response occurs at the same time after the
  stimulus, noise is random
• Add together lots of responses (approx. 2000)
• Noise cancels out but signal doesnt
• Good recording conditions required
• Still relaxed subject/sleeping baby
• Low electrical noise

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ABR Uses

• Threshold estimation
• Accurate to within 15dB of behavioural
  thresholds
• For babies and other difficult-to-test subjects
• Suspected non-organic loss

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ABR Threshold estimation
Subject 1
Normal thresholds
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 2
50dBnHL
30dBnHL
40dBnHL
Raised AC thresholds
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 2
38dBnHL
28dBnHL
Normal BC thresholds conductive loss
18dBnHL
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 3
60dBnHL
40dBnHL
50dBnHL
Raised AC thresholds
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 3
60dBnHL
50dBnHL
Raised AC thresholds
40dBnHL
30dBnHL
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 3
60dBnHL
50dBnHL
Raised AC thresholds
40dBnHL
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 3
57dBnHL
47dBnHL
Raised BC thresholds sensorineural loss
37dBnHL
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 4
85dBnHL
70dBnHL
Raised AC thresholds
50dBnHL
0.12µV/div, 2.0ms/div
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ABR Threshold estimation
Subject 4
Raised BC thresholds sensorineural loss
67dBnHL
0.12µV/div, 2.0ms/div
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ABR - Uses

• Otoneurological investigation
• To investigate retro-cochlear pathology e.g.
  acoustic neuroma
• Latency of waves compared to normative data
• Inter-peak intervals also assessed

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ABR - Uses
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Otoacoustic emissions
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OAEs What are they?

• Normal cochlear function
• Outer hair cells change their length in response
  to sound stimuli
• Purpose is to increase cochlear frequency
  discrimination
• Causes vibrations in the middle ear and TM
• These can be recorded as OAEs in the ear canal
• Generated by OHCs

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OAEs - Stimuli

• Approximately 60 of people generate OAEs
  spontaneously but of limited clinical value
• Two main types of evoked OAEs
• Transient evoked by a click stimulus
• Distortion product non-linear response to two
  stimulus tones

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OAEs - Stimuli
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OAEs - Recording

• Recorded by placing a probe in the ear canal
• Provides stimulus (80dB)
• Records OAE response
• Recording requires
• Good probe fit
• Quiet environment
• Response only recordable with normal ear canal
  and middle ear function

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OAEs - Recording
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OAEs - Recording

• Response usually displayed in frequency bands
• Signal strength compared to noise level

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OAEs - Uses

• OAEs not a hearing test
• High correlation between sensory hearing loss and
  OAE absence demonstrates that most sensory losses
  involve OHC dysfunction
• OHCs are vulnerable to noise, oxygen deprivation
  and ototoxic agents
• Robust OAEs normally present only if hearing
  threshold 20dBHL (transient stimuli)

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OAEs - Uses

• Universal Newborn Hearing Screening
• OAEs widely used in screening and many published
  reports
• In low risk new-born babies most hearing loss is
  sensory (involves loss of OHCs)
• Will be picked up by OAEs
• For babies at risk of IHC/neural dysfunction ABR
  is required as well

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OAEs - Uses

• General audiology
• Objective test
• Selectively tests OHC function
• Monitoring for ototoxicity
• Can monitor OHC function for effects of
  ototoxicity
• Common ototoxic agents with OHC effects include
  Gentamycin and Cisplatin

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OAEs - Uses
Subject 1
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OAEs - Uses
Subject 1

• Possible non-organic loss

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OAEs - Uses

• Subject 2
• CF patient treated with Tobramycin

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OAEs - Uses
Subject 2
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Questions?