Physiology of the ABR

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Physiology of the ABR

• Auditory Brainstem Response
• Megan D. Ford, M.A., CCC-A

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Sullivan, 2000
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Auditory Brainstem Response(ABR)

• Waveforms of ABR first labeled with Roman
  numerals by Jewett and Williston (1971).
• The summed responses of the synchronous firing of
  large numbers of VIIIth nerve and auditory
  brainstem neurons.

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Auditory Brainstem Response(ABR)

• By measuring the amplitude and latencies of each
  of the five major waves of the ABR, one can
  objectively estimate hearing levels, screen for
  retrocochlear pathology and monitor the VIIIth
  nerve intraoperatively.

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Auditory Brainstem Response

• Click or toneburst stimuli generate a response
  that travels the auditory pathway
• The response is measured using surface electrodes
  and plotted as five peaks and troughs
• The positive peaks are generated from multiple
  axonal pathways in the auditory brainstem

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Components of the ABRWaves I and II

• True action potentials
• Wave I (AKA AP action potential of the EcochG)
  generated from distal portion of the VIIIth nerve
• Specifically, VIIIth nerve firing due to activity
  in the basal portion of the cochlea
• Wave II generated from the proximal VIIIth nerve,
  as it enters the brainstem

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Components of the ABR Wave III

• Second-order neuron activity
• Peak comes from in or near the cochlear nucleus
• Trough arises from the trapezoid body
• Generated in the caudal portion of the auditory
  pons

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Components of the ABR Wave IV

• Often seen tagging along on Wave V
• Arises from pontine third-order neurons, mostly
  located in the superior olivary complex
• Contributions from cochlear nucleus and nucleus
  of the lateral lemniscus

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Components of the ABR Wave V

• Positive peak related to the termination of the
  lateral lemniscus as they enter inferior
  colliculus
• Negative trough attributed to dendritic
  potentials within the inferior colliculus

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Lets not forget the Cochlear Microphonic

• An alternating current potential that mocks the
  stimulus
• Not to be confused with stimulus artifact
• Arises from the outer hair cells

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Cochlear Microphonic
Can be seen by alternating the polarity of the
stimulus (Rarefaction versus Condensation)
Berlin, et al, 2002
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Interpreting the Results
Yoshinaga, et al, 2003
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Evaluating the ABR response

• Analyze the absolute latency of wave V and
  compare to
• Normative data
• The other ear (Interaural difference)
• Results that fall out of the normal range suggest
  retrocochlear pathology

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Evaluating the ABR response

• Analyze interpeak intervals (I-III, III-V, I-V)
  and compare to
• Normative data
• The other ears interpeak intervals
• Results that fall out of the normal range suggest
  retrocochlear pathology

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Evaluating the ABR response

• Absent response in the involved ear
• Absent wave I usually correlates with cochlear
  pathology
• Absent wave III-V or absent ABR is suggestive of
  retrocochlear pathology

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Evaluating the CM response

• When CM is present in the absence of ABR
  waveforms, may indicate auditory neuropathy
• Often, a present CM correlates with robust
  oto-acoustic emissions

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Considerations in Interpretation

• Many factors affect the ABR response
• Degree and type of hearing loss
• Intensity of signal
• Rate of the signal
• Type of signal (click versus toneburst)
• Recording montage
• Pathology

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Bibliography

• Hall JW III Handbook of Auditory Evoked
  Responses. Needham Heights, MA Allyn Bacon
  1992
• Jacobson J Principles Applications in Auditory
  Evoked Potentials. Needham Heights, MA Allyn
  Bacon 1994
• Scott M Auditory Brainstem Response Audiometry.
  eMedicine. http//www.emedicine.com/ent/topic473.h
  tm 2005
• Webster D Neuroscience of Communication. San
  Diego, CA Singular Publishing 1995

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fordm_at_ummhc.org

• Questions?