The Auditory Brainstem Response

 ​​THE AUDITORY BRAINSTEM RESPONSE/AUDITORY EVOKED POTENTIALS
  • electrical potentials from the auditory nervous system
  • auditory brain stem response are generated by the activity in structures of the ascending auditory pathways that occurs during the first 8-10ms after a transient sound such as a click sound has been applied to the ear
  • the electrode placement used for recording the ABR, the stimuli used to elicit the responses and how the recorded potentials are processed.
  • Earlobe, Mastoid or Concha?​
  • ABRs recorded between electrodes placed at the vertex Cz and the earlobe on the side that is stimulated; five or six vertex-positive peaks
  • numbered I, II, III, IV, V, VI there is a distinct individual variation in the wave shape of the ABR. even in individuals with normal hearing.​
  • The classic "textbook" ABR is evoked by a 100-us click of moderately high intensity level yielding several positive-to-negative waves within 10msec after stimulus onset
  • Waves are labeled using Roman Numerals I-V
  • Waves I, III and V are clinically the most useful for neurodiagnostic purposes
  • Wave V is most useful clinically when estimating behavioral thresholds
  • As a general rule, waves I, III and V have mean latencies at ~1.5, 3.5 and 5.5 msec
  • Interpeak latencies of ~2msec for I-III and III-V and 4msec for I-V
  • The amplitude can range from 0.1 to 1 uV
  • The ABR becomes adult-like at approximately 3 years of age​
  • ​The ABR seems to favor clicks in the 1kHz - 4kHz range
 
  • NEURAL GENERATORS OF THE ABR; Involve structures within the brainstem and the auditory nerve
  • Wave/Peak I distal portion of the auditory nerve
  • Wave/Peak II proximal portion of the auditory nerve
  • Wave/Peak III brainstem nuclei; cochlear nucleus (likely to have more than one anatomical contributor)
  • Wave/Peak IV ascending auditory pathway (midline brain structures; acoustic stria, trapezoid bodies and superior olivary complex; likely to have more than one anatomical contributor) 
  • Wave/Peak V the termination of the lateral lemniscus with the inferior colliculus on the contralateral side (likely to have more than one anatomical contributor)
​
  • ANESTHESIA
  • ABR - Auditory Brainstem Response - No anesthetic constraints but sensitive to ischemia
​
  • ELECTRODE PLACEMENT
  • CZ/FPZ/A1/A2​/GND (avoid mastoid placement due to potential site of surgery)
  • Be mindful of electrode noise, place ground in shoulder or skull
  • Plug electrodes into proper pod sites
  • Impedance check
    • look for <5kOhms
  • Place insert headphones
  • be sure to place flush with the external auditory canal and inserted far enough in to activate the tympanic membrane
  • otoscopy
  • history; otitis media, ear wax, occlusion, etcetera
  • Equipment set up
  • Stimulus - Click
  • Transducer - Inserts
  • Polarity
    • Condensation/Rarefaction
  • Duration - 100 usec/0.1ms
  • Intensity > or = 70 dB, could be 80dB or higher to elicit responses
  • Rate 21.1 - 39.1 (not a multiple of 60Hz)
  • Filter setting 30-1500/3000 or 100-1500/3000
    • Sweeps >1000 - enough to adequately overcome the Signal to Noise Ratio and replicate!
  • Analysis time is 15ms for peds and 11ms for adults, however, a 15 ms window with 1.5 ms/div yields 10 gratcules
  • Reverse Polarity
  • add waveforms together
  • Pick Peaks
    • Wave I Latency
    • Wave II Latency
    • Wave III Latency
    • Wave IV Latency
    • Wave V Latency
    • IWI
  • Principle ABR generators have a certain spatial orientation and direction that indicate placement of electrodes
  • Waves I & III seem to have generators in a more horizontal arrangement, whereas wave V has a more vertical arrangement
    • ​vectors
  • The most common clinical placement for a one-channel ABR setup involves a high forehead placement (Fz) for the non-inverting electrode and earlobe or mastoid of the stimulation ear for the inverting reference
  • The ground electrode placement is a matter of preference but it is usually placed on the contralateral ear or the lower forehead ​
  • ​Absolute and relative latency measures of the ABR are the most useful for neurodiagnostic applications
  • Latency prolongations that arise are due to neuronal slowing (injury, compression, obstruction) and/or poor neural synchrony, including hearing loss
  • Given that abnormal latency prolongations are what we often look for in the analysis and interpretation of ABRs, absolute latencies (time of peak appearance relative to stimulus onset) interwave (interpeak/IWI) latencies (relative differences between subsequent peaks) and wave V interaural latency differences (OLDs or IT5) are the most commonly reported information​
  • The Wave V derived using rapid stimulus repetition rate (61.1/sec) was normal for the right ear, but shifted in latency in the left ear from 5.66 ms to 6.26 ms (>.4 ms) and had poor repeatability

Key Facts

  • The ABR collectively is considered a far-field, early exogenous potential, often grouped with electrocochleography for its short latencies within 10 msec of stimulus onset
  • Jewett and colleagues correctly described the sequence of ABR waveform components as responses arising from the auditory nerve and various auditory brainstem structures
  • Clinical applications are in estimating behavioral thresholds in infants and detecting neurologic abnormalities of the eight cranial nerve and brainstem
  • The compound action potential of the ECochG is essentially one and the same with wave I of the ABR
  • The ABR is highly influenced by changes in the stimulus parameters​​
  • ​The various peaks of the ABR are likely produced by action potentials forming stationary dipoles based on their travel from the inner ear synapse to the inferior colliculus

Waveform Morphology

  • The classic "textbook" ABR is evoked by a 100-us click of moderately high intensity level yielding several positive-to-negative waves within 10msec after stimulus onset
  • Waves are labeled using Roman Numerals I-V
  • Waves I, III and V are clinically the most useful for neurodiagnostic purposes
  • Wave V is most useful clinically when estimating behavioral thresholds
  • As a general rule, waves I, III and V have mean latencies at ~1.5, 3.5 and 5.5 msec
  • Interpeak latencies of ~2msec for I-III and III-V and 4msec for I-V
  • The amplitude can range from 0.1 to 1 uV
  • The ABR becomes adult-like at approximately 3 years of age

Neural Generators

  • Involve structures within the brainstem and the auditory nerve
  • Wave I arises from the distal portion of the auditory nerve
  • Wave II is generated from the proximal portion of the auditory nerve
  • Wave III is generated from the cochlear nucleus
    • Wave III is ​likely to have more than one anatomical contributor
  • Wave IV is generated from midline brainstem structures (acoustic stria, trapezoid bodies and superior olivary complex)
    • Wave IV is ​​likely to have more than one anatomical contributor
  • Wave V is generated from the termination of the lateral lemniscus within the inferior colliculus on the contralateral side
    • Wave V is ​​likely to have more than one anatomical contributor​​

Stimulus Types

  • The ABR can be evoked by virtually any stimulus that is abrupt in nature, such as 100 usec clicks with an essentially instantaneous onset, tone bursts with rise times of a few milliseconds and even gaps in broadband noise with fall times of a few milliseconds
  • The abruptness causes a synchronous discharge of numerous auditory nerve fibers
  • Broad clicks cause a spectral splattering of frequencies
    • ​The ABR seems to favor clicks in the 1kHz - 4kHz range
  • ​Tone Bursts are short duration sinusoids at typical octave audiometric frequencies from 500-4000Hz

Polarity

  • Stimuli can be presented in one of two polarities; condensation or rarefaction
  • Waveform morphology may change in the same individual and produce different waveform morphology in different individuals
  • One way to counteract stimulus artifact is to present the stimuli using alternating polarity

Intensity

  • The ABR will express its full complement of waves at moderately high intensity levels and can show up to seven waves in some cases
  • For neurodiagnostic purposes, intensity levels from 70 - 90 dB are not uncommon
  • When decreasing the intensity level, the earlier Waves (I & III) tend to drop out first and Wave V will often remain and prolong in latency when all other waves have disappeared​
  • Decreasing the intensity level can drastically reduce the overall amplitudes of all waves and correspondingly increase their latencies
  • A 100 usec duration click stimulus with abrupt rise time will produce the largest ABR amplitudes and the shortest latencies
  • Slower rising tone burst envelopes will produce somewhat smaller ABR amplitudes and increased latencies
  • Frequency specific tone bursts will appreciably stimulate different regions of the cochlea such that the effect of the longer travel time from the base to the apex of the cochlea can be observed in the recordings as well
  • Without a doubt, various pathologies will differentially affect the ABR morphology, sometimes in very predictable ways whereas others may not

Stimulation Rate

  • A General rule is that there can be no more than one stimulus presented in the same analysis window, or else overlapped and time-shifted responses will be averaged.
  • The stimulus rate is used with odd and decimal numbers to avoid multiples of the 60-Hz line nose
  • High rates of stimulus can be used to stress the auditory system and see how well it performs
    • This method is typically used to find tumors
    • However, this method is likely to miss tumors smaller than 1 cm 

Contralateral Masking

  • Although there is a greater interaural attenuation for insert earphones, they do not completely eliminate the need for contralateral masking
  • If there is a significant asymmetry between ears, contralateral masking may be warranted
  • The general rule is to apply contralateral masking anytime that you think there will be crossover

Electrode Montage

  • Principle ABR generators have a certain spatial orientation and direction that indicate placement of electrodes
  • Waves I & III seem to have generators in a more horizontal arrangement, whereas wave V has a more vertical arrangement
  • The most common clinical placement for a one-channel ABR setup involves a high forehead placement (Fz) for the non-inverting electrode and earlobe or mastoid of the stimulation ear for the inverting reference
  • The ground electrode placement is a matter of preference but it is usually placed on the contralateral ear or the lower forehead 

Measurement Parameters

Latency

  • ​Absolute and relative latency measures of the ABR are the most useful for neurodiagnostic applications
  • Latency prolongations that arise are due to neuronal slowing (injury, compression, obstruction) and/or poor neural synchrony, including hearing loss
  • Given that abnormal latency prolongations are what we often look for in the analysis and interpretation of ABRs, absolute latencies (time of peak appearance relative to stimulus onset) interwave (interpeak/IWI) latencies (relative differences between subsequent peaks) and wave V interaural latency differences (OLDs or IT5) are the most commonly reported information

Clinical Utility

Monaural
  • Absolute Latencies for waves I, III and V
    • Measurement of latency at peak relative to stimulus onset
    • Wave V latency may be the most useful
    • Latencies for I, III and V are 1.5, 3.5 and 5.5 msec respectively
    • Waves I, III are commonly used but gender specific local norms are ideal
    • Any absolute latency that exceeds 2 standard deviations is considered diagnostically significant (retrocochlear)
  • Interwave (interpeak) latency interval (IWI) for waves I-III, III-V and I-V
    • Calculate the difference between subsequent waves
    • 2 msec for I-III and III-V and 4 msec for wave I-V are commonly used but gender specific local norms are ideal
    • Any IWI that exceeds 2 standard deviations is considered diagnostically significant
  • V/I amplitude ratio
    • divide wave V amplitude by wave I amplitude
    • <0.75 is considered diagnostically significant
Binaural
  • Interaural latency difference
    • Calculating difference in wave V latency between ears
    • 0.2 to 0.4 msec is considered diagnostically significant

Case History Example

  • 46 y/o female w/ sudden sensorineural hearing loss in left ear
  • No tinnitus
  • No Vestibular symptoms
  • Mild, high frequency loss in the right ear
  • trough shaped, mild to moderately sensorineural hearing loss left ear
  • Wave V latencies and interpeak latencies (I‐III, III‐V, I‐V) were normal bilaterally
  • Wave V was evident in the intensity series for the right ear, but was not well defined below 80 dB nHL for the left ear
  • The Wave V derived using rapid stimulus repetition rate (61.1/sec) was normal for the right ear, but shifted in latency in the left ear from 5.66 ms to 6.26 ms (>.4 ms) and had poor repeatability

Interpretation

  • A significant wave V latency shift (>.4 ms) using a stimulus rate of 73.1/sec lends itself to retrocochlear pathology
  • One diagnosis would be a tumor compressing the VIIIth cranial nerve, thus inducing hearing loss.
  • High resolution fast spin echo T2 axial and coronal images of the internal auditory canals were obtained. Thin section T1 axial images were taken before and after injection of gadolinium with thin section coronal sections after contrast. A 'well‐circumscribed 6 x 10 mm mass' was identified on the left side, which was predominantly within the internal auditory canal. The right internal auditory canal was normal

References

Auditory electrophysiology: Atcherson, S. and Stoody, T. (2012). Auditory Electrophysiology; A Clinical Guide. New York: Thieme.
Instructions for the test
  • You have been selected for an ABR/Auditory Brainstem Response test, it is a non-invasive test and the only thing that you need to do is relax. I will be cleaning and prepping areas of your skin with alcohol and a mild exfoliant. Please let me know if you have any allergies.
Otoscopy
  • Ensure that the patient has clear canals
Clean the skin with an alcohol pad
  • The forehead and mastoid sites
  • Use soap and water if allergic to alcohol
Prep the sites with nuprep
  • use a cotton swab to prepare these sites
  • clean again with alcohol
Place electrodes
  • use enough electrolytic material
  • tape sites to head
  • be mindful of electrode noise
Plug in electrodes to proper sites
  • use guides on equipment
Place insert headphones
  • be sure to place flush with the external auditory canal
Set up the equipment
  • Stimulus - Click
  • Transducer - Inserts
  • Polarity
    • Condensation
    • Rarefaction
  • Duration - 100 usec/0.1ms
  • Intensity > or = 70 dB
  • Rate 21.1 - 39.1 (not a multiple of 60Hz)
  • Filter setting 30-1500/3000 or 100-1500/3000
    • Sweeps >1000 - enough to adequately overcome the Signal to Noise Ratio and replicate
  • Analysis time is 15ms for peds and 11ms for adults
  • Montage - Fz, Ai, Ac
Check for the impedance
  • want them to be below 5kohms and within limits of each other
Reverse Polarity
  • add waveforms together
Pick Peaks
  • Wave I Latency
  • Wave II Latency
  • Wave III Latency
  • Wave IV Latency
  • Wave V Latency
  • IWI
  • etc!
Remove equipment and clean