The Auditory Brainstem Response
Key Facts
- clinical applications in estimating behavioral thresholds in infants and detecting neurologic abnormalities of the eight cranial nerve and brainstem
- Jewett and colleagues who correctly described the sequence of ABR waveform components as responses arising from the auditory nerve and various auditory brainstem structures
- 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
- the compound action potential of the ECochG is essentially one and the same with wave I of the ABR
- highly influenced by changes in the stimulus parameters
- 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
Description
Waveform Morphology
- 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
- 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 from the proximal portion of the auditory nerve
- Wave III from the cochlear nucleus
- likely to have more than one anatomical contributor
- likely to have more than one anatomical contributor
- Wave IV from midline brainstem structures (acoustic stria, trapezoid bodies and superior olivary complex)
- likely to have more than one anatomical contributor
- likely to have more than one anatomical contributor
- Wave V from the termination of the lateral lemnisus within the inferior colliculus on the contralateral side
- likely to have more than one anatomical contributor
- likely to have more than one anatomical contributor
- Viewed in another way, 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
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
- abruptness causes a synchronous discharge of numerous auditory nerve fibers
- broad clicks cause a spectral splattering of frequencies
- ABR seems to favor clicks in the 1kHz - 4kHz range
- 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
- 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 ~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
Stimulation Rate
- 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.
- Used with odd and decimal numbers to avoid multiples of the 60-Hz line nose
- High rates can be used to stress the auditory system and see how well it performs
- typically used to find tumors
- likely to miss tumors smaller than 1 cm
- typically used to find tumors
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
- 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 noninverting 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 prologations 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
Binaural
- 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
- 1.5, 3.5 and 5.5 msec respectively
- for 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
- Interwave (interpeak) latency interval (IWI) for waves I-III, III-V and I-V
- calculate 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