ABR - Diagnosis Flashcards
Evoked potentials definition
electrical signals generated by the nervous system in response to a stimulus
What are 2 short-latency evoked potentials?
ECochG and ABR
In general, what is the ABR (auditory brainstem response)?
a relatively quick and reliable, objective clinical test that measures the functional integrity of the auditory peripheral and brainstem structures (tests from the cochlea to the brainstem)
ABR is probably the most widely used AEP in which clinical scenarios?
- estimating behavioral thresholds in infants
- detecting neurologic abnormalities of the VIII CN and brainstem
The important thing to remember about the ABR is that it…
makes INFERENCES about hearing (it’s not an actual hearing test!!)
What type of stimulus does the ABR test protocol typically use?
clicks
How does the ABR actually measure hearing?
it measures hearing based on the electrical output from the different structures along the auditory pathway
What are the 3 main categories of clinical applications for the ABR?
- hearing screening (for early ID of HL)
- estimation of hearing thresholds
- neuro-otologic applications
What are some examples of neuro-otologic applications of the ABR?
- determining site/level of a lesion
- monitoring of comatose patients
- intraoperative monitoring
Describe the morphology of the ABR.
a series of + and - peaks (biphasic) of waves (components) after the onset of a high-intensity, brief stimulus (click)
How are the components of the ABR typically identified/labeled?
with Roman numerals; I-VII
T/F: At higher intensities, all ABR components are visible with wave V being the largest.
True
What are the characteristics of waves II, IV, VI, and VII?
they’re typically smaller and quite variable, and can’t always be recorded in all patients (thus limiting their clinical application)
Which waves of the ABR are usually analyzed for diagnostic purposes?
waves I, III, and V
What are 3 types of stimuli the ABR can use?
- clicks
- chirps
- tone bursts
Characteristics of click stimuli:
- abrupt onset
- short duration
- broad spectrum (to evoke responses from numerous nerve fibers along the cochlear partition)
What is the range of sound that click stimuli usually evaluate?
2000 to 4000 Hz
Characteristics of tone burst stimuli:
- provide fx specific info
- requires a longer recording window to capture specific responses
What is an example of a clinical scenario where you would use tone burst stimuli in an ABR?
when building a baby’s audiogram to determine thresholds for cochlear implantation
Why do tone burst stimuli require a longer duration for recording?
because the response has a longer latency in comparison to click stimuli
Characteristics of chirp stimuli:
- more efficient than clicks because it evokes larger amplitude ABRs
- this advantage is dependent on cochlear fx specificity
Testing parameters for chirp stimuli:
either fx specific, or in a 2000 to 4000 Hz range
What is the reason the ABR can be used to determine the site/level of a lesion?
the ABR waveform reflects the neural activity that originates in the different anatomical structures along the neural pathway
Wave I of the ABR correlates to which structure?
the part of the auditory nerve closest to the cochlea
Wave II of the ABR correlates to which structure?
the part of the auditory nerve closest to the brainstem
What are two terms we use to describe an ABR waveform?
latency and amplitude
What are the two types of latency in reference to ABR testing?
absolute and relative
Absolute latency definition
- the neural conducting velocity, essentially, how fast a stimulus is passing through the anatomical structure
- on the waveform, it’s where each peak appears (I, II, III, etc.)
Relative latency definition
consists of inter peak latency and intramural latency differences
Interpeak latency definition
the latency between waves, such as I-III, III-V, or I-V
interaural latency difference definition
comparison of the right vs left side of the brain
What are some factors that affect amplitude of ABR waveforms?
- temporal synchrony/neural activity coherence in the responding structures
- the number of elements contributing to the response
A smaller amplitude of an ABR wave signifies…
lower temporal synchrony
A larger amplitude of an ABR wave signifies…
a greater number of neurons being activated
What is the amplitude ratio of Wave V to wave I typically used for in clinical scenarios?
because wave V is typically larger than wave I, any low ratio may be indicative of a pathological abnormality
In regards to latency of waveforms, what two components appear to generate waves I and II of the ABR?
the auditory nerve (distal and proximal portions respectively)
In regards to latency on waveforms, what component appears to generate wave III?
the ipsilateral cochlear nucleus
In regards to latency on waveforms, what component appears to generate wave IV?
the superior olivary complex
In regards to latency on waveforms, what component appears to generate wave V?
the rostral part of the contralateral Lateral Lemniscus, proximal to its termination in the inferior colliculus (the LL inputs to the contralateral IC)
What are 4 stimuli factors that can affect ABR waveforms?
- intensity
- frequency
- stimulation rate
- polarity
Characteristics of intensity of a stimulus
- influences latency and amplitude
- a change in intensity affects waves I-IV most prominently, while wave V becomes more robust
An increase stimulus intensity results in…
a decreased latency
A decreased stimulus intensity results in…
an increased latency
Amplitude and stimulus intensity are directly proportional, meaning…
increased intensity will increase the amplitude (and vice versa)
Characteristics of frequency of a stimulus
- tone burst and chirps are fx specific
- the absolute latency should decrease as the fx increases
- wave V is easily IDed in all 4 fxs (500, 1k, 2k, 4k Hz)
- waves I and III are only IDed in 2k and 4k Hz
Characteristics of the rate of a stimulus
- repetition rate = rate at which stimuli are presented in a time unit
- changing of rate is used to evaluate the nature of neural adaptation
- increased presentation rate increases latency, and reduces amplitude of all components
Why do we want to evaluate neural adaptation in ABR through presentation rates of stimuli?
this value could be different in normal hearing patients vs those with auditory nerve or brainstem pathologies (good for diagnostics)
Characteristics of polarity of a stimulus
- rarefaction = neural activity occurs about half a period earlier when compared with condensation; latency is also shorter
- alternating = can be used to eliminate/reduce stimulus artifacts at high stimulus levels
Studies show that rarefaction produces…
more robust and shorter latency, thus increasing auditory sensitivity
Studies show that stimuli of condensation elicit…
earlier responses
Analog filtering is performed on the input signal to accomplish what for ABR?
to remove frequency components that are not part of the signal of interest
What filter setting is commonly used for supra threshold measures in ABR?
band-pass setting of 100 - 3000 Hz
What filter setting is commonly used for measures near threshold in ABR?
usually a more open, band-pass setting of 30 or 50 to 3000 Hz is used (this helps to capture the broad, lower-fx wave V near the threshold)
What kind of impedance readings are necessary for obtaining quality ABR waveforms?
low (below 5 kiloOhms) and balanced
What should the value of the inter-electrode impedance be for obtaining quality waveforms?
when the values are within 2k Ohms of each other
Why do contralateral waveforms not display wave I?
- because they don’t activate the auditory nerve ( they only display waves III-VI)
- it also enhances wave V
What causes a post-auricular muscle artifact (PAM)?
contraction or movement of muscles located behind the ear, particularly the post-auricular muscles
What type of masking is used for ABR, typically?
white noise, since the click is a broadband stimulus
How much masking would be needed during an ABR test?
it has to be enough to elevate the hearing threshold in the non-test ear; this prevents cross over of the signal
T/F: Masking affects the latency or amplitude of an ABR response.
false
ABR masking conditions for AC
if the latencies of waves I and V are abnormal, then you need to mask
ABR masking conditions for BC
if wave I is absent or delayed, masking is needed
A caudal to rostral course of maturation means what?
that the auditory nerve matures before the brainstem
Newborn ABR morphology
- characterized by 3 prominent, almost equal amplitude peaks as early as 32 weeks conceptional age
- emergence of other components occurs by about 3 months of age
The absolute latencies of waves III and V continue to decrease with little change in wave V’s absolute latency after how many weeks?
40
When does Wave I latency reach adult values?
as early as 3 months old
When does wave V and IPLs reach adult values?
they continue to decrease and reach adult values by 18-24 months
ABR latencies continue to … throughout early childhood before … and then start to … as we age.
decrease; stabilization; increase
The amplitude of wave I has been reported to decrease with what 2 factors?
age and HL
Female ABRs consistently show…
shorter absolute latencies, and larger amplitudes throughout waves III-V (in comparison to males)
What are some other factors that are included in gender differences between ABR waveforms?
- hearing sensitivity
- body temperature
- smaller head size
- smaller brainstem distances
- hormonal differences
Small changes in body temperature can impact neural activity by…
influencing the events that lead to action potential generation, neurotransmitter release, synaptic transmission, and neural conduction velocity
T/F: An ABR screening is recommended for babies needing care in the NICU for more than 5 days.
True (they could be at risk for late-onset progressive HL or ANSD)
The JCIH recommends that a repeat screening be performed under what circumstances?
if the infant doesn’t pass the initial screening before hospital discharge (preferably several hours after the first)
When does ABR indicate brain death in comatose patients?
- when wave I is present by itself
- when the absence of wave I is accompanied by a documented auditory end organ injury
- when previously recorded potentials are no longer detectable
Reasons to use ABR for intraoperative monitoring
- hearing preservation (for procedures near the auditory system)
- having the info can be handy when discussing postoperative outcomes for hearing
- establishing preoperative baselines
- continuous monitoring of neural functions in auditory structures during the procedure
Hearing is often considered at risk during procedures concerning lesions in which area of the brain?
the cerebellopontine angle area
AEP testing during surgical procedures can also help do what?
facilitate development and improvement of surgical techniques
List the components for clinical interpretation of ABR.
- identification of waveforms (presence or absence of waves I, III, and V)
- latencies of waves I, III, and V
- inter peak intervals of I-III, III-V, and I-V
- right left differences of the inter peak intervals
- amplitude ratio of IV/V:I
ABR that could indicate conductive HL
- all waves’ absolute latencies would be delayed
- inter peak intervals would still be normal
ABR that could indicate SNHL
- mild HL would present with reduced amplitudes (anything worse, they’d be decreased further/absent)
- some waves might still be present
- normal latencies
What are the norms for latency of waves I, III, and V?
I = 2.06 ms, III = 3.57 ms, V = 5.53 ms
What are the norms for inter peak intervals of waves I, III, and V?
I-III = 1.51 ms, III-V = 1.96 ms, I-V = 3.47 ms
ABR criteria for retrocochlear dysfunction
- absence of all waves I-V that are unexplained by extreme HL determined by audiometric testing
- absence of all waves following I, II, or III
- abnormal prolongation of inter peak intervals I-III, III-V, and I-V
- abnormal diminished V/I amplitude ratio, especially when accompanied by other abnormalities
- abnormally increased differences between the 2 ears (interaural differences) when not explained by unilateral or asymmetric middle and/or ear dysfunction
Audiometric profile for patient with an acoustic neuroma
- higher fx hearing loss usually occurs first
- unilateral loss
- tinnitus and/or dizziness on one side
- SRT fine
- WRS not consistent with pure tones
Abnormalities of Wave I on ABR can indicate…
- peripheral auditory dysfunction (conductive or cochlear)
- pathology involving the distal auditory nerve
- intracranial pathology due to the cochlea receiving blood supply from the internal auditory artery
What condition can be indicated by a poorly formed/absent wave I, but a clear wave V?
high-frequency HL
Indications of abnormal I-III inter peak intervals
- prolongation reflects dysfunction between the auditory nerve and the lower pons (on the stimulated side)
- acoustic neuromas
- demyelinating disease
- brainstem tumors
- vascular lesions on the brainstem
Indications of abnormal III-V inter peak intervals
- reflects dysfunction between the lower pons and mesencephalon (usually, but not always, on the ipsilateral side to the lesion)
- demyelinating disease
- tumor
- vascular disease
