Week 7: Stacked ABR

Question 1

protocols for ABR

Answer 1

1) threshold determination protocol
2) neurological protocol
3) IOM

Question 2

purpose of neurodiagnostic protocol

Answer 2

• to rule out problems with brainstem system

- –VIII lesions and NFII

Question 3

when to use neurodiagnostic protocol

Answer 3

when trying to see if you are looking at an audiologic or neurologic manifestation
—to rule out a retrocochlear lesion

Question 4

audiologic manifestations that would make you want to use a neurological protocol

Answer 4

• asymmetrical hearing loss
• high frequency HL (schwannoma affects HF first)
• red flags:
• –tinnitus
• –unilateral loss
• –balance

Question 5

other pathologies which will make you want to do neurologic testing

Answer 5

• demyelization diseases
• –52% have abnormal V and I-V IPL
• parkinsons disease
• meningitis
• ANSD
• hydroencephaly
• vascular abnormalities
• head injury
• coma and brain death

Question 6

three things you judge an ABR response based on

Answer 6

• morphology/waveform configuration
• –poor morphology
• –no visible peak/wave
• latency measures
• –prolonged absolute and/or IWL
• –abnormal IL5 differences
• amplitude measure
• –abnormal wave V to I amplitude ratio

Question 7

neurodiagnostic protocol (5 things to look at)

Answer 7

• intensity effect
• rate effect
• polarity effect: condensation vs rarefaction
• electrode difference: ipsi and contra
• ear difference

Question 8

intensity effect on neurodiagnostic ABR

Answer 8

*use higher intensity level (>70 dB HL) to identify waves I, III, an V

Question 9

rate effect with neurodiagnostic ABR

Answer 9

want to start with the slow rate to have a normal ABR to see overall brainstem integrity and then increase the rate to see how the morphology changes due to stress. 11.1 is the typical slow rate of 27.1 but the fast rate needs to be more stressful so closer to 66.6. expect to see the latency increase and the amplitudes to decrease

Question 10

VBI

Answer 10

vertibrobasalar insuffucuency which is a problem with blood flow. This gives pts vertigo and ABR could help see this

Question 11

polarity effect on neurodiagnostic ABR

Answer 11

want to change polarity to see if condensation or rarefaction is better and also to confirm if what you are seeing is a domination of the CM

Question 12

is wave V better with ipsi or contra

Answer 12

contra

Question 13

inter-aural latency difference

Answer 13

IT-5; the latency of wave 5 between each ear which you need to compare to see if there is a lesion; should be within 0.2 ms

Question 14

interpeak latency difference

Answer 14

latency between the peak of I-V, I-III, and III-V in the same ear
I-V should be around 4 ms

Question 15

amplitude ratio of I to V normative data value

Answer 15

should be greater than 1.0

Question 16

IT5 latency comparison

Answer 16

• one of the best ABR measures
• no intersubject variability if compared between ears
• IT5> 0.2 ms after correcting for a loss would mean an VIII nerve tumor

Question 17

limitation of IT5 latency measures

Answer 17

• insensitive to:
• –small tumors
• –bilateral tumors
• affected if non-tumor side has moderate HL (less than 50 dB which is when you would start using a correction factor) making a delayed latency
• still affected by background noise and poor SNR

Question 18

limitations of I-V or I-III IPL

Answer 18

1) cochlear HL
- –poor or absent wave I
- –HF HL
- —–prolonged wave I
- —–shortening of I-V which leads o false negative results
2) insensitive to small tumors than are under a cm big
- –IT5
3) when compared to non-tumor population
- –false positive (low specificity) if a nontumor patient has normally long I-V intervals that exceed the diagnostic criterion= false Dx of a tumor
- –false negative (low sensitivity) if a patient has normally short IPL, then you could miss a tumor because even though the wave is delayed, it could still be within normal limits

Question 19

how to overcome the limitation of I-V, I-III delay

Answer 19

• intra-aural comparison of I-V and I-III delay
• –gives better sensitivity and specificity
• limitations:
• –small tumor, click stimuli
• –cochlear HL, need wave I in both ears
• –insensitive to bilateral tumors
• use derived-band ABR or Stacked ABR

Question 20

wave V/I amplitude ratio

Answer 20

• looking at is wave V amplitude abnormally small?
• –if so is this due to a HL or a tumor?
• wave V/I ratio < 0.5 micro volts indicates wave V amplitude is abnormally small, indicating the presence of a retrocochlear pathology
• –normal is 0.5 or larger

Question 21

limitation of wave V/I amplitide ratio

Answer 21

the high variability of wave V

• small response:
• –far fields are < 1 micro volt
• –poor SNR
• cochlear HL reduces waves I and V amplitude
• identification of wave V
• phase cancellation of neural response
• –with clicks, more phase cancellation from the apex of the cochlea–less synchronization at the apex– the response reflects activity from the most basal regions
• poor sensitivity (false negatives) and specificity (false positives)
• –ratio<1= tumor in 44% of tumor cases
• –ration >1= tumor in >50% of cases
• using two highly variable measures (I and V) and depends on the presence of both of these waves
• sensitive to hearing configuration because high frequency will affect wave I but not low frequency HL
• correct for differences in cochlear loss between ears

Question 22

limitations of ABR in general (6)

Answer 22

• severity of loss
• –peripheral loss confounds ABR results)
• morphology— judgement not measurement
• marking ABR waves— variable interpretation
• not sensitive to small lesions
• disorders above the brainstem are not predicted
• diagnosis of pathology cannot be made
• –abnormal findings indicate a retrocochlear lesion, but not the type of pathology

Question 23

what is the solution to a click ABR with a small lesion

Answer 23

• derived response
• aka stacked ABR
• –click with high pass masking but looks at the magnitude of wave V instead of latency

Question 24

how stacked ABR is required for successful detection of small acoustic tumors

Answer 24

• gives an auditory signal that will stimulate all frequency regions of the cochlea (click)
• has a method for separating out responses from the different frequency regions of the cochlea (derived band ABR technique)
• a procedure of adding the responses together to approximate total neural activity (stacking method)
• –when the amplitude is smaller shows where the tumor is affecting (location is based off of affected frequency range)
• —–doesnt change intensity so can just look at the smaller amplitude and know it is from the tumor

Question 25

derived band ABR summary

Answer 25

• neural contributions from different regions of the cochlea are obtained
• derived-band ABR represent activity from more specific frequency regions than moderate-to-high level toneburst-evoked ABRS

Question 26

stacked ABR summary

Answer 26

• the stacked ABR is formed by temporally aligning wave V of the derived band ABRs and the summing the responses
• aligning the derived-band ABRs eliminates phase cancellation of lower frequency activity
• the stacked ABR amplitude reflects activity from all frequency regions of the cochlea, not just the high frequencies
• **reduction of any neural activity due to a tumor, even a small tumor, will result in a reduction of the stacked ABR amplitude

Question 27

standard vs stacked ABR sensitivity

Answer 27

• 95% sensitivity (presumably with stacked)
• the IT5 and I-V measures have < 5% specificity
• but the stacked ABR has 83% specificity

Question 28

what is stacked ABR better at

Answer 28

1) detecting small tumors

2) decreasing the number of misdiagnosed non-tumor patients (decreased number of false positives referred fro MRI)