Retrocochlear Assessment: Stacked ABR

Question 1

Describe the standard ABR test for acoustic tumor detection.

Answer 1

• IT5: Interaural time delay for wave V
• I-V Delay: Latency difference between waves I and V
• Detects nearly all medium and large acoustic tumors
• BUT, misses 30-50% of small (<1 cm) acoustic tumors

Question 2

What did Eggermont et al. (1980) state about the standard ABR test for acoustic tumor detection?

Answer 2

-“ABR yields high detection scores: up to 90% The larger the tumor, the easier its detection. It is likely that small tumors (<1 cm) will be missed.”

Question 3

What are the consequences of failure to detect small tumors?

Answer 3

-All patients with suspicious clinical hearing and balance symptoms are sent for magnetic resonance imaging (MRI)

Question 4

What are drawbacks of MRI screening for acoustic tumor detection?

Answer 4

• Relatively expensive ($2,100)
• Not available everywhere
• Invasive, anxiety-producing, and uncomfortable test for some patients
• Cannot be sued on patients with implanted metal devices or materials
• Most patients tested do not have a tumor

Question 5

Why do standard ABR measures often fail to detect small tumors?

Answer 5

• Small tumors exert less pressure and affect a smaller number of neural fibers than larger tumors
• Small tumors may not exert enough pressure to cause clinical symptoms
• Therefore, ABR measures often fail to detect small tumors because these measures are dominated by activity from a subset of CN VIII ANFs that may not be affected by the small tumor

Question 6

Why is wave V latency dominated by HF ANFs in the standard ABR?

Answer 6

• LF contributions are phase-cancelled
• A tumor in the LF regions likely will yield normal standard ABR measures (wave V latency)
• Will be missed

Question 7

What is the rationale behind the stacked ABR?

Answer 7

• If we divide the whole auditory nerve into 5 groups, could use the activity from the groups for a new ABR measure
• 5 groups could represent frequency regions of the cochlea that span the whole frequency range of the whole ANF
• Presence of a tumor would reduce contributions from affected groups in the AN (abnormal ABR measures)
• Because this new measure involves contributions from essentially all nerve fibers, it doesn’t matter where the tumor is located or which fibers are affected as long as there has been compromise of a sufficient number of ANFs

Question 8

What stimuli are used in the stacked ABR?

Answer 8

• WB clicks
• HP masking noise (varied cutoff frequencies)
• Lowering the HP cutoff results in more masking, allowing for isolation of the AN region to be tested
• As more masking is added, stacked ABR responses will reflect activity from lower and lower frequency regions of the cochlea (peak activity in each response will be delayed in time due to the TW delay down the cochlea)
• Results in derive-band ABRs

Question 9

What do derive-band ABRs result in?

Answer 9

• Neural contributions from different frequency regions of the cochlea
• Adding all derived band ABR waveforms essentially yields the standard ABR response

Question 10

What is the Stacking Technique?

Answer 10

• The stacked ABR is formed by first temporarily aligning wave V of the derived-band ABRs, then summing the response
• Aligning the derived-band ABRs eliminated the phase cancellation of LF activity
• Thus, the stacked ABR amplitude reflects activity from all frequency regions of the cochlea, not just the HFs
• Reduction of any neural activity due to a tumor, even a small tumor, will result in a reduction of the stacked ABR amplitude

Question 11

How can physiological noise be minimized in the ABR?

Answer 11

• Physiological noise needs to be sufficiently minimized or the results will be difficult to interpret
• Estimation of un-averaged noise in a block of sweeps in a derived-band
• Weighted averaging for all blocks of sweeps in a derived-band (Bayesian weighting)
• Termination of averaging when residual noise level is low (20 nV)

Question 12

What is Bayesian weighting?

Answer 12

-Those blocks of sweeps that have the lowest physiological noise are given the greatest weight in the average

Question 13

Compare specificity between the standard and stacked ABR.

Answer 13

STANDARD ABR (IT5 and I-V Delay)

• Specificity: <5%
• Sensitivity: 95%

STACKED ABR
-Specificity: 83%

Question 14

What is the stacked ABR better at doing better than the standard ABR?

Answer 14

• Detecting small tumors
• Decreasing the number of misdiagnosed non-tumor patients (i.e. decreasing the number of false positives referred for MRI)