What is the cross-setion of Internal Auditory Canal (IAC)?
- Facial nerve
- Superior vestibular nerve
- Inferior vestibular nerve
- Acoustic nerve
What happens when there’s a tumor in IAC?
Medium or large tumor in IAC can affect part of the auditory nerve, but not all of it
- May affect the fibers that are adjacent to the auditory nerve
What are the standard ABR measures for acoustic tumor detection?
IT5= interaural time delay for Wave V
- Difference between sides should not exceed 0.2-0.3 ms
I-V delay= latency difference between Wave I and V
- If the delay exceeds a certain criterion value, this measure is positive for a tumor
Can the ABR detect acoustic tumors?
The larger the tumor, the easier its detection
- Detects nearly all medium and large acoustic tumors
Misses 30-50% of small (<1 cm) acoustic tumors
- All patients with suspicious clinical hearing and balance symptoms are sent for MRI
What are the drawbacks fof screening with an MRI?
- Relatively expense ($2,100)
- Not available everywhere
- Invasive, anxiety producing, and uncomfortable test for some patients
- Cannot be used on patients with implanted metal devices or materials
- Most patients tested do not have a tumor
Why do standard ABR measures often fail to detect small tumors?
Small tumors exert less pressure and affect a smaller # of neural fibers than larger tumors
Not the only factors:
- Many small tumors exert enough pressure to cause clinical symptoms
- Many small tumors are detected by standard ABR measures
ABR measures are dominated by activity from a subset of 8th nerve fibers that may not be affected by the small tumor
How does the auditory system respond to click stimulus ABR?
- Click stimulus is a wide band acoustic signal with as much low-frequency energy
- Click-evoked ABR contains neural activity representing all frequency regions of the cochlea (not just HFs)
- Standard ABR wave V latency is dominated by HF regions b/c lower-frequency contributions are phase-cancelled
What are derived band ABRs?
Neural contributions from different frequency regions of the cochlea can be obtained using the derived-band ABR method
What is the stacking technique?
Formed by first temporally aligning wave V of the derived-band ABRs, then summing the responses
Aligning the derived band ABRs eliminates phase cancellation of the lower frequency activity
- 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
How can you minimize physiological noise in ABRs?
Estimation of unaveraged noise
Weighted averaging (Bayesian weighting)
- Weights block of sweeps
- Blocks of sweeps that have the lowest physiological noise are given the greatest weight in average
Termination of Averaging when residual noise level is low (20 nV)
- Stop averaging when the estimated noise in the average reaches a low level so that most of what we measure is response and not physiological noise
Describe the standard vs. stacked ABR measures?
The IT5 and I-V delay measures have less than 5% specificity
- IT5 and I-V delay correctly identify less than 5 out of every 100 non-tumor patients
But the Stacked ABR has 83% specificity
- Stacked ABR correctly identifies 83 out of every 100 non-tumor patients
Why is stacked ABR better than standard ABR?
Stacked ABR appears to have better sensitivity and specificity than the standard ABR for small tumors
In other worlds, the stacked ABR is better at:
- Detecting small tumors
- Decreasing the # of misdiagnosed non-tumor patients (i.e., decreasing the # of false-positives referred for MRI)
