Auditory monitoring for preservation of the function of the auditory nerve
description
Transcript of Auditory monitoring for preservation of the function of the auditory nerve
Auditory monitoring for preservation of the function of
the auditory nerve
Recording of auditory evoked potentials in operations in the
posterior fossa
Different conventions for display of ABR
NEURAL GENERATORS OF ABR:
Peak I: distal auditory nerve• Peak II: central auditory nerve• Peak III: mainly cochlear nucleus• Peak IV: unknown• Peak V: termination of the lateral lemniscus in the contralateral inferior colliculus
Intraoperative monitoring of auditory evoked potentials
• Interpretable response must be obtained in the shortest possible time
• Techniques are different from clinical use of ABR
Recording parameters for obtaining an interpretable responses in the shortest
possible time:
• High stimulus rate
• High sound intensity
• Optimal filtering
• Optimal electrode placement
• Reduction of electrical interference
Recording parameters for obtaining an interpretable responses in the shortest
possible time:
• High stimulus rate
• High sound intensity
• Optimal filtering
• Optimal electrode placement
• Reduction of electrical interference
Effect of stimulus rate on amplitude of ABR
Gain from increasing the stimulus rate
Recording parameters for obtaining an interpretable responses in the shortest
possible time:
• High stimulus rate
• High sound intensity
• Optimal filtering
• Optimal electrode placement
• Reduction of electrical interference
Earphones
Recording parameters for obtaining an interpretable responses in the shortest
possible time:
• High stimulus rate
• High sound intensity
• Optimal filtering
• Optimal electrode placement
• Reduction of electrical interference
Auditory brainstem evoked potentials (ABR):
The latencies of the peaks are important
therefore: Enhance the peaks by using
optimal filtering
Digital filtering can enhance the waveform of the ABR
Recording parameters for obtaining an interpretable responses in the shortest
possible time:
• High stimulus rate
• High sound intensity
• Optimal filtering
• Optimal electrode placement
• Reduction of electrical interference
Dipoles of ABR
Recording parameters for obtaining an interpretable ABR in the shortest possible time:
• High stimulus rate
• High sound intensity
• Optimal filtering
• Optimal electrode placement
• Reduction of electrical interference
Find the source of interference
Recording directly from the auditory nerve
Provides nearly instantaneous monitoring of neural conduction in the auditory
nerve
Cotton wick recording electrode
ABR:
Vertex-neck
Earlobe-earlobe
Auditory nerve:
ABRVertex-neck Unfiltered
Filtered
CAP from CNVIII Distally
Near brainstem
Normal CAP
Injury from heat
Response from auditory nerve
Before surgical manipulations
Solid lines: Rarefaction clicks
Dashed lines: Condensation clicks
After surgical manipulations
Waveform of the CAP recorded from the exposed CNVIII depends on pre-existing hearing loss
The recording electrode is difficult to keep in place on the
CNVIII
Recording from the surface of the cochlear nucleus
Recording from the cochlear nucleus:
Recording electrode is placed in the lateral recess of the
fourth ventricle
TUMOR
CN VIII
FLOCCULUS
FORAMEN OF LUSCHKA
WICK ELECTRODE
ELECTRODE WIRE UNDER DURA SUTURES
CN IX & CN X
ELECTRODE WIRE
CHOROID PLEXUS
FROM: MOLLER ET AL 1994
Auditory nerve injuries affect speech discrimination more than cochlear injuries