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
Earlobeearlobe
Auditory
nerve:
ABR
Vertex-neck
Unfiltered
Filtered
CAP from
CNVIII
Distally
Near
brainstem
Response from auditory nerve
Normal CAP
Injury from
heat
Before surgical
manipulations
After surgical
manipulations
Solid lines:
Rarefaction clicks
Dashed lines:
Condensation clicks
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
ELECTRODE
WIRE UNDER
DURA
SUTURES
CN VIII
CN IX & CN X
FLOCCULUS
FORAMEN
OF LUSCHKA
WICK
ELECTRODE
ELECTRODE
WIRE
CHOROID
PLEXUS
FROM: MOLLER ET AL 1994
Auditory nerve injuries affect speech
discrimination more than cochlear injuries