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4_auditorynerv

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									   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:
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
                       ELECTRODE
                       WIRE UNDER
TUMOR
                       DURA
                       SUTURES


CN VIII

                       CN IX & CN X
FLOCCULUS




                       ELECTRODE
                       WIRE
FORAMEN
OF LUSCHKA
                       CHOROID
WICK
                       PLEXUS
ELECTRODE


             FROM: MOLLER ET AL 1994
Auditory nerve injuries affect speech
discrimination more than cochlear injuries

								
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