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