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

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