Neurophysiological mechanisms of conduction impairment of the auditory nerve during cerebellopontine angle surgery.
OBJECTIVE
Intraoperative auditory brainstem response (ABR)-monitoring is useful for hearing preservation in patients undergoing cerebellopontine angle surgery. Prolongation of the latency of wave V, for example, is observed under surgical stress such as cerebellar retraction. We analyzed intraoperative ABR findings to study the neurophysiological mechanism(s) underlying latency prolongation.


METHODS
The ABR recorded during microvascular decompression surgery was studied in 18 patients with hemifacial spasm. We measured each trace of the ABR records, both the latency of each wave and some interpeak latencies. We also analyzed their waveforms especially in the early component, to assess changes during surgery.


RESULTS
The latency of wave V varied with cerebellar retraction. The delayed latency of wave V was correlated with the prolonged interpeak latency of waves I-III. An additional wave (designated wave I') between waves I and II was appeared; it was accompanied by a prolongation in the latency of wave V. Wave I' contributed to prolongation of the interpeak latency of waves I-III, resulting in a delay in the latency of wave V. Chronological analysis revealed that the minimum latency of wave I' was the same as wave IN, suggesting that wave I' arose near the porus acusticus internus (PAI).


CONCLUSIONS
Our study showed that cerebellar retraction may result in conduction impairment of the auditory nerve near the PAI, suggesting that the Obersteiner-Redlich zone is an electrophysiologically vulnerable site and wave I' is derived from the change in the vector of wave IN.


CONCLUSIONS
Our findings may provide neurophysiological evidence to support the theoretical model of ABR generators by Scherg and von Cramon.
Sato
Sumito
S
Department of Neurosurgery, Kitasato University School of Medicine, 1-15-1 Kitasato, Sagamihara, Kanagawa 228-8555, Japan. sumito@ba.mbn.or.jp
Yamada
Masaru
M
Koizumi
Hiroyuki
H
Onozawa
Yuya
Y
Shimokawa
Naomi
N
Kawashima
Emi
E
Fujii
Kiyotaka
K
D000161:Acoustic Stimulation / Q000379:methods; D000328:Adult; D000368:Aged; D002530:Cerebellopontine Angle / Q000601:surgery*; D019299:Decompression, Surgical; D004569:Electroencephalography / Q000379:methods; D016057:Evoked Potentials, Auditory, Brain Stem / Q000502:physiology*; D005260:Female; D019569:Hemifacial Spasm / Q000503:physiopathology* / Q000601:surgery*; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D016343:Monitoring, Intraoperative; D011930:Reaction Time / Q000502:physiology; D012189:Retrospective Studies; D055815:Young Adult