Axonal injury in auditory nerve observed in reversible latency changes of brainstem auditory evoked potentials (BAEP) during cerebellopontine angle manipulations in rats

• Published in: Hearing research 2002-11, Vol.173 (1), p.91-99
• Main Authors: Sekiya, Tetsuji, Shimamura, Norihito, Yagihashi, Akinori, Suzuki, Shigeharu
• Format: Article
• Language: English
• Subjects: Amyloid beta-Protein Precursor - metabolism; Animals; Auditory nerve; Axonal injury; Axons - metabolism; Axons - pathology; Biological and medical sciences; Brainstem auditory evoked potential; Cerebellopontine Angle; Cerebellum - physiology; Cochlear Nerve - metabolism; Cochlear Nerve - pathology; Electrodiagnosis. Electric activity recording; Evoked Potentials, Auditory, Brain Stem - physiology; Hearing; Intraoperative monitoring; Investigative techniques, diagnostic techniques (general aspects); Male; Medical sciences; Nervous system; Pressure; Rats; Rats, Sprague-Dawley; Reaction Time; β-amyloid precursor protein
• ISSN: 0378-5955; 1878-5891
• DOI: 10.1016/S0378-5955(02)00633-0

Intraoperative monitoring of brainstem auditory evoked potentials (BAEP) has been widely utilized to reduce the incidence of postoperative hearing disturbance due to cerebellopontine angle manipulations. The prolongation of wave V of BAEP is usually used as a criterion to warn the surgeons to modify their surgical maneuvers. However, it is not known whether all neuropathological changes are avoided if BAEP latency intraoperatively returns to the baseline level or some neuropathological changes ‘silently’ occur even if BAEP normalizes. The aim of this study was to experimentally clarify this point that would be important for the long-term prognosis of patients’ hearing. The cerebellopontine angle portion of the auditory nerve was quantitatively compressed in the rats and reversible prolongation of BAEP latency was reproduced just as it occurs during surgery in humans. Twenty-four hours after the compression, the auditory nerve was removed for β-APP immunostaining to investigate the degree of axonal injury. The results of the present study disclosed that axonal injury occurred even in the cases where the intraoperative normalization of prolonged wave IV (equivalent to wave V in humans) latency had been obtained. Therefore, the interpretation of BAEP changes based only on the prolongation of the latency of BAEP was not enough to prevent the auditory nerve from developing morphological changes. Changes in the amplitude of wave V of BAEP appears to be more sensitive than its latency change as an intraoperative indicator for axonal injury in the auditory nerve.