Abstract
Electrocochleographic potentials from the internal auditory meatal portion of the cochlear nerve (IAM-EcoG) and brain-stem auditory evoked potentials (BAEP) from the vertex were simultaneously recorded during cerebellopontine angle (CPA) manipulations (retractions of the cerebellar hemisphere or the cochlear nerve) of dogs. The same BAEP changes as in human subjects were obtained from the animal models. IAM-EcoG showed graded deteriorations in accordance with BAEP changes. The results are as follows: 1) Prolongation of the I-V interpeak latency (IPL) of BAEP, the most common finding during CPA manipulations, is the reflection of prolongation of the I-II IPL, which is caused by conduction impairment or block of the nerve impulses between the extracranial portion of the cochlear nerve and the brain-stem. The operative manipulations act as stretch or compression injuries to the cochlear nerve in the CPA and bring about acute traumatic cranial nerve root lesion, a retrocochlear lesion. 2) The obliteration of all BAEP components including wave I cannot be caused by conduction block. This is caused by occlusion of the internal auditory artery (IAA). BAEP wave I and IAM-EcoG P1-N1 complex are important indicators for the cochlear blood flow during surgical interventions. 3) As possible causes of IAA obstructions, mechanical distortions of the relationship between the anterior inferior cerebellar artery (AICA) and IAA at their junctional portions, mechanical vasospasms of the AICA-IAA complex, and ensuing no-reflow phenomena are discussed. 4) The EcoG is expected to be a useful intraoperative, real-time monitor during CPA surgery which can detect rapid changes derived from cochlear insufficiency. This real timability can overcome the disadvantages of BAEP.
