Changes of the auditory system after cerebellopontine angle manipulations.

The current tendency in acoustic neuroma surgery to attempt the preservation of hearing function and the problem of accidental hearing loss caused by microsurgical neurovascular decompression operations for hemifacial spasm or trigeminal neuralgia prompted us to study the exact surgical vulnerability of the auditory system. The surgical procedures for operation on the cerebellopontine angle of dogs were carried out according to the sequence of the posterior fossa transmeatal operation for acoustic neuroma. The operation was tentatively divided into three stages: (a) craniectomy and dural opening, (b) cerebellar retraction, and (c) identification of the cochlear nerve in the unroofed internal auditory canal (IAC). The postoperative behavior of the auditory system was evaluated electrocochleographically (EcochG) and histologically. Overzealous retraction of the cerebellar hemisphere caused transient disturbance of the EcochG pattern. Mechanical stretching of both the cochlear nerve and the internal auditory artery may cause a disturbance in the synchronized discharge of the cochlear neurons. Various manipulations at the porus acusticus internus or the IAC (such as pinching the nerve with forceps or electrocoagulation) produced thoroughly distorted EcochG patterns. From the histological findings, the main causative factor for these labyrinthine damages was considered to be vascular insufficiency. The current need for neurosurgical operations to preserve hearing is discussed in the light of these findings.