THE CHANGES IN COCHLEAR BLOOD FLOW AND AUDITORY BRAINSTEM RESPONSE IN THE PRESSURE-INDUCED ANIMAL MODEL OF ACOUSTIC NEUROMA

Abstract

Changes in cochlear blood flow (CoBF) and auditory brainstem response (ABR) in a pressureinduced animal model of acoustic neuroma were examined. A suboccipital approach was used to expose the right cerebello-pontine angle in guinea pigs. Under surgical microscope, the bundle of nerves and vessels at the entrance of the internal auditory meatus was exposed without retraction. The two pressure points, one anterior to and the other posterior to the center of the bundle were separately compressed by a pressure probe (1mm in diameter). CoBF from the basal turn or second turn of the right cochlea was measured with a laser Doppler flowmeter. ABR was recorded from the electrodes placed on the vertex and the right mastoid process. With compression, the changes in CoBF and ABR were found in a total of 19 animals. We classified these changes into three types based mainly on CoBF. In type I (n=9), an increase rather than a decrease of CoBF was noted, and an increase in the I-II inter-peak latency with a decrease in the amplitudes of wave II-IV in ABR were observed. Those changes were mainly attributed to the blockage of cochlear nerve. In type II (n=6), CoBF was completely stopped and all waves of ABR disappeared during compression. This suggested the presence of cochlear ischemia. After relaxation of compression both CoBF and ABR recovered, but I-II inter-peak latency remained delayed. CoBF in type III (n=4) decreased and then slowly recovered. In type III, all waves transiently disappeared, and wave I reappeared with recovery of CoBF. The changes in type III were caused by damage to both the artery and the nerve. In addition, the changes in CoBF and ABR were closely related to the pressure points. The changes in type I were often found in compression of the anterior pressure point, whereas the changes in type II are associated with the posterior pressuring point (p<0.05). The results indicate that the cochlear nerve or the internal auditory artery is more susceptible to damage by compression of an anterior or posterior pressure point, and that the compression position is an important determinant in the type of auditory dysfunction and the degree of hearing loss.