Perilymph Displacement by Cerebrospinal Fluid in the Cochlea

Abstract

On histological examination, the cochlear aqueduct of guinea pigs has a large open lumen, unlike the tissue-filled periotic duct of primates. Hence, the guinea pig's large patent periotic duct appears capable of accommodating a bulk flow of cerebrospinal fluid from the subarachnoid space into the scala tympani if an egress for this fluid was provided by experimentally perforating the cochlea. In order to measure this flow the cochlea in anesthetized guinea pigs was surgically exposed and a single opening was drilled in the basal turn of the scala tympani. All of the displaced perilymph was collected in a calibrated, silicone-coated micropipette sealed into this hole, and; was measured directly by recording the advance of the meniscus in the micropipette. The displacement rates were typically 0.8 to 1 μliter/min, but could be temporarily increased by short periods of respiratory depression and changes in body orientation. From a consideration of anatomical and hydrodynamic relationships, it is concluded that a major source of the fluid which displaces perilymph in the opened guinea pig cochlea is cerebrospinal fluid which enters the scala tympani via the cochlear aqueduct. These results indicate that if a single opening is made into the cochlea at the apex, all of the perilymph in scala tympani (8.0 utters) would be displaced in approximately 10 min. Furthermore, these results have important implications for the technique of intracochlear injection used to study the effects of ions and drugs on the cochlea, because the continuous displacement of perilymph from the opened cochlea consequently removes the test substance previously injected into the scala tympani.