Perilymph volume flow in the fistulized cochlea

It has been previously reported that in guinea pigs the rate of perilymph volume flow is extremely low (<2 nl/min) when the cochlea is in the physiological, sealed state. In the present study, flow rates were monitored when the otic capsule was perforated. Flow was measured by comparing dispersal of a tracer (trimethylphenylammonium) in both directions along scala tympani with ion selective microelectrodes. Perforation of the cochlear apex produced a high rate of flow averaging 0.4 μl/min. The rate was reduced considerably when cerebrospinal fluid (CSF) pressure was released by opening the dura at the foramen magnum. Under these conditions, the mean rate of 6.9 nl/min. Blockage of the cochlear aqueduct was even more effective, reducing flow to a mean value of 1.4 nl/min. In the latter condition, the flow rate was not significantly different from the normal, sealed cochlea condition. These results demonstrate that the high rates of flow in the fistulized cochlea arise from a nonphysiologic entry of CSF into scala tympani through the cochlear aqueduct. For experimental purposes, this nonphysiologic flow can be reduced by releasing CSF pressure, or better still, by occluding the cochlear aqueduct. [Work supported by NIH Program Project Grant No. PO1 NS24372.]