Dynamics of inner ear pressure change caused by intracranial pressure manipulation in the guinea pig.

Previous studies have shown that pressure changes in the cerebrospinal fluid compartment are transmitted to the inner ear. The main route for pressure transfer is the cochlear aqueduct. about which little is known with regard to its dynamic properties. In the present study, sudden intracranial pressure changes (square waves and short pulses) were created in guinea pigs by means of an electronically controlled infusion system. Simultaneously with pressure manipulation, hydrostatic pressure was monitored in both the peridural space and the perilymphatic compartment of the inner ear. The onset of an inner ear pressure change following manipulation of intracranial pressure was immediate. Inner ear pressure increased or decreased without a measurable time lag, and equalized within a few seconds. During square wave intracranial pressure manipulation, inner ear pressure equalized somewhat more slowly after pressure increase than after pressure decrease. To a first approximation, the pressure equalization curves for the inner ear could be fitted with a single exponential function, rising or falling with a time constant in the range 1-3 s, and the system can be described as a low-pass filter composed of a constant compliance and a constant flow resistance. Detailed analysis, however, showed small deviations from a purely exponential recovery process. With a more complicated (non-linear) model, almost perfect fits to the inner ear pressure equalization curves could be obtained. This non-linearity may be a consequence of the dependence of the compliance and, or flow resistance on pressure.