Effect of static middle-ear and intracranial pressure changes on differential electrocochleographic response.

In an animal model, we examined the extratympanic electrocochleographic response to static pressure changes (middle-ear pressure and intracranial pressure [ICP]) with attention to the summating potential (SP), action potential (AP), and the SP/AP ratio. With a closed middle ear, raised or lowered middle-ear pressure and raised or lowered ICP resulted in congruent increases of the SP and the SP/AP ratio, while the AP remained at a steady voltage. With a closed middle ear, raising or lowering ICP by positioning also had the effect of raising or lowering middle-ear pressure. With an open middle ear, raising or lowering middle-ear pressure resulted in congruent increases of the SP and the SP/AP ratio and, though the AP remained steady, it showed much higher voltage values. With an open middle ear, the baseline SP and baseline SP/AP ratio were reduced, but the reduction of the SP/AP ratio was mainly due to an increased AP. With an open middle ear, the SP, the SP/AP ratio, and the AP did not change appreciably during positioning-induced ICP changes. This suggests that ICP changes by positioning are not very effective in the rat when the middle ear is open. Thus, the effect of ICP changes with the middle ear closed are mainly due to positioning-induced pressure changes in the middle ear. Our findings confirm that static middle-ear pressure is critical for the cochlea and that good eustachian tube function is essential to keeping the pressure gradient across the round-window membrane as small as possible, the latter possibly assisted by a Windkessel function of the round-window membrane. However, relatively small pressure changes in the middle ear can overwhelm it, which suggests a very limited Windkessel function. In Ménière's disease, sensitivity to static pressure changes is possibly exaggerated. The insertion of a middle-ear ventilation tube, as practiced in treating selected cases of Ménière's disease, could restore Windkessel function. It has a stabilizing effect, as the inner ear appears less pressure-sensitive. This effect decreases Ménière's disease symptoms, but it is not without consequences. After the insertion of a middle-ear ventilation tube, a reduced SP/AP ratio is no evidence of improved inner-ear function. Its consequences are enlarged AP voltages, very likely loss of middle-ear dampening, and possible increased ringing.