HYPOTHESIS:To assess the effects of positive and negative middle ear pressures on auditory threshold. BACKGROUND:Nonatmospheric middle ear pressures can alter auditory threshold by their effects on tympanic membrane and ossicular chain mobility. METHODS:Experiments were conducted on guinea pigs by inducing alterations in pressure (positive and negative) with a syringe connected to the middle ear bulla cavity, the magnitude of the pressure being assessed with a water manometer. Elevated middle ear fluid pressures were also induced by attaching a saline-filled vertical tube to the saline-filled middle ear. The effect of these altered middle ear air and fluid pressures were assessed by recording auditory nerve-brainstem evoked responses. RESULTS:There was no effect on auditory threshold of positive middle ear air pressures (up to 250 mm H2O). A negative middle ear air pressure of -50 mm H2O induced a significant 9.5-dB threshold elevation, whereas more negative pressures (up to -150 mm H2O) did not induce an additional threshold elevation. Filling the middle ear cavity with saline induced a 10- to 16-dB elevation, whereas additional fluid pressures (up to 200 mm H2O) did not induce further elevations. CONCLUSION:The major factor inducing threshold elevation in serious otitis media is not the alteration in middle ear pressure but rather the reduction in the volume of compressible air in the middle ear by the fluid.