Impact of negative middle ear pressure on distortion product otoacoustic emissions

Abstract

Negative middle ear pressure (NMEP) is a disorder of the middle ear in which the tympanic membrane is retracted and pulled inward. Structures of the middle ear are compressed, and there is an increase in middle ear impedance. NMEP can modify estimates of auditory function such as Distortion Product Otoacoustic Emissions (DPOAE), which are generated in the cochlea when two primary tones (f₁, f₂ with f₂> f₁) are presented simultaneously to the ear and detected in the ear canal. Inter-modulation between primary tones produces distortion products at predictable frequencies not present in the original signal. The 2f₁-f₂ DPOAEs measured in the ear canal are a composite of two components both having the same frequency, but originating via different mechanisms from different cochlear locations and having different phase properties. One component is generated by the nonlinear response of the basilar membrane (BM) to the overlapping traveling waves generated by the primary tones. The other linear reflection component arises when a portion of energy generated at the overlap region travels to its own characteristic place on the BM, where it is partially reflected by pre-existing perturbations.

NMEP was voluntarily induced when subjects performed the Toynbee maneuver (subjects to pinch their noses and swallow until a sensation of fullness is felt). Logarithmic frequency sweeping primaries (primary levels L₁/L ₂ = 65/65, 70/70, 75/75 dB SPL) producing 2f₁-f₂ from 320-2560 Hz were presented during interleaved trials normal middle ear pressure and subject-induced NMEP. The sweeping primary procedure provided detailed information about DPOAE level as a function of frequency and permitted estimates of the composite DPOAE and the extraction of the two major components. Extraction of the levels and phases of the sweeping primaries provided information about the changes in middle ear function.

At the primary levels tested, the nonlinear distortion component dominated the composite DPOAE. NMEP reduced the levels of the composite DPOAE, generator, and reflection components from 1060 Hz to 3537 Hz. Separation of the components reduced variability and permitted more reliable evaluation of the impact on NMEP on DPOAE.