Mechanisms of hearing loss resulting from middle-ear fluid

Fluid in the middle ear, a defining feature of otitis media with effusion (OME), is commonly associated with a 20- to 30-dB conductive hearing loss. The effects and relative importance of various mechanisms leading to conductive hearing loss were investigated in a human temporal bone preparation. Umbo velocity in response to ear-canal sound was measured with a laser vibrometer while saline and silicone fluids of viscosity 5-12,000 cSt were introduced into the middle ear to contact part or all of the tympanic membrane (TM) and fill part or all of the middle ear. At low frequencies, reductions in umbo velocity (deltaVU) of up to 25 dB depended on the percentage of the original middle-ear air space that remained air-filled, which suggests that the primary mechanism in hearing loss at low frequencies is a reduction of the admittance of the middle-ear air space due to displacement of air with fluid. At higher frequencies, deltaVU (of up to 35 dB) depended on the percentage of the TM contacted by fluid, which suggests that the primary mechanism at high frequencies is an increase in tympanic membrane mass by entrained fluid. The viscosity of the fluid had no significant effect on umbo velocity. deltaVU for the fluid-filled middle ear matched hearing losses reported in patients whose middle ear was believed to be completely filled with fluid. The difference between deltaVU for a partly-filled middle ear and hearing losses reported in patients whose middle ear was believed to be incompletely fluid-filled is consistent with the reported effect of middle-ear underpressure (commonly seen in OME) on umbo velocity. Small amounts of air in the middle ear are sufficient to facilitate umbo motion at low frequencies.