Effects of middle-ear static pressure on pars tensa and pars flaccida of gerbil ears

It has long been known that static pressure affects middle-ear function and conventional tympanometry uses variations in static pressure for clinical assessment of the middle ear. However, conventional tympanometry treats the entire tympanic membrane as a uniform interface between the external and m iddle ear and does not differentiate the behavior of the two components of the tympanic membrane, pars tensa and pars flaccida. To analyze separately the different acoustic behavior of these two tympanic membrane components, laser Doppler velocimetry is used to determine the motion of each of these two structures. The velocities of points near the center of p. tensa and p. flaccida in response to the external-ear sound pressure at different middl e-ear static pressures were measured in nine gerbil ears. The effect of mid dle-ear static pressure on the acoustic response of both structures is simi lar in that nan-zero middle-ear static pressures generally reduce the veloc ity magnitude of the two membrane components in response to sound stimuli. Middle-ear under-pressures tend to reduce the velocity magnitude more than do middle-ear over-pressures. The acoustic stiffness and inertance of both p. tensa and p. flaccida are altered by static pressure, as shown in our re sults as changes of transferfunction phase angle. Compared to p. tense, p. flaccida showed larger reductions in the velocity magnitude to small over- and under-pressures near the ambient middle-ear pressure. This higher press ure sensitivity of p. flaccida has been found in all ears and may link the previously proposed middle-ear pressure regulating and the acoustic shuntin g functions of p. flaccida. We also describe, in both p. tensa and p. flacc ida, a frequency dependence of the velocity measurements, hysteresis of vel ocity magnitude between different directions of pressure sweep and asymmetr ical effects of over- and under-pressure on the point velocity. (C) 2001 Pu blished by Elsevier Science B.V.