TYMPANIC MEMBRANE-CHANGES IN EXPERIMENTAL PURULENT OTITIS-MEDIA

The stiffness properties of the gerbil tympanic membrane (TM) were inv estigated during the early course of experimental purulent otitis medi a produced by middle ear inoculation (n = 49) with type 3 Streptococcu s pneumoniae. In a first approach the acoustic admittance and suscepta nce were measured in vitro with tympanometry and in a second approach the tympanic membrane displacement in response to static pressure was measured with moire interferometry. A histological assessment of the t ympanic membrane was made concurrently. The acoustic admittance and su sceptance reduced during the initial days post-inoculation. These redu ctions seem to correlate with an oedema simultaneously developing in t he pars tensa. The pressure-induced displacement increased with time o f disease, i.e. the mechanical stiffness of the TM reduced. Local defo rmation zones, or 'weak spots' appeared in the inferior half of the pa rs tensa in three out of eight cases measured at 3 or 4 days post-inoc ulation. The loss of stiffness in the pars tensa may affect the furthe r course of otitis media, and the 'weak spots' are possible precursors of retraction pockets and/or perforations. The stiffness reductions m ay be caused by changes on a sub-microscopical, molecular level.