Tympanic membrane changes in experimental purulent otitis media

The stiffness properties of the gerbil tympanic membrane (TM) were investigated during the early course of experimental purulent otitis media produced by middle ear inoculation (n = 49) with type 3 Streptococcus pneumoniae. In a first approach the acoustic admittance and susceptance 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 tympanic membrane was made concurrently. The acoustic admittance and susceptance reduced during the initial days post-inoculation. These reductions seem to correlate with an oedema simultaneously developing in the pars tensa. The pressure-induced displacement increased with time of disease, i.e. the mechanical stiffness of the TM reduced. Local deformation zones, or 'weak spots' appeared in the inferior half of the pars tensa in three out of eight cases measured at 3 or 4 days post-inoculation. The loss of stiffness in the pars tensa may affect the further course of otitis media, and the 'weak spots' are possible precursors of retraction pockets and/or perforations. The stiffness reductions may be caused by changes on a sub-microscopical, molecular level.