Hearing loss in relation to physical properties of middle ear effusions

Hearing loss was measured in cats after ligation of the eustachian tube (ET) on one side to produce middle ear effusions. Auditory nerve responses to broad-band clicks were recorded from the ear canal. Plots of N1 response amplitude and latency versus click sound pressure level (SPL) were constructed. Hearing loss was determined as the average displacement along the SPL axis of the latency plot, relative to a previously recorded control series. Viscosity of effusion was measured with a cone and plate viscometer. Specific gravity was measured using a 1 cc pycnometer. Middle ear pressure was assessed tympanographically. After ET ligation, all cats developed negative middle ear pressure within three days and flat (type B) tympanograms within seven days. Hearing loss also developed early and became maximal at about three weeks. Tympanocentesis yielded either thin fluid or glue-like effusions in nearly equal numbers. The data indicate there is not a significant difference between the amount of hearing loss associated with thin fluids or with glue. Viscosity is not clearly related to the duration of time the fluid is present in the middle ear. There is a significant positive correlation between the amount of hearing loss and the volume of fluid present in the middle ear. Thus, the amount of hearing loss appears to depend only on the volume of effusion and is not significantly dependent on other physical properties of the fluid.