Effects of effusion in the middle ear and perforation of the tympanic membrane on otoacoustic emissions in guinea pigs.

The influence of fluid in the middle ear and of perforation and closure of the tympanic membrane (TM) on otoacoustic emissions (OAEs) was evaluated in guinea pigs. Click-evoked otoacoustic emissions (CEOAEs) and distortion product otoacoustic emissions (DPOAEs) were measured after the auditory bulla was opened. Neither OAE level changed significantly when fluid filled only half the space in the bulla, but both OAE levels disappeared when fluid completely filled the bulla. These changes were reversible. Thus, the presence of fluid in the bulla influenced CEOAE and DPOAE levels only when its volume filled more than half the space of the bulla. Changes in both CEOAE and DPOAE levels were affected by the size of the perforation of the tympanic membrane. For the smallest perforation, the reduction in both CEOAE and DPOAE levels was restricted to the lower frequencies. However, as the size of the perforation increased, a decrease in high-frequency function occurred. Thus, the results indicate that the magnitude of both OAEs was proportional to the size of the perforation. Both OAE levels improved after the perforation was closed. Because CEOAEs were more sensitive than DPOAEs to perforation and closure of the TM, DPOAEs may be better suited for OAE measurement in ears with perforated TMs.