Variation in gross and histological appearance of the canine pars flaccida

The tympanic membrane is divided into the pars flaccida (PF) and pars tensa. Otoscopic differences in the appearance of the PF in the dog (flat vs. bulging) have been reported anecdotally. The purpose of this study was to histologically examine the canine PF to determine if differences exist between a flat and a bulging PF. We hypothesized that differences in the PF may be structural or due to an increased pressure in the middle ear. Four adult canine cadavers were used (two had bilateral bulging PF, two had bilateral flat PF). The ear specimens were fixed in 10% buffered neutral formalin, decalcified, routinely processed and stained with haematoxylin and eosin, toluidine blue (staining for mast cells), and Verhoeff-van Gieson (staining for elastic fibres). One blinded investigator evaluated each PF histologically. Seven ears (three with bulging PF, four with flat PF) were evaluated in the study. The PF was identified in all seven ears. No histological differences that distinguished a bulging PF from a flat PF were identified. Six ears were evaluated for the presence of mast cells and elastic fibres. Less than one mast cell per x40 field was identified in the PF in five of six ears. No elastic fibres were identified in any of the six PF. Based on the results of this study, it appears unlikely that there is a structural difference that accounts for a bulging PF. Therefore, there may be increased middle ear pressure in dogs with a bulging PF.