Middle ear volume and pressure effects on tympanometric middle ear pressure determination: model experiments with special reference to secretory otitis media

Objective: Middle ear pressure (Pm) measured by tympanometry has revealed high negative values in patients with secretory otitis media (SOM) in contrast to direct measurement. This may be explained by errors in tympanometry caused by volume displacement of the tympanic membrane (TM) affecting the volume of the middle ear (Vm) and the Pm according to Boyle’s Law. Such errors are susceptible to the size of Vm. Methods: A realistic middle ear model based on previous clinical studies of normal pressure-volume relations of the middle ear system (MES) was constructed. In this model non-linear behaviour and hysteresis of the MES was imitated and Pm as well as Vm could be controlled. Results: Tympanometrically estimated Pm decreased on average 38 daPa, when Vm was changed from 21 to 1 cm3. The decrease was most pronounced, when Vm became smaller than 5 cm3. Moreover, tympanometry showed a linear numerical overestimation of Pm by a factor 2.31 compared with model Pm. Conclusion: A curve fit was derived describing the tympanometric Pm as a function of Vm. This demonstrated that tympanometric Pm approached −∞ daPa, when middle ear volume approached 0 cm3, which indicates that negative tympanometric recordings and B curves can be found in ears with normal Pm entirely due to very small Vm’s. This explains the discrepancy between direct and tympanometric measurements of Pm in SOM, since the effusion replaces the air filled expandable volume resulting in a very small ‘functional’ Vm. Numerical overestimation of Pm by tympanometry was explained by hysteresis, which reflected the viscoelastic properties of the MES. These results question the significance of negative Pm’s as a pathogenetic factor in SOM.