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 (P(m)) 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 (V(m)) and the P(m) according to Boyle's Law. Such errors are susceptible to the size of V(m).


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 P(m) as well as V(m) could be controlled.


RESULTS
Tympanometrically estimated P(m) decreased on average 38 daPa, when V(m) was changed from 21 to 1 cm(3). The decrease was most pronounced, when V(m) became smaller than 5 cm(3). Moreover, tympanometry showed a linear numerical overestimation of P(m) by a factor 2.31 compared with model P(m).


CONCLUSIONS
A curve fit was derived describing the tympanometric P(m) as a function of V(m). This demonstrated that tympanometric P(m) approached -infinity daPa, when middle ear volume approached 0 cm(3), which indicates that negative tympanometric recordings and B curves can be found in ears with normal P(m) entirely due to very small V(m)'s. This explains the discrepancy between direct and tympanometric measurements of P(m) in SOM, since the effusion replaces the air filled expandable volume resulting in a very small 'functional' V(m). Numerical overestimation of P(m) by tympanometry was explained by hysteresis, which reflected the viscoelastic properties of the MES. These results question the significance of negative P(m)'s as a pathogenetic factor in SOM.
Gaihede
M
M
Department of Otolaryngology, Holstebro Central Hospital, DK-7500, Holstebro, Denmark. mgaihede@dadlnet.dk
D000158:Acoustic Impedance Tests; D004432:Ear, Middle / Q000503:physiopathology*; D006801:Humans; D008954:Models, Biological; D010034:Otitis Media with Effusion / Q000503:physiopathology*; D011312:Pressure