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Middle ear volume and pressure effects on tympanometric middle ear pressure determination: model experiments with
1Department of Otolaryngology, Holstebro Central Hospital, DK-7500, Holstebro, Denmark. mgaihede@dadlnet.dk
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).
Conclusion:
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.
Insights
Tympanometry may inaccurately show negative middle ear pressure in secretory otitis media due to small middle ear volume. This study explains discrepancies and questions the role of negative pressure in the condition.
Area of Science:
- Otolaryngology
- Biophysics
- Medical Acoustics
Background:
- Tympanometry measures middle ear pressure (P(m)) but often shows high negative values in secretory otitis media (SOM).
- This contrasts with direct measurements and may stem from tympanometric errors related to middle ear volume (V(m)) and Boyle's Law.
- The accuracy of tympanometry is influenced by the V(m), particularly its size.
Purpose of the Study:
- To investigate the impact of middle ear volume (V(m)) on tympanometric measurements of middle ear pressure (P(m)).
- To explain the discrepancy between tympanometric and direct P(m) measurements in patients with SOM.
- To evaluate the role of negative P(m) as a pathogenetic factor in SOM.
Main Methods:
- Construction of a realistic middle ear model (MES) incorporating non-linear behavior and hysteresis.
- Simulation of varying middle ear pressure-volume relationships within the model.
- Controlled manipulation of V(m) and P(m) to assess tympanometric accuracy.
Main Results:
- Tympanometric P(m) decreased significantly with decreasing V(m), especially below 5 cm³.
- A numerical overestimation of P(m) by a factor of 2.31 was observed with tympanometry compared to the model.
- A derived curve fit indicated tympanometric P(m) approaches negative infinity as V(m) approaches zero.
Conclusions:
- Very small V(m) can lead to negative tympanometric readings and B curves even with normal P(m).
- The effusion in SOM reduces functional V(m), explaining discrepancies in P(m) measurements.
- Hysteresis in the MES accounts for numerical overestimation of P(m) by tympanometry, questioning the pathogenetic significance of negative P(m) in SOM.