Tympanometry versus direct middle ear pressure measurement in an artificial model: is tympanometry an accurate method to measure middle ear pressure?

Abstract

Indirect tympanometric pressure measurements of the middle ear (ME) do not correspond to direct pressure measurements. Tympanometry is an accepted method for estimating ME pressures. These pressure values are different, however, when measurements are made directly. Negative pressures were induced by volume changes in an artificial ME model with various "mastoid" volumes. Both tympanometric and direct pressure measurements were obtained simultaneously. A substantial difference was demonstrated between tympanometric and direct pressure measurements. There was a linear correlation between tympanometry and direct pressures between -46.8 and -93.6 mmH2O, with a tympanometric overestimation of 40% to 20%, respectively. The smaller the "mastoid" volume, the greater the overestimation. The correlation was not linear for direct pressures between zero and -46.8 mmH2O. Tympanometric overestimation was greatest (i.e., 4-14-times the actual pressure) in the pressure range between zero and -12 mmH2O. Tympanometry does not yield precise ME pressure values. In a model with a larger "mastoid" and an actual pressure range of -46.8 to -93.6 mmH2O tympanometric readings are somewhat less unreliable (error of 40%-20%). However, tympanometry is in error of more than 400% in a model with a small mastoid in which the actual ME pressure is only slightly negative (i.e., range between -3.9 and -46.8 mmH2O). Therefore, this study implies that tympanometry has little use for measuring ME pressure especially in chronic ears or their sequelae that are usually characterized by having small negative pressures and small mastoids.