Tympanometric experiments on retracted ear drums--does tympanometry reflect the true middle ear pressure?

Tympanometry in ears with retracted or partially atelectatic tympanic membranes does not reflect the true middle ear (ME) pressure. The position of the tympanogram peak depends on the size of a retraction pocket and the remaining ME gas volume. Thus tympanometry in such cases cannot be used for measurements of ME pressure.

To prove the hypothesis that tympanometry in ears with retraction pockets or atelectasis does not measure ME pressure correctly.

Tympanometry was performed in a simplified artificial ME model, in which different 'tympanic membranes' with pockets could be integrated.

We found a shift of the tympanogram peaks, either towards negative or positive pressures, depending on the position of the pocket: in case of an 'inward' direction (towards the ME like a retraction pocket) the peak was in the negative pressure range, in case of an 'outward' direction (bulging into the ear canal) the shift was towards positive pressures. The shift increased with decreasing volume behind the tympanic membrane. The effect cannot be simply be explained by hysteresis but by the air 'cushion' that is 'trapped' behind the membrane, limiting its excursions during the tympanometric sweep.