Tympanometry accurately measures middle ear underpressures in monkeys

Tympanometry is useful for evaluating middle ear (ME) status, but its accuracy in estimating true ME pressure has been questioned. We evaluated the accuracy of tympanometry in 6 monkeys. Direct application and measurement of ME pressure were achieved with a probe introduced into the mastoid antrum, and tympanometry was done over a large range of applied ME pressures. For all ears, tympanometric pressure was a linear function of applied pressure. At large overpressures, the tympanometric pressure was approximately 40 mm H2O greater than the applied pressure, but there was little error in the measurement for applied underpressures. The measurement error was proportional to the ME pressure multiplied by the ratio of the extant volume displacement of the tympanic membrane to ME volume. These results show that in monkeys, tympanometry provides an accurate, relatively unbiased estimate of ME underpressure and suggest that the measurement error for tympanometry can be predicted for MEs of other species.