Effects of acute negative middle ear pressure on hearing. New answers to old questions and a review of the literature

Structural stiffening in the human middle ear due to static pressure: Finite-element analysis of combined static and dynamic middle-ear behavior

The vibration response of the middle ear (ME) to sound changes when static pressure gradients are applied across the tympanic membrane (TM). To date, it has not been well understood which mechanisms lead to these changes in ME vibration response. In this study, a 3D finite-element model of the human ME was developed that simulates the sound-induced ME vibration response when positive and negative static pressures of up to 4 kPa are applied to the TM. Hyperelasticity of the soft-tissue components was considered to simulate large deformations under static pressure. Some ME components were treated as viscoelastic materials to capture the difference between their static and dynamic stiffness, which was needed to replicate both static and dynamic ME behavior. The change in dynamic stiffness with static preload was simulated by linearization of the hyperelastic constitutive model around the predeformed state. For the preloaded harmonic response, we found that the statically deformed ME geometry introduced asymmetry in the vibration loss between positive and negative pressure, which was due to the TM cone shape. As opposed to previous assumptions, the prestress in the ME due to static pressure had a substantial impact on the vibration response. We also found that material nonlinearity led to a higher stiffening at the umbo but a less pronounced stiffening at the footplate compared to the linear elastic condition. The results suggest that flexibility of the incudomalleolar joint (IMJ) enhances the decoupling of static umbo and footplate displacements, and that viscosity and viscoelasticity of the IMJ could play a role in the transfer of sound-induced vibrations from the umbo to the footplate. The components of the incudostapedial joint had minimal effect on ME mechanical behavior.

Stapedial Reflex and Ears With High Static Acoustic Admittance

Purpose

To evaluate modified acoustic reflex diagnostic protocols for a group of individuals ( n = 9) with high peak compensated static acoustic admittance (Y tm ) tympanograms.

Method

A modified procedure designed to improve acoustic stapedius reflex threshold (ASRT) measurements in individuals with high-admittance tympanograms was employed in both an experimental and a control group. ASRTs were measured at 0.5, 1.0, and 2.0 kHz, ipsilateral and contralateral. Measurements were obtained within each condition for 7 ear canal pressures that were set to 0, ±50, ±100, and ±150 daPa (relative to tympanometric peak pressure [TPP]).

Results

Though measuring ASRTs at −50 daPa (relative to TPP) in the high-admittance and normal groups did not result in significantly better thresholds than at TPP, the absent reflex rate was reduced when the ear canal pressure was changed by −50 daPa during ASRT measurements.

Conclusions

Based on this sample, it is suggested that a patient presenting with high peak compensated static acoustic admittance (peak Y tm ≥ 2.1 mmho) undergo ASRT evaluation with the ear canal pressure set to −50 daPa (relative to TPP).

Effect of middle ear pressure change on middle ear mechanics

The effect of graded variations in middle ear pressure on ossicular vibration was measured in 15 normal human temporal bone specimens. The displacement amplitude of the umbo and stapes head was measured at 16 frequencies between 0.2 kHz and 3.5 kHz at a constant sound pressure of 134 dB SPL at the tympanic membrane (TM) using a non-contacting video measuring system. Both negative and positive pressures decreased umbo and stapes vibration at low frequencies and slightly increased the vibration at higher frequencies. The effects were greater for negative pressure than for positive pressure. The change in stapes vibration was less than that of the umbo at low frequencies, but increased at higher frequencies. In some temporal bones, a small positive pressure produced improvement in stapes vibration at all frequencies. These effects were thought to be primarily due to an increased stiffness of the TM and a damping of ossicular vibration, due to stretching of the ossicular suspensory ligaments and the annular ligament of the footplate.

Effects of middle ear pressure on frequency representation in the central auditory system

Changes in middle ear pressure (MEP) are known to produce an attenuation of sound transmission through the outer and middle ear, but the effects on frequency representation in the auditory system have not previously been studied. This issue is of particular interest because of changes in MEP occurring during episodes of otitis media. We have investigated the effect of changes in MEP on the tuning of neurons in the inferior colliculus (IC) of the gerbil to calibrated tone stimulation of the contralateral, pressurized ear. Both negative and positive non-atmospheric MEP produced an elevation of neural thresholds that was inversely related to IC neuron best frequency (BF). A robust, linear relationship was found between BF at atmospheric MEP (control) and BF at -20 daPa MEP. Higher resolution analysis was performed on a sub-sample of neurons that had particularly stable BFs with repeated, control MEP. For the majority of these neurons, alternation of MEP between control and -20 daPa had no effect on BF. However, a few neurons showed small (up to 5%), significant shifts in BF with -20 daPa MEP. These results are consistent with previous reports of the effects of MEP on spontaneous otoacoustic emissions. We conclude that non-atmospheric MEP acts as a high-pass filter on the input to the cochlea, but does not change the frequency organization of the auditory system to any marked extent.

Middle ear pressures in patients with nasopharyngeal carcinoma and their clinical significance

There is a paucity of studies investigating middle ear pressures (MEPs) in patients with nasopharyngeal carcinoma (NPC). This paper prospectively examines MEPs in patients with nasopharyngeal carcinoma before and after radiotherapy and discusses their clinical significance. Newly diagnosed patients with nasopharyngeal carcinoma were studied before and at three to 12 months (mean 7.5 months) after radiotherapy. MEPs were measured by tympanometry. Thirty-three patients completed the study. The mean MEP before and after radiotherapy was -55.2 mm water (range -250 to 45 mm water) and -73.1 mm water (range -215 to 35 mm water) respectively. About two-thirds of assessable ears had an increase in negative MEPs after irradiation and the rest had less negative MEPs after irradiation. Those ears which developed post-irradiation middle ear effusions were found to have pre-irradiation negative middle ear pressures of at least -45 mm water. These findings provide possible explanations for some unexplained audiometric observations previously made in patients with nasopharyngeal carcinoma after radiotherapy. Tympanometry before radiotherapy may prove to be useful in identifying ears with a high risk of developing post-irradiation middle ear effusion.