Intracochlear Pressures in Simulated Otitis Media With Effusion: A Temporal Bone Study

Abstract

Hypothesis: Simulated otitis media with effusion reduces intracochlear pressures comparable to umbo velocity.

Background: Otitis media with effusion is a common cause of temporary hearing loss, particularly in children, producing deficits of 30 to 40 dB. Previous studies measured the effects of simulated effusion on ossicular mechanics; however, no studies have measured cochlear stimulation directly. Here, we compare pressures in the scala vestibuli and tympani to umbo velocity, before and after induction of simulated effusion in cadaveric human specimens.

Methods: Eight cadaveric, hemi-cephalic human heads were prepared with complete mastoidectomies. Intracochlear pressures were measured with fiber optic pressure probes, and umbo velocity measured via laser Doppler vibrometry (LDV). Stimuli were pure tones (0.1-14 kHz) presented in the ear canal via a custom speculum sealed with a glass cover slip. Effusion was simulated by filling the mastoid cavity and middle ear space with water.

Results: Acoustic stimulation with middle ear effusion resulted in decreased umbo velocity up to ∼26 dB, whereas differential pressure (PDiff) at the base of the cochlea decreased by only ∼16 dB.

Conclusion: Simulating effusion leads to a frequency-dependent reduction in intracochlear sound pressure levels consistent with audiological presentation and prior reports. Results reveal that intracochlear pressure measurements (PSV and PST) decrease less than expected, and less than the decrease in PDiff. The observed decrease in umbo velocity is greater than in the differential intracochlear pressures, suggesting that umbo velocity overestimates the induced conductive hearing loss. These results suggest that an alternate sound conduction pathway transmits sound to the inner ear during effusion.

Figure 1

A) Illustration of the experimental setup and measurement procedures. Each ear was prepared with a mastoidectomy, atticotomy, and extended facial recess to expose the cochlear promontory and ossicular chain. Stainless steel guide tubes for P_SV and P_ST fiber optic pressure sensors, and a stainless -steel cannula for fluid insertion, were implanted in dental cement in appropriate orientations to access appropriate middle ear structures. A stainless steel aural speculum was modified to accept speaker and microphone probe tubing, and the opening covered with a microscope cover glass to provide a sealed acoustical environment. Umbo velocity was measured with an LDV directed through the cover glass. B) photograph of the speculum, microphone, speaker, and cannula tubing, relative to the ear canal and mastoidectomy. C) photomicrograph showing the intracochlear pressure probe (P_SV & P_ST) locations, relative to the stapes and round window. Retroreflective glass beads are visible on the stapes, and alginate dental impression material is visible at the base of each pressure probe.

Figure 2

Baseline stapes velocity acoustic functions (H_Stapes = V_Stapes/P_EAC) for each specimen. Gray bands represent the range of responses reported previously (24).

Figure 3

Baseline acoustic function magnitudes for individual specimen, calculated for umbo velocity, scala vestibuli pressure, scala tympani pressure, and the differential intracochlear pressure. Gray bands represent the range of responses reported previously (H_Umbo (10); H_SV/ST/Diff (21)).

Figure 4

Top: Mean (± standard error of the mean, SEM) transfer function magnitudes (i.e. response magnitude normalized to sound pressure level in the ear canal) recorded before (Baseline) and after insertion of water into the middle ear (Effusion), as a function of stimulation frequency. Responses are superimposed onto the range of responses observed in prior literature (same as in Fig. 3). Bottom: the change in response magnitude following insertion of water into the middle ear (in dB re: Baseline). Frequency bands assessed in statistical comparisons are indicated at the top of the each axis, and significant changes during effusion relative to baseline (assessed with Student’s t-tests) are indicated with an asterisk.