Measurements of human middle- and inner-ear mechanics with dehiscence of the superior semicircular canal

Abstract

Objectives: (1) To develop a cadaveric temporal-bone preparation to study the mechanism of hearing loss resulting from superior semicircular canal dehiscence (SCD) and (2) to assess the potential usefulness of clinical measurements of umbo velocity for the diagnosis of SCD.

Background: The syndrome of dehiscence of the superior semicircular canal is a clinical condition encompassing a variety of vestibular and auditory symptoms, including an air-bone gap at low frequencies. It has been hypothesized that the dehiscence acts as a "third window" into the inner ear that shunts acoustic energy away from the cochlea at low frequencies, causing hearing loss.

Methods: Sound-induced stapes, umbo, and round-window velocities were measured in prepared temporal bones (n = 8) using laser-Doppler vibrometry (1) with the superior semicircular canal intact, (2) after creation of a dehiscence in the superior canal, and (3) with the dehiscence patched. Clinical measurements of umbo velocity in live SCD ears (n = 29) were compared with similar data from our cadaveric temporal-bone preparations.

Results: An SCD caused a significant reduction in sound-induced round-window velocity at low frequencies, small but significant increases in sound-induced stapes and umbo velocities, and a measurable fluid velocity inside the dehiscence. The increase in sound-induced umbo velocity in temporal bones was also found to be similar to that measured in the 29 live ears with SCD.

Conclusion: Findings from the cadaveric temporal-bone preparation were consistent with the third-window hypothesis. In addition, measurement of umbo velocity in live ears is helpful in distinguishing SCD from other otologic pathologies presenting with an air-bone gap (e.g., otosclerosis).

FIG. 1

(A), Preoperative audiogram of a 35-year-old woman with vertigo and left-sided conductive hearing loss due to an SCD. There is a low-frequency air-bone gap. (B), Postoperative audiogram of the SCD patient shown in A 3 months after a middle-fossa procedure wherein the SCD was repaired with bone wax and temporalis fascia. Note the resolution of the air-bone gap (courtesy of Dr. Dennis Poe).

FIG. 2

(A), Schematic representation of air-conducted sound transmission in the normal ear. In the normal ear, air-conducted sound via the ear canal produces motion of the tympanic membrane and ossicles that sets the stapes into back-and-forth motion. The resulting pressure difference between the oval and round windows leads to motion of the cochlear partition, resulting in perception of air-conducted sound. Because the bony walls of the inner ear and the cochlear fluid are essentially incompressible, the net inward motion of the stapes is balanced by an equal outward motion of the round window. The endolymphatic compartment is shaded in light gray. The perilymphatic compartment is in white. (B), Schematic representation of the mechanism of hearing loss for air-conducted sounds due to SCD. In the presence of an SCD, it is hypothesized that some fraction of the fluid volume displaced by the oscillating stapes is shunted through the dehiscent canal away from the cochlea, resulting in a decrease in the sound activating the cochlea and an elevation of air-conducted hearing thresholds. Thus, a decrease in round-window motion is predicted. The SCD is also predicted to lower the input impedance of the inner ear, which should result in an increased motion of the stapes and, in turn, an increased motion of the umbo.

FIG. 3

Schematic drawing showing the temporal-bone preparation to explore the effects of SCD. A sound source was placed in the external auditory canal. An SCD was created in the anterior limb of the superior canal. Umbo velocity (Vu) was measured on the tympanic membrane via an opening in the anterior wall of the external auditory canal. Stapes velocity (Vs) and round-window velocity (Vrw) were measured through a posterior tympanotomy/facial recess approach. The velocity of fluid within the exposed dehiscence (Vscd) was also measured.

FIG. 4

Sound-induced fluid velocity within the SCD (Vscd), normalized by ear canal sound pressure in eight bones (solid curves): magnitude (top) and phase (bottom). The dashed curve is the mean velocity measured at the same location after the SCD was patched in the eight bones, and the dotted curve represents the mean noise level determined by measuring the sound-induced motion of the temporal bones.

FIG. 5

(A), Round-window velocity (Vrw) normalized by ear-canal sound pressure in a typical bone in the baseline condition (solid curve), after creating a 2-mm² SCD (dot-dashed curve), and after the SCD was patched (dotted curve). Top: magnitude; bottom: phase. (B), Ratio of Vrw with SCD to baseline (ΔVrw) in eight bones (dotted curves). Top: magnitude in dB; bottom: phase. The solid curve and shaded area are the mean and the 95% CI.

FIG. 6

(A), Stapes velocity (Vs) normalized by sound pressure in a typical bone in the baseline condition (solid curve), after creating a 2-mm² SCD (dot-dashed curve), and after the SCD was patched (dotted curve). Top: magnitude; bottom: phase. (B), Ratio of Vs with SCD to baseline (ΔVs) in eight bones (dotted curves). Top: magnitude in dB; bottom: phase. The solid curve and shaded area are the mean and the 95% CI.

FIG. 7

(A), Umbo velocity (Vu) normalized by sound pressure in a typical bone in the baseline condition (solid curve), after creating a 2-mm² SCD (dot-dashed curve), and after the SCD was patched (dotted curve). Top: magnitude; bottom: phase. (B), Ratio of Vu with SCD to baseline (ΔVu) in eight bones (dotted curves). Top: magnitude in dB; bottom: phase. The solid curve and shaded area are the mean and the 95% CI.

FIG. 8

Ratio of mean Vu in 29 live ears with SCD normalized by the mean of 56 normal subjects (solid curve with circles), as well as the mean ΔVu and its 95% CI of cadaveric ears with SCD obtained from Figure 7B. Top: magnitude in dB; bottom: phase.