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

Superior semicircular canal dehiscence (SCD) syndrome is a recently recognized entity where patients often present with vestibular symptoms typically provoked by sound and/or static pressure (1,2). As clinical experience with SCD accrues, it has become apparent that some patients with SCD present with unexplained air-bone gaps of 5 to 50 dB, with or without vestibular symptoms (3–6). Our investigations and those of others have shown that the clinical presentation in such cases can so closely mimic that observed in otosclerosis that surgical exploration and unsuccessful stapedectomy procedures have been performed mistakenly in some instances (3–5). It is somewhat puzzling that an air-bone gap on audiometry can be produced by a pathologic condition affecting the inner ear rather than the middle ear. Definitive proof that the air-bone gap in these patients is indeed the result of the SCD is the closure of the gap when the dehiscence is repaired (3,7). Figure 1A shows a preoperative audiogram from a 35-year-old patient with SCD who presented with both vertigo and an air-bone gap. The SCD was repaired via a middle-fossa approach, wherein the SCD was plugged with bone wax and temporalis fascia. Figure 1B is an audiogram performed 3 months postoperatively showing closure of the air-bone gap. 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 ... Our research group and others have proposed a mechanism to explain the air-bone gap in SCD (1,2,5,8). The SCD is hypothesized to introduce a mobile “third window” into the inner ear (in addition to the oval and round windows). In the normal ear, air-conducted sound sets the stapes into back-and-forth motion as a result of the motion of the tympanic membrane, malleus, and incus (Fig. 2A). Because the normal vestibular system has no window, no sound-induced fluid motion is produced within the semicircular canals. In the presence of an SCD (Fig. 2B), it is hypothesized that a part of the fluid volume displaced by the oscillating stapes is shunted through the opened superior canal, resulting in measurable fluid velocity within the dehiscence. Inasmuch as the inner-ear fluid volume is displaced away from the cochlea, this results in a decrease in the sound activating the cochlea and an elevation of air-conducted hearing thresholds, which should manifest as a decrease in round-window velocity. The third-window hypothesis also predicts that the shunt path through the SCD will lower the inner-ear input impedance, which should result in increased stapes and umbo velocities. 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 ... The goals of the present study were (1) to develop a cadaveric temporal-bone preparation of SCD to allow us to investigate the pathways of air-conducted sound transmission in SCD and test the predictions of the third-window hypothesis and (2) to assess the potential clinical usefulness of measurements of umbo velocity using laser-Doppler vibrometry as a diagnostic test for patients with SCD who present with an air-bone gap.