A superior semicircular canal dehiscence-induced air-bone gap in chinchilla

Abstract

An SCD is a pathologic hole (or dehiscence) in the bone separating the superior semicircular canal from the cranial cavity that has been associated with a conductive hearing loss in patients with SCD syndrome. The conductive loss is defined by an audiometrically determined air-bone gap that results from the combination of a decrease in sensitivity to air-conducted sound and an increase in sensitivity to bone-conducted sound. Our goal is to demonstrate, through physiological measurements in an animal model, that mechanically altering the superior semicircular canal (SC) by introducing a hole (dehiscence) is sufficient to cause such an air-bone gap. We surgically introduced holes into the SC of chinchilla ears and evaluated auditory sensitivity (cochlear potential) in response to both air- and bone-conducted stimuli. The introduction of the SC hole led to a low-frequency (<2000 Hz) decrease in sensitivity to air-conducted stimuli and a low-frequency (<1000 Hz) increase in sensitivity to bone-conducted stimuli resulting in an air-bone gap. This result was consistent and reversible. The air-bone gaps in the animal results are qualitatively consistent with findings in patients with SCD syndrome.

Figure 1

Cochlear partition response to compressional bone conducted sound if A) the RW and OW are of equal impedance, B) the RW and OW have different impedances, C) the fluid volume of the vestibule and semicircular canals is taken into consideration. Note that the largest cochlear partition motion is in response to greatly different impedances between the SV and ST. The solid lines in the figure represent the normal state of the inner ear and the dashed lines represent the compressed inner ear. Adapted from Békésy Figure 6-17(Békésy, 1960, p.145).

Figure 2

A schematic illustrating the location of the SCD and superior canal in the middle-ear air space; the placement of earphone in the ear canal; the placement of the CP electrode in the round window niche; and the BC stimulator affixed to the skull.

Figure 3

A) The CP recorded from an ear in the patch-unpatch (SCD Open)-repatch states. A low frequency increase in CP is observed between 250 Hz and 500 Hz. A shift in phase is also observed above 1500 Hz in the SCD open condition. B) The CP in response to BC after the data is smoothed using the loess smoothing algorithm.

Figure 4

The mean change in BC-CP between A) the patched and repatched conditions and B) the SCD open and patched conditions. The 95% confidence intervals as well as the means for both magnitude and phase are plotted (n=7). The large 95% confidence intervals around the phases in Figure 4A at low and high frequencies are, in part, due to uncertainties in phase unwrapping at the lowest and highest frequencies.

Figure 5

The effect of SCD on bone-conducted cochlear potential (n=20). The mean CP from seven ears in which reversibility was established as well as the mean CP from 13 ears in which reversibility was not determined are shown. The mean and 95% confidence interval of the pooled data is also illustrated.

Figure 6

The effect of dehiscence size on CP in response to bone conducted sound stimuli. A) CP in response to three different dehiscence sizes in an example ear. B) the mean effect of SCD size on BC-CP (n=5).

Figure 7

Changes in CP in response to both AC and BC sound stimuli in an example ear. A) A reversible decrease in low-frequency sensitivity to BC sound is observed. B) A reversible increase in sensitivity to AC sound is observed.

Figure 8

The mean change in AC-CP/P_TM. A) between the patch and repatch state and B) between the SCD open and the patched state along with the 95% confidence intervals (n=7).

Figure 9

The mean air-bone gap with the 95% confidence intervals (n=7).