Three-Dimensional Vibration-Induced Vestibulo-ocular Reflex Identifies Vertical Semicircular Canal Dehiscence

Abstract

Vertical semicircular canal dehiscence (VSCD) due to superior canal dehiscence (SCD) or posterior canal dehiscence (PCD) of the temporal bone causes vestibular and cochlear hypersensitivity to sound. This study aimed to characterize the vibration-induced vestibulo-ocular reflex (ViVOR) in VSCD. ViVORs in one PCD and 17 SCD patients, confirmed by CT imaging reformatted in semicircular canal planes, were measured with dual-search coils as binocular three-dimensional eye rotations induced by skull vibrations from a bone oscillator (B71-10 ohms) at 7 ms, 500 Hz, 135-dB peak-force level (re: 1 μN). The ViVOR eye rotation axes were computed by vector analysis and referenced to known semicircular canal planes. Onset latency of the ViVOR was 11 ms. ViVOR from VSCD was up to nine times greater than normal. The ViVOR's torsional rotation was always contraversive-torsional (the eye's upper pole rotated away from the stimulated ear), i.e. its direction was clockwise from a left and counterclockwise from a right VSCD, thereby lateralizing the side of the VSCD. The ViVORs vertical component distinguishes PCD from SCD, being downwards in PCD and upwards in SCD. In unilateral VSCD, the ViVOR eye rotation axis aligned closest to the dehiscent vertical semicircular canal axis from either ipsilateral or contralateral mastoid vibrations. However, in bilateral VSCDs, the ViVOR eye rotation axis lateralized to the ipsilateral dehiscent vertical semicircular canal axis. ViVOR was evoked in ossicular chain dysfunction, even when air-conducted click vestibulo-ocular reflex (VOR) was absent or markedly reduced. Hence, ViVOR could be a useful measurement to identify unilateral or bilateral VSCD even in the presence of ossicular chain dysfunction.

Costimulation of the Horizontal Semicircular Canal during Skull Vibrations in Superior Canal Dehiscence Syndrome

“…The presence of horizontal components during vibration using a hand-held vibrator was also described in previous studies, although the intensities were variable. White et al [2007] reported that 2 of 4 unilateral SCD patients, who were confirmed by anatomical and functional studies, showed strong horizontal nystagmus -8°/s and 20°/s, respectively -beating to the SCD side during vibration stimulation, while mastoid vibration unpredictably induced either excitatory or inhibitory horizontal nystagmus [Manzari et al, 2008] or the SPV of the horizontal component of the nystagmus was negligible [Aw et al, 2011]. We also observed inhibitory horizontal nystagmus in 1 patient and negligible in 2 of the 10 subjects.…”

“…The vibrator used in this study was a commercially available and readily accessible device, which is appropriate for office examinations with Frenzel goggles. The nystagmus was sufficiently strong to be visible under Frenzel goggles, while the excursion of the eyeball observed in the study by Aw et al [2011], by bone oscillator, was at best 0.2°, which could not be recorded without a magnetic search coil system. Thus, the methods in our study can be considered more practical, providing adjuvant information to assist diagnosis of SCD syndrome; that is, the lesion side can be suspected when fast components of ViVOR direct to the side of autophony and lowfrequency conductive hearing loss.…”

“…This was also true in SCD patients in our study and previous reports. Aw et al [2011] also advocated the efficacy of ViVOR for identifying unilateral or bilateral vertical semicircular canal dehiscence. The ViVOR was consistent with excitation of the affected-side SC and is evoked in ossicular chain dysfunction, even when air-conducted click VOR was absent or markedly reduced.…”

“…Vestibular and neck muscle spindle afferents could be variably stimulated by mastoid vibration, depending on the type of vibrator and the characteristics of the vibration stimuli (such as frequency, size of contact area, pressure over the contact area, battery-powered or AC-driven). Mastoid or skull vibration also induces variable amounts of eye movement, although mostly weak nystagmus, even in healthy subjects without SCD, depending on the area of vibration and type of vibrator [Aw et al, 2011; al. , 2007].…”

“…It has also been tested in patients with SCD [Aw et al, 2011;Manzari et al, 2008;White et al, 2007]. Previous studies using vibration stimuli demonstrated relatively stronger downbeat torsional nystagmus with a few or absent horizontal components in most cases, which is apparently different from the nystagmus in unilateral vestibular loss patients [Aw et al, 2011;Manzari et al, 2008;White et al, 2007].…”