Costimulation of the horizontal semicircular canal during skull vibrations in superior canal Dehiscence syndrome.

A sound- and pressure-induced vestibuloocular reflex (VOR) has been described as vertical and torsional in superior canal dehiscence (SCD), and the rotational axes of induced VOR have been assumed to fit with the axis of the affected superior semicircular canal (SC). However, it has been difficult to characterize the pattern of vibration-induced VOR (ViVOR). We aimed to characterize the pattern of ViVOR by comparing the intensity and the axis of ViVOR with several clinical parameters of SCD. Ten symptomatic SCD patients were recruited. SCD size and location were measured on a reformatted image in the plane of the SC. Unilateral vibratory stimulation (100 Hz) was applied to the mastoid surface. ViVOR were recorded using 3D videooculography. The median 3D velocity of ViVOR was measured and the 3D vector trajectory plotted for reference against the axes of the human semicircular canals. A correlation between the magnitude of ViVOR and the size of SCD was evaluated. We also compared the location of SCD with the vertical-to-torsional component ratio of the ViVOR. ViVOR were present in 7 patients; 6 patients showed a substantial horizontal component in the excitatory direction in addition to strong torsional and weak vertical components. The computed rotational axes of ViVOR were located mostly between the axes of the ipsilateral SC and horizontal canal (HC) with a variable deviation to the axis of the ipsilateral posterior canal (PC). The magnitude of ViVOR was not related to the size of the SCD. The vertical-to-torsional component ratio of ViVOR tended to decline as the dehiscence was closer to the common crus. In SCD, mastoid vibration may stimulate the affected-side HC and PC as well as the SC. SCD can be suspected when excitatory horizontal torsional ViVOR direct to the side of the auditory symptoms.