Bone vibration-induced nystagmus is useful in diagnosing superior semicircular canal dehiscence

To explore vestibular integrity by bone vibration-induced nystagmus (VIN) during bedside examination. Disease or dysfunction of vestibular end organs would reduce or eliminate their contribution to total eye movement response to VIN.

It is assumed that vibration of the mastoid (at a frequency of 100 Hz) stimulates all vestibular end organs (semicircular canals and otolith structures). Previous studies have described oculomotor responses to vestibular activation by vibratory stimulus. Stimulation of individual semicircular canals produces eye movement in the plane of the stimulated canal. Vibratory stimulation of otolith structures (utricular macula) produces changes in ocular torsional position. Superior semicircular canal dehiscence (SCD) is responsible for nontypical cochleovestibular symptoms. Diagnosis is difficult without resorting to imaging, high-resolution computed tomography (HRCT) being the only tool providing reliable assessment. To avoid unnecessary referral for expensive imaging, it would be useful to have a simple screening test to select cases for HRCT.

Video eye movements were recorded in complete darkness in patients diagnosed with SCD. The eye movement responses to long-lasting unilateral vibratory stimulation applied to the mastoid surface were evaluated.

VIN evoked a response in all cases, mostly demonstrating excitation of the affected side. The pattern of vertical, torsional and horizontal eye velocity and eye position was evaluated with three-dimensional infrared video-oculography (50 Hz sampling).

Analysis of VIN, recently proposed to study transmission of excitatory stimuli by bone conduction, may be appropriate for altered immittance caused by dehiscence. This promises to be an interesting new field of research.