Bone Vibration-Induced Nystagmus Is Useful in Diagnosing Superior Semicircular Canal Dehiscence

Abstract

Objective: 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. Background: 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. Methods: 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. Results: 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). Conclusion: 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.

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

“…Our study again confirmed that normal subjects generated a miniscule downward ViVOR. However, ViVOR from VSCD was enlarged, up to nine times greater in magnitude, substantiating vestibular hypersensitivity to skull vibration (Manzari et al 2008;Welgampola et al 2009). In our study, the ViVOR was always an excitatory response from the VSCD.…”

“…In contrast, nystagmus induced by 2 s, 100 Hz vibrations from a hand-held vibrator could either be excitatory or inhibitory and, therefore, could not lateralize the dehiscent side nor distinguish PCD from SCD (Manzari et al 2008). This is because excitatory ViVOR would be in the opposite directions to inhibitory ViVOR and not being able to reliably predict whether the response would be excitatory or inhibitory would hamper the usefulness of relying on nystagmus eye rotation directions to identify the VSCD based on Ewald's first law (Ewald 1892).…”

“…Although high-resolution temporal bone computed tomography (CT) provides anatomical evidence of a semicircular canal dehiscence, it can overestimate the occurrence of the bony dehiscence (Williamson et al 2003;Aw et al 2006;Freidmann et al 2010). Hence, measurement of vestibular hypersensitivity to air-conducted sound (Minor et al 1998;Aw et al 2006Aw et al , 2010Brantberg et al 2006) or skull vibrations (Manzari et al 2008;Welgampola et al 2009) provides a functional diagnosis of a VSCD. However, in the presence of ossicular chain dysfunction, skull vibration may be useful as a diagnostic aid.…”

“…However, in the presence of ossicular chain dysfunction, skull vibration may be useful as a diagnostic aid. However skull vibrations induced by a hand-held vibrator unpredictably induced either excitatory or inhibitory nystagmus in SCD (Manzari et al 2008). Since skull vibrations are believed to be transmitted to both labyrinths, such vibrations may have difficulty identifying the location of the semicircular canal dehiscence.…”

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

“…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]. Skull vibration, however, also unpredictably induced either excitatory or inhibitory nystagmus in some SCD patients [Manzari et al, 2008].…”