Eye movements in patients with superior canal dehiscence syndrome align with the abnormal canal

Background: The superior canal dehiscence (SCD) syndrome consists of sound- or pressure-induced nystagmus and vertigo caused by a defect in bone overl ying the superior semicircular canal. The SCD syndrome is diagnosed based u pon characteristic symptoms, signs, and findings on CT imaging of the temor al bones. However, SCD syndrome is often misdiagnosed as perilymphatic fist ula (PLF), and the symptoms of sound- and pressure-induced vertigo are more commonly attributed to the vestibular utricle, rather than to the superior semicircular canal. This study explored the role of the superior canal and the utricle in the pathophysiology of SCD syndrome. Methods: Three-dimensi onal scleral search coils were used to record eye movements in 11 patients with SCD syndrome. Results: Ten patients developed nystagmus with upward to rsional slow phases characteristic of superior canal activation when loud t ones were presented to the affected ear or when the patients performed a Va lsalva maneuver. Visual fixation led to a suppression of the nystagmus and the appearance of a sustained torsional deviation of the eyes. Two patients also had sound-evoked head movements in the same direction as the ocular s low phases. The response of the affected superior canal to rapid head rotat ions was tested in nine patients. The response was diminished in those with large (greater than or equal to5 mm) defects in the bone overlying the sup erior canal. Conclusions: The evoked eye movements in patients with SCD syn drome arise from the superior canal, not the utricle. The syndrome is recog nized by the characteristic nystagmus evoked by tones or maneuvers that cha nge middle ear or intracranial pressure. Examination for this nystagmus sho uld be performed under conditions that prevent visual fixation.