Background: The superior canal dehiscence (SCD) syndrome consists of sound- or pressure-induced nystagmus and vertigo caused by a defect in bone overlying the superior semicircular canal. The SCD syndrome is diagnosed based upon characteristic symptoms, signs, and findings on CT imaging of the temoral bones. However, SCD syndrome is often misdiagnosed as perilymphatic fistula (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-dimensional scleral search coils were used to record eye movements in 11 patients with SCD syndrome.

Results: Ten patients developed nystagmus with upward torsional slow phases characteristic of superior canal activation when loud tones were presented to the affected ear or when the patients performed a Valsalva 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 slow phases. The response of the affected superior canal to rapid head rotations was tested in nine patients. The response was diminished in those with large (≥5 mm) defects in the bone overlying the superior canal.

Conclusions: The evoked eye movements in patients with SCD syndrome arise from the superior canal, not the utricle. The syndrome is recognized by the characteristic nystagmus evoked by tones or maneuvers that change middle ear or intracranial pressure. Examination for this nystagmus should be performed under conditions that prevent visual fixation.