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Patients with superior canal dehiscence (SCD) syndrome experience vertigo and oscillopsia with loud sounds and/or stimuli that result in changes in middle ear or intracranial pressure. Findings on temporal bone CT were analyzed to determine if a developmental abnormality is associated with the syndrome.Temporal bone CT scans [0.5 mm collimation and projections into the superior semicircular canal (SC) plane] were used to compare the bone overlying the SC in patients with SCD syndrome (20 unilateral, 7 bilateral) and in 88 patients without SCD syndrome who had undergone temporal bone CT for evaluation of other otologic disorders (controls).The thickness of bone overlying the SC in the controls measured 0.67±0.38 mm (mean±SD). For individual control subjects, the thickness of bone on one side was correlated with that on the other side (r=0.43; p<0.0001). The thickness of bone overlying the SC on the intact side in patients with unilateral dehiscence measured 0.31±0.23 mm, and was thinner than that noted in the controls (p<0.0001).These findings support the notion that there is a developmental abnormality underlying SCD syndrome. When dehiscence is found on one side, the contralateral side is likely to be thin.