Superior semicircular canal dehiscence syndrome

• Defect in the arcuate eminence on coronal CT—loss of bony covering over the superior semicircular canal
• High-resolution thin-slice CT (0.5 mm) showing discontinuity in the superior semicircular canal arch in the coronal plane
• MRI T2 SPACE sequences showing fluid signal where bone should be covering the superior semicircular canal

• Coronal high-resolution CT (0.5 mm slice thickness) is the gold standard, demonstrating a focal defect in the arcuate eminence with disruption of the normal bony cortex overlying the superior semicircular canal.
• The dehiscence appears as a well-defined area of absent bone with sharp margins, typically located in the region of the superior semicircular canal dome.
• Thin-slice technique (0.5 mm) significantly improves detection rates to approximately 93%, compared to only 50% with 1 mm slices, because very thin intact soft tissue coverings can mimic dehiscence on thicker slices.
• MRI using thin volumetric T2 SPACE sequences can also demonstrate the defect with reported sensitivity of 100% and specificity of 97%, making it an excellent confirmatory test, though CT remains superior for surgical planning and defining bony anatomy.
• Post-operative MRI assessment using T2 SPACE sequences allows evaluation of the integrity of any reconstructed or repaired bony covering of the superior semicircular canal.
• The adjacent perilymph and endolymph within the semicircular canal show normal T2 hyperintensity, which becomes directly exposed to the intracranial space through the dehiscence.

• Thin or intact but very delicate soft tissue covering mimicking dehiscence on CT—this is why CT has a 10% false positive rate and careful technique with thin slices and clinical correlation are essential.
• Erosive otosclerosis affecting the otic capsule—distinguished by bilateral involvement and irregular erosive pattern rather than a focal arcuate eminence defect.
• Prior mastoid surgery or cholesteatoma causing bone erosion—distinguished by clinical history and pattern of bone loss (non-specific erosion vs. focal arcuate eminence defect).
• Congenital anterior semicircular canal dehiscence—extremely rare, distinguished by location (anterior canal rather than superior canal).
• Incomplete ossification of the superior semicircular canal (normal variant in infants)—distinguished by patient age (pediatric) and normal developmental timing.

When reporting, clearly state: "There is a focal defect in the bony covering (arcuate eminence) of the superior semicircular canal, best visualized on coronal thin-section CT. This finding is consistent with superior semicircular canal dehiscence. Correlation with clinical presentation (vestibular symptoms, Tullio phenomenon, or conductive hearing loss) is recommended to confirm the diagnosis of superior semicircular canal dehiscence syndrome, as this radiologic finding alone can occur in asymptomatic individuals."

• Using thick-slice CT (1 mm) leads to only 50% detection sensitivity; always use thin slices (0.5 mm) for accurate detection of the bony defect.
• Diagnosing superior semicircular canal dehiscence syndrome on imaging alone without clinical correlation—approximately 10% of asymptomatic individuals have radiologic dehiscence, so clinical symptoms (Tullio phenomenon or Hennebert's sign) are essential for diagnosis.
• Mistaking a thin but intact soft tissue covering for true dehiscence due to CT's inherent limitation in resolving very thin intact structures; this accounts for the false positive rate and emphasizes the value of MRI confirmation.
• Forgetting to evaluate both ears in coronal planes, as dehiscence may be bilateral in some cases and involvement can be asymmetric in symptom severity.