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Superior Semicircular Canal Dehiscence Syndrome

Summary

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  • Rare inner ear disorder characterised by absence of bone overlying the superior semicircular canal
  • Presents with vestibular and auditory symptoms triggered by sound or pressure changes
  • Diagnosis confirmed by high-resolution CT imaging of temporal bones1

Pathophysiology

  • Absence of bone over superior semicircular canal creates a "third window" in the inner ear
  • Abnormal communication between inner ear and middle cranial fossa
  • Results in:
    • Pressure-induced displacement of endolymph
    • Abnormal activation of vestibular system
    • Altered sound transmission to cochlea

Demographics

  • Prevalence estimated at 0.5-2% in general population
  • Typically presents in middle-aged adults (40-50 years)
  • No significant gender predilection
  • Bilateral involvement in 25-50% of cases

Diagnosis

  • Clinical presentation:
    • Vertigo and oscillopsia induced by loud sounds (Tullio phenomenon)
    • Autophony (hearing one's own voice abnormally loud)
    • Pulsatile tinnitus
    • Chronic disequilibrium
  • Diagnostic tests:
    • Vestibular evoked myogenic potentials (VEMPs) with lowered thresholds
    • Audiometry showing low-frequency air-bone gap
    • Fistula test may be positive

Imaging

  • High-resolution CT of temporal bones:
    • Key diagnostic modality
    • Axial and coronal planes with <1mm slice thickness
    • Findings:
    • Absence of bone over superior semicircular canal
    • Direct communication between inner ear and middle cranial fossa
  • MRI:
    • May show fluid signal extending from superior semicircular canal to middle cranial fossa
    • Useful for ruling out other causes of vestibular symptoms

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  • A 30-year-old patient presented with noise-induced dizziness.
  • Cone beam CT showed dehiscence of the right superior semicircular canal.
  • The dehiscence resolved following a transmastoid resurfacing with bone pate.

Treatment

  • Conservative, or surgical canal plugging/resurfacing for disabling symptoms. Diagnosis rests on thin-section CT reformatted along/perpendicular to the canal to avoid overcalling dehiscence from partial-volume averaging

Differential diagnosis

Differential Diagnosis Differentiating Feature
Tegmen tympani dehiscence Bony defect in the tegmen tympani (roof of middle ear) rather than the superior semicircular canal arch on CT; different anatomical location
Superior canal thinning (without dehiscence) Thin but intact bone covering the superior canal on thin-section CT; no true gap; partial volume averaging artefact
Enlarged vestibular aqueduct syndrome Enlarged endolymphatic duct >1.5 mm at midpoint measurement on CT; cochlear involvement
Otosclerosis Lucent halo around cochlea ("halo sign") on CT; fenestral or retrofenestral involvement; no dehiscence of semicircular canal
Cholesteatoma Expansile soft tissue mass with bone erosion in middle ear or mastoid on CT; opacification of middle ear

  1. Steenerson et al. Superior Semicircular Canal Dehiscence Syndrome. 2020. Seminars in neurology - Open in new tab. ↩