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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 bones

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 dehisence of the right superior semicircular canal.
  • The dehisence resolved following a transmastoid resurfacing with bone pate.

Treatment

  • Conservative management:
    • Avoidance of symptom-triggering activities
    • Vestibular rehabilitation
  • Surgical options:
    • Canal plugging: Occlusion of dehiscent canal with bone wax or fascia
    • Resurfacing: Repair of dehiscence with cartilage, fascia, or bone graft
    • Middle fossa craniotomy approach most common
  • Outcomes:
    • Surgical success rates of 80-90% for symptom improvement
    • Potential complications include sensorineural hearing loss and vestibular dysfunction

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