Superior semicircular canal dehiscence (SSCD) is a defect in the bone overlying the superior (anterior) semicircular canal at the arcuate eminence. Described by Minor (1998), this condition creates a 'third window' effect causing a combination of vestibular and cochlear symptoms. Prevalence in cadaver studies is reported as 0.5-0.7%, but the proportion of symptomatic cases is much lower. Bilateral dehiscence is found in 30-40% of patients. Clinical symptoms include sound-induced vertigo (Tullio phenomenon), Valsalva-induced vertigo, autophony (hearing one's own voice), pulsatile tinnitus, and loss of otoacoustic emissions due to increased bone conduction. Diagnosis is established by demonstrating bone defect on coronal reformats of high-resolution temporal bone CT. Surgical treatment (plugging or resurfacing) provides cure in symptomatic cases.
Age Range
20-60
Peak Age
40
Gender
Equal
Prevalence
Uncommon
The pathophysiology of SSCD is based on the 'third window' concept. The normal inner ear has two windows: oval window (stapes footplate) and round window. The pressure differential between these two windows enables cochlear fluid movement and hearing occurs. In SSCD, the bone defect creates a third window — acoustic energy escapes through the dehiscence to the middle cranial fossa instead of passing through the cochlea. This third window produces two main consequences: (1) Cochlear symptoms: bone conduction increases while air conduction decreases — 'pseudo-conductive hearing loss' occurs (air-bone gap is detected on audiogram despite normal middle ear). (2) Vestibular symptoms: sound waves or pressure changes (Valsalva, cough, straining) directly stimulate the endolymphatic fluid of the superior SCC through the dehiscence — ampullary receptors are activated causing vertigo/nystagmus (Tullio phenomenon). On CT, dehiscence appears as bone defect — coronal reformats show absence of bone continuity over superior SCC roof. Partial volume effect can falsely show bone covering thinner than 0.5 mm as dehiscence — therefore 0.5 mm thin-section CT and Stenvers/Poschl reformats are required.
Combination of sound-induced vertigo (Tullio phenomenon) clinical finding with bone defect in superior SCC roof on coronal CT reformats is diagnostic for SSCD. This clinical-radiological correlation establishes diagnosis and is sufficient for surgical planning.
Defect in bone covering over the arcuate eminence of the superior semicircular canal. Best seen on coronal reformats. Poschl (oblique coronal parallel to SCC plane) and Stenvers (oblique sagittal perpendicular to SCC plane) reformats improve diagnostic accuracy.
Report Sentence
A defect measuring approximately ___ mm in the bone covering over the arcuate eminence of the superior semicircular canal on the ___ side is identified on coronal reformats, consistent with SSCD.
Middle ear and ossicular chain are normal — conductive hearing loss in SSCD is 'pseudo-conductive' and there is no middle ear pathology. This finding is important to exclude otosclerosis.
Report Sentence
Middle ear cavity and ossicular chain are normal; air-bone gap detected on audiogram can be explained by SSCD 'third window' effect.
Thinning of bone covering over superior SCC on contralateral side (<0.3 mm). Indicates bilateral SSCD or 'impending dehiscence.' Contralateral side should always be evaluated.
Report Sentence
Bone covering over the superior semicircular canal on the contralateral side is significantly thinned (___ mm); consistent with bilateral SSCD or 'impending dehiscence.'
Direct continuity of superior SCC lumen with middle cranial fossa CSF on T2-weighted CISS/FIESTA sequences. Complementary finding supporting CT. Thin-section T2 sequences show inner ear fluid structures in detail.
Report Sentence
Direct continuity of the superior semicircular canal lumen with middle cranial fossa CSF space is observed on T2 CISS/FIESTA sequences; supporting SSCD diagnosis.
Poschl (oblique coronal parallel to SCC plane) and Stenvers (oblique sagittal perpendicular to SCC plane) reformats confirm dehiscence. Standard coronal sections carry risk of false-positive results due to partial volume effect — oblique reformats eliminate this risk.
Report Sentence
Dehiscence of the superior semicircular canal roof is confirmed on Poschl and Stenvers reformats (defect size: ___ mm); SSCD diagnosis is established.
Criteria
Unilateral dehiscence. 60-70% of cases. Symptoms on affected side.
Distinct Features
Contralateral side normal or thin bone covering. Surgery unilateral plugging/resurfacing. Contralateral side under surveillance.
Criteria
Bilateral dehiscence. 30-40% of cases. Usually one side more symptomatic.
Distinct Features
Both sides must be evaluated on CT. Surgery first on more symptomatic side. Second side in second session if needed.
Criteria
Bone defect present on CT but no clinical symptoms. Considered anatomic variant.
Distinct Features
Treatment unnecessary. Observation only. VEMP test may show subclinical dysfunction. Partial volume effect should be excluded (0.5 mm sections).
Distinguishing Feature
Congenital atresia shows closure of EAC with bony plate; SSCD shows bone defect in superior SCC roof. Anatomical location is completely different. Atresia causes conductive, SSCD pseudo-conductive hearing loss.
Distinguishing Feature
CT is normal in Bell's palsy with facial nerve enhancement as MRI finding; SSCD shows bone defect in superior SCC on CT. Clinical symptoms differ — facial paralysis in Bell's, vertigo and hearing loss in SSCD.
Distinguishing Feature
Fibrous dysplasia shows expansile ground-glass density bone lesion; SSCD shows bone defect (bone absence). In FD bone expands, in SSCD bone is missing. Two completely different pathologies.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralSSCD diagnosis is established with high-resolution temporal bone CT (0.5 mm) and confirmed with VEMP (vestibular evoked myogenic potential) test. Treatment is determined by symptom severity: mild symptoms — symptom management and avoidance of triggers; moderate-severe symptoms — surgical treatment. Surgical options: (1) Transmastoid canal plugging — most commonly performed, low morbidity; (2) Middle fossa craniotomy with resurfacing — more invasive but better hearing preservation. Surgical success rate is 90-95% with symptoms usually completely resolving.
Surgical treatment (plugging or resurfacing) provides cure in symptomatic SSCD patients. Treatment is unnecessary in asymptomatic cases. High-resolution temporal bone CT is the gold standard for diagnosis. VEMP test (vestibular evoked myogenic potential) supports the diagnosis.