3.9 Superior Semicircular Canal Dehiscence Syndrome
Key Features
Superior semicircular canal dehiscence (SSCD) occurs when there is an absence of intact bone overlying the superior semicircular canal.
Presentation can include the Tullio phenomenon, autophony, and hyperacusis.
A third mobile window leads to low-frequency conductive loss with intact acoustic reflex.
Vestibular-evoked potentials (VEMP) can demonstrate reduced thresholds and large peak-to-peak amplitudes.
Treatment is primarily surgical (resurfacing and/or canal plugging).
Superior semicircular canal dehiscence (SSCD), or semicircular canal dehiscence syndrome (SCDS), was first described by Minor in 1998 and is defined by the absence of bone overlying the superior semicircular canal, leading to vestibular and auditory symptoms. Patients with SSCD can present with conductive hearing loss (CHL) with intact acoustic reflexes, conductive hyperacusis, pulsatile tinnitus, and Tullio phenomenon.
Epidemiology
The incidence of SCDS is unknown. However, temporal bone studies suggest that the anatomic incidence of dehiscence of the superior canal is 0.5%.
Clinical
Signs and Symptoms
Clinically, signs and symptoms may range from very mild to severe. Patients with SSCD may exhibit some or all of the following:
Tullio phenomenon: Vertigo or dizziness induced by loud sound, with oscillopsia and nystagmus
Autophony: The person′s own speech or other self-generated noises, such as chewing, are perceived as unusually loud in the affected ear.
Dizziness, disequilibrium, or true vertigo
Hyperacusis
Conductive hearing loss, often in the low-frequency range
Aural fullness
Pulsatile tinnitus
Headache
Differential Diagnosis
With a healthy or normal-appearing middle ear, the presence of normal acoustic reflex should help discriminate otosclerosis with stapes immobility from possible SSCD. Obviously, one must exclude other identifiable causes of CHL, such as tympanic membrane perforation, ossicular chain problems, or middle ear effusion. The differential diagnosis for dysequilibrium is broad. Noise-induced vertigo raises suspicion for SSCD. Perilymphatic fistula and an enlarged vestibular aqueduct are possible conditions associated with an inner ear third mobile window. Ménière′s disease, patulous auditory (eustachian) tube, and migraine may have some overlapping findings.
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