Superior Canal Dehiscence
- Author: Wayne T Shaia, MD; Chief Editor: Arlen D Meyers, MD, MBA more...
Superior canal dehiscence syndrome (SCDS) is a newly described condition in which vestibular symptoms are elicited by sound or pressure secondary to a dehiscent superior semicircular canal. More than 70 years have passed since Tullio and Hennebert described their findings of sound-induced and pressure-induced vestibular activation.
Since then, the Tullio phenomenon, wherein vestibular symptoms are induced by loud sounds, has been associated with syphilis, perilymphatic fistula, congenital deafness, Ménière disease, head trauma, Lyme disease, cholesteatomas with labyrinthine fistula, and fenestration operations. The Hennebert sign of vestibular symptoms due to changes in external auditory canal pressure is frequently found in conjunction with the Tullio phenomenon (as in perilymphatic fistula, syphilis, Ménière disease).
In 2000, Minor found a series of patients with positive Tullio and Hennebert signs. He was the first to relate these positive findings directly to an anatomical defect of the superior semicircular canal that was detected with high-resolution computed tomography (HRCT). Minor theorized that, when the bone over the canal becomes thin or dehiscent, it acts as an additional window for the vestibular system, allowing pressure and noise changes to induce vestibular activity. SCDS, as it has become known, is now recognized and treated by otolaryngologists and neurotologists throughout the world.
The cochleovestibular system has 2 functional windows. The oval window, which houses the footplate of the stapes, allows sound to enter the inner ear (vestibule) and to be carried via hydroacoustic waves through the perilymph. This allows the mechanical wave to be transduced into neural activity, and, thus, sound is perceived.
The function of the round window is more controversial. It is thought to have several roles. Its first role is thought to involve the release of sound and mechanical energy from the scala tympani. Another proposed role is its participation in the secretion and absorption of substances in the inner ear. The round window may also play a role as a defense mechanism of the inner ear.
These 2 windows of the inner ear work together to regulate hearing and balance. When a dehiscence in the superior semicircular canal is created, a third-window effect is thought to take place. As a result, endolymph within the labyrinthine system continues to move in relation to sound or pressure, which causes an activation of the vestibular system. The intracranial pressure transmission to the round window may also result in increased compliance of the inner ear from stretching of the round window membrane. This pressure transmission may also result in a frank round window (or oval window) fistula.
The true incidence of persons with symptomatic SCDS is currently unknown. One study of 1000 cadaveric temporal bones revealed that a dehiscence of bone that overlies the superior canal was present in approximately 0.5% of temporal bone specimens. In an additional 1.4% of the specimens, the bone was markedly thin (≤ 0.1 mm) compared with the normal bone.
SCDS has no racial bias.
SCDS appears to affect males and females equally.
In 2000, Minor reported that, in his original series of 17 patients, the median age at diagnosis was 40 years (range, 27-70 y).
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