Superior canal dehiscence

Superior canal dehiscence
Classification and external resources
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Superior canal dehiscence syndrome (SCDS) is a rare medical condition of the inner ear, leading to hearing and balance symptoms in those affected. The symptoms are caused by a thinning or complete absence of the part of the temporal bone overlying the superior semicircular canal of the vestibular system. There is evidence that this rare defect, or susceptibility, is congenital. There are also numerous cases of symptoms arising after physical trauma to the head. It was first described in 1998 by Dr. Lloyd B. Minor of Johns Hopkins University, Baltimore, USA.

Superior canal (SCD) can affect both hearing and balance to different extents in different people.

Symptoms of SCDS include:

According to current research, in approximately 2.5% of the general population the bones of the head develop to only 60–70% of their normal thickness in the months following birth. This genetic predisposition may explain why the section of temporal bone separating the superior semicircular canal from the cranial cavity, normally 0.8 mm thick, shows a thickness of only 0.5 mm, making it more fragile and susceptible to damage through physical head trauma or from slow erosion. An explanation for this erosion of the bone has not yet been found.

The presence of dehiscence can be detected by a high definition (0.6 mm or less) coronal CT scan of the temporal bone, currently the most reliable way to distinguish between superior canal dehiscence syndrome (SCDS) and other conditions of the inner ear involving similar symptoms such as Ménière's disease and perilymphatic fistula. Other diagnostic tools include the vestibular evoked myogenic potential or VEMP test, videonystagmography (VNG), electrocochleography (ECOG) and the rotational chair test. An accurate diagnosis is of great significance as unnecessary exploratory middle ear surgery may thus be avoided. Several of the symptoms typical to SCDS (e.g. vertigo and Tullio) may also be present singly or as part of Ménière's disease, sometimes causing the one illness to be confused with the other. There are reported cases of patients being affected by both Ménière's disease and SCDS concurrently.

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