superior semicircular canal dehiscence syndrome
Superior semicircular canal dehiscence syndrome (SCDS) is an inner ear abnormality, where a clinical disequilibrium phenomenon is associated with the absence of the bony covering of the superior semicircular canal (SSCC).
Notably, this CT finding has also been described in ~10% of individuals without these clinical features; this may be because CT produces a significant false positive rate due to its inherent inability (with current technology) to resolve very thin intact soft tissue coverings .
Current literature suggests that the formal term superior semicircular canal dehiscence syndrome should be reserved for patients who have both the clinical and radiological features.
The incidence of superior semicircular canal dehiscence syndrome increases with age.
Patients may experience vestibular and visual symptoms. The classic presentation is Tullio phenomenon: vertigo and nystagmus induced by loud noises. The same symptoms may be brought on by sudden changes in pressure, e.g. Valsalva maneuver. In some, there is a history of longstanding disequilibrium and unsteadiness .
It can also present with auditory dysfunction, with symptoms including hearing loss, autophony, pulsatile tinnitus and hyperacusis . The deafness is conductive, mimicking otospongiosis and could explain some cases of persistent conductive hearing loss after uneventful stapedectomy. Some patients experience oscillopsia .
Dehiscence of the SSC forms a "third window" into the inner ear, in addition to the round and oval windows. This allows motion of the endolymph to be induced by sound and pressure stimuli .
Thin slice petrous temporal bone CT demonstrates a defect in the arcuate eminence (bony covering) of the SSC, best seen in the coronal plane. Reducing the slice thickness from 1 mm to 0.5 mm may improve detection rates from 50 to 93% .
MRI is not usually required for the initial diagnosis of SCDS, but there is research showing it may be as good and possibly better than CT. Broweays et al. in 2013 found that MRI in 112 patients had a sensitivity of 100%, specificity of 96.5%, positive predictive value of 61.1%, and negative predictive value was 100% compared to CT thereby making it an excellent 'rule out' test. This said, however, most surgeons would be reluctant to operate without a CT, as it is far superior in defining the bony anatomy.
MRI is more often currently used to follow up patients that have had surgery, allowing assessment of the integrity of the bony covering of the SSC. A thin volumetric T2 SPACE is the best sequence.
Treatment and prognosis
Surgical procedures are performed through a middle cranial fossa approach. Plugging or resurfacing the superior semicircular canal has been documented to resolve or improve the symptoms . Canal plugging is thought to achieve long-term control more often than resurfacing .
In pediatric cases it more commonly presents with a mild deafness, and a watch-and-wait approach to treatment is advised in children .
History and etymology
It was initially reported by American otolaryngologist Lloyd B Minor (fl. 2018) in 1998 with further data published in 2005 .