Superior semicircular canal dehiscence syndrome

Superior semicircular canal dehiscence syndrome (SCDS) is a recently described inner ear abnormality, where a clinical disequilibrium phenomenon is associated with the absence of the bony covering of the superior semicircular canal (SSC).

Notably, this CT finding has also been described in ~10% of individuals without these clinical features ¹; 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 ¹¹. 

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.

Epidemiology

The incidence of superior semicircular canal dehiscence syndromeincreases with age.

Clinical presentation

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 manoeuvre. In some, there is a history of longstanding disequilibrium and unsteadiness ¹. 

It can also present with auditory dysfunction, with symptoms including hearing loss, autophony, pulsatile tinnitus and hyperacusis ¹³. The deafness is conductive, mimicking otospongiosis and could explain some cases of persistent conductive hearing loss after uneventful stapedectomy ⁷. Some patients experience oscillopsia ¹¹.

Pathology

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 ².

Radiographic features

CT

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% ².

MRI

MRI is not usually required for the initial diagnosis ofSCDS, but there is research showing it may be as good and possibly better than CT. Broweays et al. ¹⁰ 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 ⁴. Canal plugging is thought to achieve long-term control more often than resurfacing ⁶.

In paediatric cases it more commonly presents with a mild deafness, and a watch-and-wait approach to treatment is advised in children ¹³.

History and etymology

It was initially reported byAmerican otolaryngologist Lloyd B Minor in (fl. 2018) in 1998 ¹² with further data published in 2005 ⁶.