Practice Essentials
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. Imaging studies are critical in the diagnosis of SCDS. Surgical correction of SCDS is reserved for patients with severe disabling symptoms.
More than 70 years have passed since Tullio and Hennebert described their findings of sound-induced and pressure-induced vestibular activation. [1] 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. [2] 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.
Symptoms of superior canal dehiscence syndrome
Patients with SCDS usually present with symptoms of sound- or pressure-induced dizziness. Chronic imbalance is another symptom of SCDS. The patient's symptoms may be re-created when sound or pressure is presented to the affected ear. Patients often control these symptoms with strict avoidance of noisy environments. In addition, some patients may hear a swishing noise when they move their eyes in a certain direction. Moreover, hyperacusis, although not specific, is found in a high percentage of patients with SCDS.
Workup in superior canal dehiscence syndrome
Imaging studies are critical in the diagnosis of SCDS. A high-resolution computed tomography (HRCT) scan of the temporal bones without contrast is required to make a definitive diagnosis.
Comprehensive audiometric evaluation is indicated in any patient with vestibular symptoms. In a patient with SCDS, key factors may be revealed. One key is the presence of normal symmetrical hearing, which helps to eliminate a retrocochlear process as a cause of vertigo.
Video-oculography can be valuable in recording the vertical and torsional eye movements specific to SCDS.
Caloric test results are usually unaffected in patients with SCDS; however, when the dehiscence is large (>0.5 mm), reduced caloric test results may be demonstrated on the affected side.
Other tests that may help in the diagnosis of SCDS include vestibular evoked myogenic potentials (VEMPs), as well as vibration of the suboccipital region with the demonstration of torsional nystagmus.
Management of superior canal dehiscence syndrome
Surgical correction of SCDS is reserved for patients with severe disabling symptoms. Operative techniques include the following:
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Traditional middle fossa craniotomy and repair of fistula
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Endoscopic craniotomy approach - In this procedure, patients undergo a middle cranial fossa craniotomy through a small, limited-access craniotomy of 2 cm or less on the affected side; the temporal lobe is gently retracted; upon elevation of the dura, a small endoscope is gently inserted, and the dehiscence is identified and resurfaced
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Transmastoid superior canal occlusion - In this approach, a mastoidectomy is performed, and the superior semicircular canal is identified near the ossicular heads; the superior semicircular canal is then ablated with a combination of tissue and fascia
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Minimally invasive approach via transcanal oval and round window reinforcement - Rather than directly addressing the dehiscent canal via a middle fossa craniotomy, an innovative approach first described by Kartush suggested that dampening the inner ear's sensitivity by reinforcing the oval and round windows may alleviate symptoms in some patients [3]
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Combination approach - In this technique, the superior canal defect is repaired via the middle fossa approach with concomitant reinforcement of the oval and round windows
Pathophysiology
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.
A retrospective study by Castellucci et al indicated that a positive correlation exists between increased SCD size and air-conducted pure-tone average (AC PTA), a low-frequency air-bone gap (ABG), AC cervical vestibular-evoked myogenic potentials (cVEMPs) amplitude, bone-conducted (BC) cVEMPs amplitude, and both AC and BC ocular VEMPs (oVEMPs) amplitude. Moreover, the investigators reported that dehiscence length negatively correlated with AC cVEMPs and oVEMPs thresholds, as well as superior canal vestibulo-ocular reflex (VOR) gain. It was also determined that compared with SCD along the ampullary arm, arcuate eminence dehiscences demonstrated lower superior canal VOR gains. In addition, BC threshold impairment was lower in association with arcuate eminence dehiscences than with dehiscences at the superior petrosal sinus. [4]
Epidemiology
Frequency
United States
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.
Race
SCDS has no racial bias.
Sex
SCDS appears to affect males and females equally.
Age
In 2000, Minor reported that, in his original series of 17 patients, the median age at diagnosis was 40 years (range, 27-70 y). [2]
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Coronal high-resolution computed tomography scan (1-mm sections) that demonstrates the presence of the superior semicircular canal (figure A, black arrow). As the scan is followed posteriorly (figures B, C, D), the bony dehiscence over the superior canal (black arrow) becomes more apparent.
