Aetiology

The aetiology of superior semicircular canal dehiscence is highly contentious, and has been much debated in the literature over the last five years. It is unclear whether it is a congenital, acquired or two-hit phenomenon.

Takahashi et al. macroscopically observed the middle cranial fossa of 42 adult cadavers and 4 fetal cadavers.Reference Takahashi, Tsunoda, Shirakura and Kitamura⁶ Six fetuses were also examined by CT; the volume data of the CT images obtained from four adults were used for comparison. The authors showed by macroscopic anatomy analysis and CT examination that in fetuses the superior semicircular canal and cochlea protrude into the cranium. In contrast, the superior semicircular canals of all adults were completely or almost entirely buried within the temporal bone. Therefore, they hypothesised that protrusion of the superior semicircular canal into the cranium during fetal development causes membranous labyrinth adhesion to the dura because the bony labyrinth develops much later than the membranous labyrinth. Such adhesion would in turn interfere with ossification and lead to a bony defect in the superior semicircular canal.

Several authors have explored the possibility that superior semicircular canal dehiscence is associated with a congenital morphological abnormality of the skull base. Patients with superior semicircular canal dehiscence frequently exhibit contralateral canal thinning and tegmen dehiscence or thinning.Reference Gracia-Tello, Cisneros, Crovetto, Martinez, Rodriguez and Lecumberri^7–Reference El Hadi, Sorrentino, Calmels, Fraysse, Deguine and Marx¹⁰ These findings provide more supporting evidence for a congenital aetiology of superior semicircular canal dehiscence.

However, Nadgir et al. reported superior semicircular canal dehiscence to be more prevalent in older age groups, suggesting an acquired aetiology.Reference Nadgir, Ozonoff, Devaiah, Halderman and Sakai¹¹ These authors assessed the prevalence of superior semicircular canal dehiscence in imaging data from multiple age groups. They retrospectively reviewed 306 consecutive temporal bone CT examinations performed between July 2005 and March 2007. Patients ranged in age from 7 months to 89 years and were subcategorised into 5 age groups. Twenty-four patients were identified with superior semicircular canal dehiscence and 137 had superior semicircular canal thinning. Each successive age category experienced a 93 per cent increase (95 per cent confidence interval (CI) 30–187 per cent) in the prevalence of superior semicircular canal dehiscence (p = 0.001). They concluded that the increased radiological prevalence of superior semicircular canal dehiscence in older age groups suggests that superior semicircular canal dehiscence is more commonly an acquired rather than a congenital condition. Despite this finding, Parlea et al. reported a patient with a solely developmental aetiology for superior semicircular canal dehiscence.Reference Parlea, Georgescu and Calarasu¹² The case report described a 46-year-old woman presenting with Tullio phenomenon. Temporal bone CT showed that she had a defect of the left superior semicircular canal caused by an enlarged superior petrosal sinus receiving drainage from a large cerebellar developmental venous anomaly.

The two-hit aetiology of superior semicircular canal dehiscence has been much discussed since described by Minor in 2000.Reference Minor¹³ Whyte Orozco et al. performed a cadaveric study to test this theory.Reference Whyte Orozco, Martinez, Cisneros, Obon, Gracia-Tello and Angel Crovetto¹⁴ They demonstrated that a thinned temporal bone consists exclusively of the inner (or endosteal) layer separating the canal from the superior semicircular conduct. They concluded that the absence of the external and middle layers, besides reducing its thickness, makes the canal susceptible to suffering a second event leading to fracture and dehiscence.

There is a controversial suggestion that superior semicircular canal dehiscence may be related to osteopenia and osteoporosis. Crovetto et al. suggested that in superior semicircular canal dehiscence there is slight osteopenia of the roof of the superior semicircular canal associated with ageing, which is more pronounced in menopausal women.Reference Crovetto-De la Torre, Whyte-Orozco, Crovetto-Martinez, Whyte-Orozco, Obon-Nogues and Martinez-Rodriguez¹⁵ Moreover, Yu et al. argued that superior semicircular canal dehiscence may be associated with osteoporosis in elderly Asian women.Reference Yu, Teich, Moonis and Wong¹⁶ No investigations into whether vitamin D deficiency is associated with superior semicircular canal dehiscence have been reported.

Overall, the lack of agreement regarding the aetiology of superior semicircular canal dehiscence syndrome suggests that the cause may be multifactorial. Further research is therefore required to obtain an improved understanding of the aetiology.

Is dehiscence size important?

The question of whether the superior semicircular canal dehiscence size correlates with the signs and symptoms of this syndrome has been thoroughly investigated over the last five years. Six key papers have been published, involving a total of 281 patients; the superior semicircular canal dehiscence size was measured intra-operatively in 85 patients and via CT images in 196 patients. The studies investigated heterogeneous variables. However, there was consensus in three studies that the air–bone gap (ABG) size is significantly related to the superior semicircular canal dehiscence size.Reference Chien, Janky, Minor and Carey^17–Reference Niesten, Hamberg, Silverman, Lou, McCall and Windsor¹⁹ Only one paper compared ocular vestibular evoked myogenic potential thresholds with superior semicircular canal dehiscence size, and found that they were significantly related.Reference Manzari, Burgess, McGarvie and Curthoys²⁰ Several reports indicated that pure tone audiogram patterns and average bone-conduction thresholds were not significantly related to superior semicircular canal dehiscence size.Reference Chien, Janky, Minor and Carey^17, Reference Pfammatter, Darrouzet, Gartner, Somers, Van Dinther and Trabalzini^21, Reference Chi, Ren and Dai²² There has been disagreement about whether there is a significant relationship between both the superior semicircular canal dehiscence size and cervical vestibular evoked myogenic potential amplitudes and the patient's presenting signs and symptoms.Reference Chien, Janky, Minor and Carey^17, Reference Manzari, Burgess, McGarvie and Curthoys^20, Reference Pfammatter, Darrouzet, Gartner, Somers, Van Dinther and Trabalzini²¹ The findings of each report are summarised in Table I.

Table I

Key studies comparing sscd size with the clinical presentation and investigation results

SSCD = superior semicircular canal dehiscence; ABG = air–bone gap; cVEMP = cervical vestibular evoked myogenic potential; oVEMP = ocular vestibular evoked myogenic potential; intra-op = intra-operatively; NA = not applicable; CT = computed tomography

Investigations

Recently, there have been a few important findings about superior semicircular canal dehiscence, particularly regarding the use of imaging and vestibular evoked myogenic potentials for its diagnosis.

Multiple papers have reported that CT evaluation alone is unreliable when the clinical context of other investigations are not taken into consideration. There is a significant false positive risk when performing CT analysis of superior semicircular canal dehiscence.

Mondina et al. observed 3 dehiscences in 37 temporal bones by CT in the Poschl and Stenvers planes. However, no dehiscence was found by microscopic analysis.Reference Mondina, Bonnard, Barreau, Darrouzet and Franco-Vidal²³ In a larger population, Tavassolie et al. compared the surgical and CT findings of 34 surgically confirmed superior semicircular canal dehiscence patients.Reference Tavassolie, Penninger, Zuniga, Minor and Carey²⁴ They concluded that CT can overestimate the dehiscence size and can result in false positives for dehiscence detection. Crovetto et al. compared the CT and cadaveric temporal bone results of 604 ears.Reference Crovetto, Whyte, Rodriguez, Lecumberri, Martinez and Elexpuru²⁵ They found that a higher incidence of superior semicircular canal dehiscence was obtained following CT scanning (3.6 per cent) compared with anatomical analysis (0.6 per cent). Masaki stressed the importance of diagnosing superior semicircular canal dehiscence based on both CT findings and clinical symptoms.Reference Masaki²⁶ When a diagnosis of superior semicircular canal dehiscence syndrome in 164 ears was based solely on CT images, 80 per cent were false positives. As CT diagnosis of superior semicircular canal dehiscence can be unreliable, Eibenberger et al. investigated whether the limits of CT resolution allow the objective detection of bone structure defects as small as 0.1 mm in diameter.Reference Eibenberger, Carey, Ehtiati, Trevino, Dolberg and Haslwanter²⁷ They used a bone cement phantom containing small, well-defined structural defects (diameter 0.1–0.4 mm) and tested both multislice and cone beam CT. The defects were reliably detected by comparing radiodensity variances between a region of interest (i.e. a region containing a hole) and a homogeneous region. Statistical noise analysis using the Fligner–Killeen test was used to predict the presence or absence of a hole (p ≥ 0.05). This analysis allowed the authors to distinguish real irregularities down to 0.1 mm (p ≤ 0.0001) from measured image noise or reconstruction errors.

It is not unusual for patients with vertigo and symptoms of semicircular canal dehiscence to be investigated using both CT and MRI modalities. Browaeys et al. attempted to determine whether MRI alone is sufficient. They examined 112 consecutive patients (i.e. 224 ears) who underwent concurrent MRI and high-resolution CT of the temporal bones.Reference Browaeys, Larson, Wong and Patel²⁸ They concluded that fast imaging employing steady-state acquisition (‘FIESTA’) MRI acquisition, with sensitivity and negative predictive values of 100 per cent, can conclusively exclude superior semicircular canal dehiscence. A negative MRI study therefore precludes the need for CT imaging to detect semicircular canal dehiscence; thus, only patients with a positive MRI study should have further CT evaluation. This approach may reduce costs and undue irradiation. To demonstrate the features seen using different imaging modalities, Figure 1 shows coronal T2-weighted MRI slices while Figure 2 shows coronal CT slices of the same patient with a right superior semicircular canal dehiscence.

Fig. 1

Coronal T2-weighted magnetic resonance imaging scan, showing the superior semicircular canals lying close to the superior aspect of the temporal bone.

Fig. 2

Consecutive coronal computed tomography images of the same patient as in Figure1, showing the right semicircular canal dehiscence. R = right; L = left

The importance of the vestibular evoked myogenic potential in diagnosing superior semicircular canal dehiscence syndrome has recently been investigated. Patients with this syndrome often show reduced vestibular evoked myogenic potential thresholds, thought to be due to the increased effectiveness of the transmission of sound energy to the sacculus caused by increased compliance of the inner ear.Reference Brantberg, Lofquist and Fransson²⁹ The most common cervical myogenic response to vestibular stimuli is assessed; however, ocular and masseter response may also be evaluated. Cervical vestibular evoked myogenic potentials are obtained by first turning the patient's head away from the non-test ear. Electrodes are then placed on the forehead, anterior neck and over the sternocleidomastoid muscle of the test side. One hundred or so narrow band clicks of high intensity are then presented via inserts to the test ear to stimulate the ipsilateral saccule. The resultant myogenic activity is detected by the electrodes and averaged (Figure 3).

Fig. 3

Graph showing the cervical vestibular evoked myogenic potential of a patient with superior semicircular canal dehiscence syndrome. The non-specific waveform from the abnormal ear is shown in blue, and the characteristic positive-negative-positive trace obtained from the normal side is shown in pink.

Milojcic et al. compared the audiometric and cervical vestibular evoked myogenic potential of ears with superior semicircular canal dehiscence with those of normal ears and of the unaffected ears of unilateral superior semicircular canal dehiscence patients.Reference Milojcic, Guinan, Rauch and Herrmann³⁰ They found that patients with superior semicircular canal dehiscence syndrome had significantly larger ABGs at 0.25 and 0.5 kHz, and significantly lower cervical vestibular evoked myogenic potential thresholds at 0.5 kHz. They combined these two features to form an adjusted cervical vestibular evoked myogenic potential threshold. This measure allowed them to better differentiate superior semicircular canal dehiscence ears from normal or unaffected superior semicircular canal dehiscence ears, which they argued may be clinically applicable.

Other studies compared the use of ocular vestibular evoked myogenic potential versus cervical vestibular evoked myogenic potential for diagnosing superior semicircular canal dehiscence syndrome. Ocular vestibular evoked myogenic potentials represent extraocular myogenic activity in response to otolith stimulation, for example by air-conducted sound or bone-conducted vibration. Electromyographic activity of the extraocular muscles can be recorded using surface electrodes placed beneath the contralateral eye. Zuniga et al. demonstrated cervical vestibular evoked myogenic potential threshold sensitivity and specificity to range from 80 per cent to 100 per cent for diagnosing this syndrome; ocular vestibular evoked myogenic potential amplitudes demonstrated a sensitivity and specificity of greater than 90 per cent.Reference Zuniga, Janky, Nguyen, Welgampola and Carey³¹ Kantner and Gurkov argued that this was because ocular vestibular evoked myogenic potentials are likely to reflect predominantly utricular function, while cervical vestibular evoked myogenic potentials assess saccular function.Reference Kantner and Gurkov³² Vestibular evoked myogenic potentials are still not used regularly for diagnosing superior semicircular canal dehiscence. However, Zuniga et al. argued that ocular vestibular evoked myogenic potential testing in response to air-conducted sound provides an excellent screening test for superior semicircular canal dehiscence without radiation exposure.Reference Zuniga, Janky, Nguyen, Welgampola and Carey³¹ In addition, the ocular vestibular evoked myogenic potential amplitude measurement requires considerably less effort and time for patients compared with the cervical vestibular evoked myogenic potential threshold measurement.

Surgical management

In 2009, Vlastarakos et al. published a meta-analysis of the efficacy and complications of the various surgical techniques for treating superior semicircular canal dehiscence syndrome.Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos³³ Their review identified a total of 64 primary operations for superior semicircular canal repair: 56 ears were operated on for vestibular complaints and 7 for auditory complaints. A total of 33 ears underwent canal plugging, 16 underwent resurfacing and 15 underwent capping. Success rates for these procedures were 97 per cent, 50 per cent and 93 per cent, respectively. The observed differences were statistically significant (p = 0.001). Resurfacing proved less effective than both plugging (p = 0.002) and capping (p = 0.01) techniques. Temporalis fascia combined with bone pâté and bone wax (for plugging), bone graft (for resurfacing) or hydroxyapatite cement (for capping) was commonly used as sealing material. Most operations were performed via a middle cranial fossa approach, with sensorineural hearing loss (SNHL) and disequilibrium being the most frequent complications.

There has been significant debate about which surgical approach is best for managing superior semicircular canal dehiscence syndrome, in particular when comparing a middle cranial fossa approach with a transmastoid approach. There have even been a few cases of superior semicircular canal dehiscence being managed via a transcanal approach. Ward et al. published hearing outcomes in 43 patients after surgical plugging of the superior semicircular canal by a middle cranial fossa approach.Reference Ward, Agrawal, Nguyen, Della Santina, Limb and Francis³⁴ They concluded that low-frequency ABGs decrease after surgical plugging as a result of both increased bone-conduction thresholds and decreased air-conduction thresholds. In their study, surgical plugging via a middle cranial fossa approach in this syndrome was associated with mild, high-frequency SNHL that persisted in 25 per cent of patients with no change in speech discrimination.

There are several reports of the use of a transmastoid approach for treating superior semicircular canal dehiscence, all with promising outcomes. Beyea et al. performed transmastoid occlusion of the superior semicircular canal in 16 patients, 13 with vestibular symptoms, 2 with pulsatile tinnitus and 1 with hyperacusis.Reference Beyea, Agrawal and Parnes³⁵ These symptoms were either greatly improved or completely resolved in 15 of the 16 patients who underwent transmastoid occlusion of the superior canal. Hearing was preserved in 14 patients and improved in 2 patients.

Both resurfacing the superior canal and the transmastoid approach for plugging the superior canal have drawbacks: plugging is invasive to the inner ear, while resurfacing requires a middle cranial fossa approach which is not without serious potential complications. Amoodi et al. therefore described their experience of treating superior semicircular canal dehiscence syndrome by resurfacing the canal defects of four patients via the transmastoid approach.Reference Amoodi, Makki, McNeil and Bance³⁶ All patients reported symptom resolution, with audiograms showing some hearing improvement in three of them.

Fiorino et al. evaluated their results of treating superior semicircular canal dehiscence by plugging and resurfacing the defect via the transmastoid approach in six patients.Reference Fiorino, Barbieri, Pizzini and Beltramello³⁷ There were no intra- or post-operative complications, and patients experienced the immediate relief of symptoms attributable to the dehiscence.

Lastly, Achar et al. performed a retrospective review of six patients with intractable symptoms due to superior semicircular canal dehiscence syndrome who were operated on via a transmastoid approach.Reference Achar, Nichani, Freeman and Lloyd³⁸ One patient underwent canal plugging with fascia and five underwent resurfacing with cymba conchal cartilage. All patients had post-operative symptom resolution and sensorineural hearing preservation; none had major complications related to the approach or surgical technique used.

Overall, a literature analysis shows that canal plugging is associated with better symptomatic success rates compared with canal resurfacing techniques. However, the incidence of post-operative SNHL is higher with canal plugging than with resurfacing techniques. Recently, there has been a drive to surgically treat superior semicircular canal dehiscence syndrome via a transmastoid approach if the dehiscence anatomy and surgical expertise allow. Since 2009, 32 cases of the use of a transmastoid approach to treating this syndrome have been published in 4 papers. The reports concluded that the day-case procedure using a transmastoid approach is a safe, effective and less-invasive technique than the standard middle fossa approach, which has many potential complications and requires a longer hospital stay. However, the transmastoid approach is a new form of management for this syndrome and experience of this technique and its outcomes is limited.

In an attempt to improve the safety and efficacy of the middle cranial fossa approach, Peng and Shaia presented a case series of 10 patients with superior semicircular canal dehiscence syndrome that was surgically managed via an endoscopic middle cranial fossa approach (unpublished data). This provided better visualisation of the defect, a smaller preauricular incision, and shorter recovery and operative times. However, this technique limited the surgeons to performing a resurfacing technique to treat the dehiscence, the brain still required retracting and the patients were still subject to the risks associated with craniotomy. In all, 93 per cent of pre-operative symptoms showed post-operative resolution.

The transcanal approach was described only recently. Silverstein et al. reported a transcanal complete round window niche occlusion for treating superior semicircular canal dehiscence syndrome in 2009.Reference Silverstein and Van Ess³⁹ This operation was performed on a 37-year-old man who presented with intermittent episodes of vertigo, left-sided hearing loss, bilateral tinnitus, left-sided aural fullness and hyperacusis, with imaging consistent with a superior semicircular canal dehiscence. These symptoms all resolved after the operation. The operation was again performed on a patient with a similar presentation; however, this patient's symptoms worsened post-operatively and so a transmastoid superior canal occlusion was performed. However, the result of the second operation was not published.

Banerjee et al. continued to treat superior semicircular canal dehiscence with round window occlusion using bone wax and fascia. He published his experience of treating two superior semicircular canal dehiscence syndrome patients with hyperacusis symptoms. Both patients reported a great post-operative reduction in symptoms, with cessation of the Tullio phenomenon. The authors concluded that this procedure is simple to perform and reversible, and that day-case surgery can be undertaken.Reference Nikkar-Esfahani and Banerjee⁴⁰ However, much more evidence is required for this technique to be widely accepted.

More recently, Silverstein et al. published a retrospective, multicentre case series of 22 patients who were treated for symptomatic superior canal dehiscence using round window tissue reinforcement or complete round window occlusion.Reference Silverstein, Kartush, Parnes, Poe, Babu and Levenson⁴¹ Their analysis revealed a significant improvement in all symptoms except for hearing loss in the 19 patients who underwent round window reinforcement. In contrast, two of the three patients who underwent the alternative treatment of round window occlusion experienced symptom deterioration requiring revision surgery.