Search strategy

Data for this review were identified by searches of Medline, Embase and Epistemonikos databases, using the search terms ‘superior semicircular canal dehiscence’ and ‘superior semicircular canal’. All papers were written in English or German language, and published from 1998 until October 2019. The references of relevant articles were also searched. We excluded case reports, conference abstracts, and papers focusing on animals and fetal development. The current literature review focused on the incidence and symptoms of superior semicircular canal dehiscence, and the diagnostic techniques and treatment options.

Presenting symptoms and pathophysiology

Superior semicircular canal dehiscence typically presents with: vertigo; oscillopsia (the illusion of a moving visual world, with an inability to stabilise visualised objects, which appear to constantly oscillate); and nystagmus, with a rotational component on the plane of the superior semicircular canal triggered by loud noises (Tullio phenomenon), or/and by manoeuvres that change the middle-ear or intracranial pressure (Hennebert sign).^{Reference Minor, Solomon, Zinreich and Zee8–Reference Martin, Chahine, Veyret, Richard, Prades and Pouget23} The patients may additionally describe a feeling of ‘blocked’ ear or aural pressure, and some drop in their hearing (Table 1). Most patients are able to identify certain triggers of their symptoms, which are usually related to either loud noises or pressure changes.^{Reference Minor, Solomon, Zinreich and Zee8–Reference Martin, Chahine, Veyret, Richard, Prades and Pouget23}

Table 1. Key presenting symptoms, clinical findings and diagnostic tests

*In an otherwise healthy individual. ^†Cone beam or digital volume tomography can better identify the bony dehiscence, given the reduced partial volume effect. VEMPs = vestibular-evoked myogenic potentials; CT = computed tomography

Many patients also report a hypersensitivity to sounds, being able to hear their own sounds very loudly. Typically, the patient with superior semicircular canal dehiscence will report autophony, but also the ability to hear movements of their eyes or even their hair very loudly; the ability of patients with superior semicircular canal dehiscence to hear their eyeballs moving is an auditory ‘hallmark’ of the disease, mostly because of the oddness of the symptom rather than its scientific background or clinical significance.^{Reference Minor, Solomon, Zinreich and Zee8–Reference Martin, Chahine, Veyret, Richard, Prades and Pouget23} This hypersensitivity audiologically corresponds to a negative bone conduction threshold in the low frequencies: a supra-threshold (Figure 2a). Essentially, the inner ear is too sensitive to bone-conducted sounds, a condition called conductive hyperacusis. This phenomenon is also responsible for the low-frequency air–bone gap (conductive hearing loss). Following this hypersensitivity, Weber tuning fork testing typically lateralises to the affected ear, and, interestingly, it can be heard even if the tuning forks are placed far from the head (Table 1).^{Reference Minor, Solomon, Zinreich and Zee8–Reference Martin, Chahine, Veyret, Richard, Prades and Pouget23}

Fig. 2. (a) Pure tone audiogram of the left (affected) ear of a patient with superior semicircular canal dehiscence, showing a bone conduction supra-threshold (thin line marked with ‘>’) up to −10 dB and an air–bone gap (gap between the bone conduction ‘>’ and the air conduction ‘×’). (b) Cervical vestibular-evoked myogenic potentials of the same patient, showing a low (abnormal) threshold in the left (L) ear (n1 and p1 waveforms) up to 65 dB, while in the right (R) ear, healthy ear cervical vestibular-evoked myogenic potentials are recorded at 100 dB.

The reported symptoms are related to the ‘third window’ effect, which, although not completely understood, provides a rational explanation of the patient's symptoms.^{Reference Minor, Solomon, Zinreich and Zee8,Reference Nadgir, Ozonoff, Devaiah, Halderman and Sakai12,Reference Sohmer, Freeman and Perez24,Reference Hiroven, Carey, Liang and Minor25} Instead of having two windows on the osseous cochlea, namely the round and the oval windows, patients with superior semicircular canal dehiscence have an additional, third one at the level of the superior semicircular canal (Figure 3). This is believed to increase the inner-ear immittance and affect the endolymphatic homeostasis, resulting in hypersensitivity of the inner ear and generating a vestibular response to lower sound level stimuli than anticipated.^{Reference Minor, Solomon, Zinreich and Zee8,Reference Nadgir, Ozonoff, Devaiah, Halderman and Sakai12,Reference Sohmer, Freeman and Perez24,Reference Hiroven, Carey, Liang and Minor25}

Fig. 3. Model of an inner and middle ear embedded in the temporal bone, with the upper part removed to enable visualisation of the middle- and inner-ear structures of an intact inner ear, showing: (a & b) the direction of travel of the acoustic energy through the oval window (OW, thick red arrows) and the round window (RW, thin blue arrows). This balance is being disturbed following the bony dehiscence of the superior semicircular canal (b), with abnormal energy transmission (dashed arrows) through the ‘third window’. PSC = posterior semicircular canal; SSC = superior semicircular canal; LSC = lateral semicircular canal

The third window allows bone-conducted sounds to reach the inner ear through the labyrinth rather than through the oval window, disseminating the acoustic energy and resulting in cochlear hypersensitivity (Figure 3). The presence of a third window lowers the impedance of the cochleovestibular system, causing lower resistance for pressure and, essentially, sound transmission.^{Reference Minor, Solomon, Zinreich and Zee8,Reference Nadgir, Ozonoff, Devaiah, Halderman and Sakai12,Reference Sohmer, Freeman and Perez24–Reference Rosowski, Songer, Nakajima, Brinsko and Merchant27} Recent studies in a chinchilla model by Hiroven et al. have shown such mechanisms of pressure sensitivity following a third window.^{Reference Hiroven, Carey, Liang and Minor25} The effect of the third window on inner-ear pressure changes as well as the ease of transmission of intracranial pressure changes through the ‘window’ can explain both the vertiginous and auditory symptoms related to superior semicircular canal dehiscence, particularly autophony and hyperacousis.^{Reference Sohmer, Freeman and Perez24–Reference Rosowski, Songer, Nakajima, Brinsko and Merchant27}

The theory and concept of the third window is not new; several middle-ear surgeons were using this concept to treat conductive hearing loss in patients with otosclerosis until 1955, prior to the introduction of stapedectomy and stapes replacement prostheses.^{Reference Lenarz and Boenninghaus28} Otosclerosis is a progressive fixation of the stapes due to abnormal bone metabolism around the footplate and osseous inner-ear capsule, causing primarily significant conductive hearing loss.^{Reference Eshraghi and Telischi29,Reference Nazarian, McElveen and Eshraghi30} Middle-ear surgeons used to create a small bony opening, a third window, on the osseous cochlea or vestibule, to improve patients’ conductive hearing loss. As the third window causes a milder conductive hearing loss in the low frequencies, the surgeons used this technique to bypass the footplate fixation, which was preventing the sound conduction. Thus, the surgeons aimed for a better hearing threshold via the transmission of sounds through the third window rather than the fixed stapes.^{Reference Lenarz and Boenninghaus28}

It is this third window effect that explains the symptoms related to superior semicircular canal dehiscence. However, it still leaves a few questions unanswered. In particular, it is unclear why some patients will experience more severe symptoms than others, and why some patients will be completely asymptomatic, with superior semicircular canal dehiscence being an incidental imaging finding.

To date, there is no known factor affecting vestibular symptom severity. The extent of the bony defect has not been associated with dizziness or oscillopsia severity. However, the size and location of the bony defect have been linked to the presence, character and severity of the auditory symptoms.^{Reference Chien, Janky, Minor and Carey31–Reference Niesten, Stieger, Lee, Merchant, Grolman and Rosowski35} It seems that most patients with a very small defect do not experience severe auditory symptoms. In contrast, patients with a wide dehiscence describe more severe autophony and have a larger air–bone gap in the low frequencies. Additionally, these patients, as well as patients with a dehiscence near the superior petrosal sinus, report more troublesome tinnitus, which can be pulsatile in nature. This happens because of transmission of the heartbeat through the middle fossa dura vessels and/or the superior petrosal sinus.^{Reference Niesten, Hamberg, Silverman, Lou, McCall and Windsor34,Reference Niesten, Stieger, Lee, Merchant, Grolman and Rosowski35}

Despite the impact of the size and location of the defect on the auditory symptoms, it still remains unclear why some individuals are asymptomatic, at least at the time of the radiological diagnosis. The presence of a very thin bony layer that is too thin to be visualised on computed tomography (CT), or even cone beam CT, because of technical and resolution limitations, is a possible explanation. Despite radiological and electrophysiological confirmation of superior semicircular canal dehiscence, in some cases there might still be a very thin area of bone overlying the superior semicircular canal, preventing the clinical appearances of superior semicircular canal dehiscence. Nonetheless, symptomatic patients with a very thin bony cover, reflecting ‘near-superior semicircular canal dehiscence’, have been reported.^{Reference Ward, Wenzel, Ritzl, Gutierrez-Hernandez, Della Santina and Minor36,Reference Lookabaugh, Kelly, Carter, Niesten, McKenna and Curtinet37}

An alternative theory is that not every patient with superior semicircular canal dehiscence will develop symptoms because of the lack of a ‘second event’.^{Reference Minor19,Reference Chilvers and McKay-Davies22,Reference Whyte Orozco, Martinez, Cisneros, Obon, Gracia-Tello and Angel Crovetto38} This theory is based on the concept that many individuals can have superior semicircular canal dehiscence but will not develop any symptoms unless there is a second event that initiates them. In the literature, these second events (possible mechanisms of symptom initiation in a patient with existing superior semicircular canal dehiscence) have included: head trauma; bone metabolism abnormalities, for example osteoporosis; increased middle-ear or intracranial pressure; recent flight; or rapid changes of pressure. Given the communication of the inner-ear space with the cerebrospinal fluid,^{Reference Pisano, Niesten, Merchant and Nakajima26,Reference Rosowski, Songer, Nakajima, Brinsko and Merchant27} even changes in the intracranial pressure can affect patients’ symptoms, acting as a ‘trigger’ for symptom initiation (the second event).

Additionally, some patients might not experience any significant symptoms, as a result of a ‘non-functional’ superior semicircular canal either, because of a previous infection (labyrinthitis causing failure, fibrosis or obliteration of the canal) or any other event that has resulted in superior semicircular canal failure. In such cases, the patients can be asymptomatic despite the radiological evidence of superior semicircular canal dehiscence.

In summary, the otherwise healthy symptomatic patient with superior semicircular canal dehiscence will typically present with pressure- or sound-generated dizziness and nystagmus, oscillopsia, autophony, and conductive hyperacusis, and will have normal otoscopic findings, and low-frequency conductive hearing loss with a bone conduction supra-threshold, while Weber's tuning fork test will lateralise towards the affected side (Table 1).

Origin

Given the available evidence, there is an ongoing debate as to whether superior semicircular canal dehiscence is a congenital, developmental or an acquired condition. There are good numbers of studies in favour of both theories, indicative of the scientific controversies and possibly the co-existence of both; the precise mechanism by which the ‘third window’ is created is unclear. Based on the current evidence, explained below, there are patients with superior semicircular canal dehiscence of a congenital origin, but also patients with a pre-existing thinning of the bony overlay of the superior semicircular canal, which gradually was either absorbed or disrupted, leading to the presence of superior semicircular canal dehiscence and its related symptoms. Careful analysis of the available studies indicates that no one theory can explain all cases; thus, it appears that superior semicircular canal dehiscence could be the result of the combination of an acquired factor and an existing congenital or developmental background (often referred to as a ‘second event’ theory).

Takahashi et al. compared macroscopically the middle cranial fossa of adult and fetal cadavers, including imaging findings.^{Reference Takahashi, Tsunoda, Shirakura and Kitamura39} They showed that the superior semicircular canals protrude into the cranium in fetal life, while in adulthood they are embedded in bone. This observation raised the hypothesis that this protrusion into the cranium could lead to adhesions between the dura and the superior semicircular canal, and increase the likelihood of a superior semicircular canal dehiscence in later life. Additionally, the bilateral presentation of superior semicircular canal dehiscence in some cases, or even the contralateral thinning of the bony cover of the superior semicircular canal reported by many authors, supports a congenital background of superior semicircular canal dehiscence. Furthermore, studies showing multiple tegmen defects and/or ear malformations in patients with superior semicircular canal dehiscence are in favour of such an origin.^{Reference Suryanarayanan and Lesser40–Reference Oh, Vivas, Hudgins and Mattox43}

A recent case–control study showed better-developed (pneumatised) temporal bones and lower-lying middle fossa dura anteriorly in patients with superior semicircular canal dehiscence, raising the possibility of a developmental cause of superior semicircular canal dehiscence.^{Reference Tikka and Kontorinis44} Furthermore, the description of superior semicircular canal dehiscence in paediatric patients, aged even younger than one year, and the decreased prevalence of thin bony cover of the superior semicircular canal with age, also support a congenital background.^{Reference Jackson, Allen, Morell, Carpenter, Givens and Kakade15,Reference Saxby, Gowdy, Fandiño, Chadha, Kozak and Sargent16,Reference Meiklejohn, Corrales, Boldt, Sharon, Yeom and Carey45,Reference Lagman, Ong, Chung, Elhajjmoussa, Fong and Wang46} Overall, certain aspects of temporal bone anatomy have been sporadically associated with superior semicircular canal dehiscence. However, it is difficult to determine whether such anatomical differences, like the thin overlying bone, have a causal effect or not.

On the other hand, the prevalence of superior semicircular canal dehiscence has been found to be higher in older patients and in those with osteoporosis, suggesting an acquired rather than a congenital origin.^{Reference Nadgir, Ozonoff, Devaiah, Halderman and Sakai12,Reference Crovetto, Whyte, Rodriguez, Lecumberri, Martinez and Fernandez47–Reference Mahulu, Fan, Ding, Quaye, Mambo and Mafuru50} For instance, an assessment of 306 temporal bone CT scans showed increased radiological prevalence in older patients, with nearly 8 per cent of the scans showing superior semicircular canal dehiscence and nearly 45 per cent showing thinning of the bony cover of the superior semicircular canal.^{Reference Nadgir, Ozonoff, Devaiah, Halderman and Sakai12} Two years later, using similar settings, a study involving temporal bone imaging of 312 patients showed slight osteopenia of the superior semicircular canal roof associated with ageing.^{Reference Crovetto, Whyte, Rodriguez, Lecumberri, Martinez and Fernandez47} Additionally, a recent study investigating bone metabolism markers concluded that such markers could be important in the clinical assessment and management of patients with superior semicircular canal dehiscence.^{Reference Nguyen, Lagman, Sheppard, Duong, Ong and Poon48}

Despite the abovementioned evidence, and the possibility of superior semicircular canal dehiscence being an acquired condition, the otic capsule undergoes little remodelling once its development is completed, mostly because of factors related to genetics of the inner ear.^{Reference Sørensen, Frisch and Bretlau51,Reference Thys and Van Camp52} Thus, the direct impact of bony changes of the temporal bone on the superior semicircular canal itself is debatable.

In addition to age-related changes of the temporal bone and bone metabolism, body mass index (BMI) and the presence of increased intracranial pressure have been examined as factors contributing to the pathophysiology of superior semicircular canal dehiscence. Although Schutt et al. showed a possible causal correlation between increased BMI, obstructive sleep apnoea and superior semicircular canal dehiscence,^{Reference Schutt, Neubauer, Samy, Pensak, Kuhn and Herschovitch11} and El Hadi et al. showed a possible association between increased intracranial pressure and superior semicircular canal dehiscence,^{Reference El Hadi, Sorrentino, Calmels, Fraysse, Deguine and Marx53} the causal impact of increased intracranial pressure is controversial. Indeed, Kuo et al. failed to identify an association between increased intracranial pressure and the presence of superior semicircular canal dehiscence.^{Reference Kuo, Bagwell, Mongelluzzo, Schutt, Malhotra and Khokhar54}

If one considers the controversial available evidence indicating a congenital or developmental origin for some cases, and an acquired origin for others, the second event theory seems to offer the most rational explanation for the origin of superior semicircular canal dehiscence. In particular, a congenital or developmental background (first event) may well be present in most cases; however, it will be an additional factor such as a head trauma, a rapid increase in middle-ear or intracranial pressure, or age-related factors that will result in the superior semicircular canal dehiscence.^{Reference Minor19,Reference Chilvers and McKay-Davies22,Reference Whyte Orozco, Martinez, Cisneros, Obon, Gracia-Tello and Angel Crovetto38,Reference Tikka and Kontorinis44,Reference Brandolini, Modugno and Pirodda55} Essentially, the second event causes disruption of the thin overlying bone, leading to the bony defect. Although a second event cannot always be identified, it is assumed that this is mostly related to the retrospective character of many of the available studies, and the gaps in the clinical information and the medical history.

A gradual progressive thinning of bone occurring over time in predisposed individuals currently offers the most logical explanation for the origin of superior semicircular canal dehiscence.

Key diagnostic batteries

Although the suspicion of superior semicircular canal dehiscence is based on purely clinical grounds and detailed medical history, it is the imaging studies, with specific focus on the high-resolution CT of the temporal bones (Figure 1), and the audiovestibular assessment, mainly vestibular-evoked myogenic potential testing, that will confirm the diagnosis (Figure 2 and Table 1).

Additional tests such as pure tone audiometry, globally used as the first-line method of hearing assessment, can show typical but not pathognomonic findings, namely a low-frequency supra-threshold (indicative of hypersensitivity to sounds) and a low-frequency air–bone gap (conductive hearing loss in the low frequencies) (Figure 2).^{Reference Minor, Solomon, Zinreich and Zee8–Reference Chilvers and McKay-Davies22}

While other tests have been sporadically described, the combination of clinical suspicion, low-frequency conductive hearing loss, the radiological presence of superior semicircular canal dehiscence and the registration of vestibular-evoked myogenic potential responses in lower than normal hearing thresholds are considered pathognomonic for superior semicircular canal dehiscence (Table 1).

Treatment options

Based on the increasing evidence on dealing with superior semicircular canal dehiscence, surgical interventions are currently being considered as a sensible therapeutic approach for the symptomatic patient. The decision-making follows detailed consultation, careful assessment of symptom severity, patients’ realistic expectations, desired outcomes and related risks. Overall, surgical intervention in a limited variety of approaches and methods has shown promising outcomes in dealing with the vestibular symptoms and oscillopsia, while the impact on the auditory symptoms, although they are mostly improved, remains unpredictable.^{Reference Minor19,Reference Ziylan, Kinaci, Beynon and Kunst20,Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos82–Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86} There is increasing evidence that surgery, in the sense of either resurfacing the bony defect of the superior semicircular canal or obliterating the superior semicircular canal, can achieve good results in the symptomatic patient with superior semicircular canal dehiscence, while the risks for associated complications, mainly irreversible severe sensorineural hearing loss and long-term imbalance, are relatively low in experienced centres.^{Reference Minor19,Reference Ziylan, Kinaci, Beynon and Kunst20,Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos82–Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86} However, the available evidence, mostly a result of the limited timeframe since the initial description of the condition and the issues with diagnosing the disease, is limited to small cohorts and retrospective case series.

In contrast, superior semicircular canal dehiscence patients who are asymptomatic or have much milder symptoms are managed conservatively, undergoing a thorough consultation and receiving generic vestibular rehabilitation advice. To date, there is no direct evidence on the efficacy of the conservative management of superior semicircular canal dehiscence, but such management is reserved for patients with milder clinical presentation, who generally do not require any active treatment.

With respect to resurfacing the bony defect, this can be achieved through either a middle fossa (intracranial) approach or a transmastoid (extracranial, through the mastoid) approach. The risks have overall been described as being slightly higher for the middle fossa approach, mainly because of the additional potential risks associated with intracranial operations, such as seizures or stroke.^{Reference Minor19,Reference Ziylan, Kinaci, Beynon and Kunst20,Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos82–Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86} However, in most available studies, there seems to be no statistically significant difference in the complication rates involving irreversible sensorineural hearing loss or permanent balance problems.^{Reference Minor19,Reference Ziylan, Kinaci, Beynon and Kunst20,Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos82–Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86}

Transmastoid obliteration of the superior semicircular canal has been gradually gaining popularity. It involves identification of the superior semicircular canal through mastoidectomy, gentle opening of the canal and its obliteration (elective cancellation or destruction of the superior semicircular canal). This has been shown to have great vestibular outcomes without higher complication rates compared with resurfacing techniques; additionally, it is an extracranial procedure.^{Reference Minor19,Reference Ziylan, Kinaci, Beynon and Kunst20,Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos82–Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86}

Both interventions described here, middle fossa or transmastoid resurfacing and transmastoid obliteration of the superior semicircular canal, can achieve resolution of the vestibular symptoms, with a potential, but not definite, positive impact on the auditory symptoms. Thus, they are considered well-established surgical techniques. The even more recently introduced obliteration of the round window (which aims to block the natural second window, the round window),^{Reference Nikkar-Esfahani and Banerjee87} despite the initial enthusiasm associated with its technical simplicity (it can be performed as a day-case procedure), has not shown adequate control of the patients’ symptoms.^{Reference Succar, Manickam, Wing, Walter, Greene and Azeredo88,Reference Ahmed, Rajagopal and Lloyd89} Ahmed et al., in 2019, reviewed the available evidence on round window obliteration; they concluded that the evidence is scant and that this procedure is unlikely to replace the better-established techniques of superior semicircular canal resurfacing or obliterating.^{Reference Ahmed, Rajagopal and Lloyd89}

An early review paper published in 2009 on the efficacy and complications of surgical approaches showed only studies with short-term follow up, and an absence of standardised assessment and documentation methods. Still, it concluded that surgery was a valid therapeutic option for symptomatic patients with superior semicircular canal dehiscence, as the overall reported outcomes were promising.^{Reference Vlastarakos, Proikas, Tavoulari, Kikidis, Maragoudakis and Nikolopoulos82} Gradually, more studies focusing on surgical outcomes were published, shedding more light on approaches and results.

More recent reviews have shown no superiority of any of the approaches. In particular, Gioacchini et al., in 2016, identified no significant differences among the different modalities; a year later, Ziylan et al. came to the same conclusion.^{Reference Ziylan, Kinaci, Beynon and Kunst20,Reference Gioacchini, Alicandri-Ciufelli, Kaleci, Scarpa, Cassandro and Re85} Additionally, they showed lower complication and revision rates, and a shorter hospital stay, with the transmastoid approach, indicating this as a method of choice in the severely affected superior semicircular canal dehiscence patient. Interestingly, a systematic review published in 2018, conducted by Nguyen et al., which included 24 retrospective studies (230 patients), showed that resurfacing through the middle fossa approach can achieve higher odds of resolving auditory symptoms compared with the transmastoid approach; however, it did not include factors beyond symptom resolution and associated hearing loss (in-patient stay or revision rates).^{Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86} The authors also commented on the better exposure that the middle fossa approach can offer compared with the transmastoid approach; this factor could reduce the risk of deafness as a complication of the surgery.^{Reference Nguyen, Lagman, Sheppard, Romiyo, Duong and Prashant86}