Superior Canal Dehiscence

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]

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]

Patients with superior canal dehiscence syndrome (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. This gaze-evoked tinnitus can be found in almost 25% of patients with SCDS. Hyperacusis is defined as an unusual sensitivity to normal everyday sounds. Although not specific, this condition is found in a high percentage of patients with SCDS.

After a general head and neck examination is performed, a detailed neurootological examination should be performed in all patients with vertigo. Other, more common, causes of vertigo and imbalance must be eliminated before SCDS can be diagnosed. Note the following tests and examinations:

• Gait test: Determine whether the patient staggers or is off-balance with gait. Typically, patients with SCDS demonstrate a normal gait pattern.

• Oculomotor examination: All patients must be assessed for an intranuclear ophthalmoplegia and other signs of multiple sclerosis. In addition, gaze-dependent nystagmus must be eliminated as a cause of the imbalance. Nystagmus of peripheral (ie, labyrinthine) origin is usually unidirectional. Nystagmus of central origin (ie, brainstem) is usually bidirectional. Patients with SCDS do not typically demonstrate nystagmus upon routine examination. Nystagmus can be induced with loud sounds or with a pressure fistula test.

• Romberg test: This test is used to evaluate peripheral sensation, dorsal column function, and midline cerebellar function. The results of this test are usually abnormal in patients with a central pathologic condition. Patients with SCDS demonstrate normal Romberg test results.

• Fukuda test: Patients who undergo this test are asked to step in place for 20-30 seconds with their eyes closed. Rotation of the patient may indicate a unilateral loss of vestibular function. The results of this nonspecific test are typically normal in patients with SCDS.

• Dix-Hallpike maneuver: The Dix-Hallpike maneuver is performed by laying a patient back suddenly with the patient's head turned to one side. The test results are considered abnormal (or positive) if the examiner sees geotropic or ageotropic rotatory nystagmus that typically lasts less than 60 seconds. An abnormal or positive Dix-Hallpike examination result is most likely due to benign paroxysmal positional vertigo (see the Medscape Reference article Benign Paroxysmal Positional Vertigo). Patients with SCDS do not typically have positive Dix-Hallpike test results.

• Head-shake test: The patient wears Frenzel lenses, and the examiner shakes the patient's head at approximately 1 Hz in the horizontal plane for 20 seconds. After the shaking is stopped, the eyes are observed for nystagmus. This test can reveal latent nystagmus and indicate which labyrinth is malfunctioning. In this test, the fast phase of nystagmus is directed toward the normal (or better-performing) labyrinth. The results of this nonspecific test may be abnormal in patients with SCDS.

• Head-thrust test: The patient is asked to gaze steadily at a target in the room while the examiner briskly moves the patient's head from side to side. If the patient's eyes remain fixed on the target, the test result is normal. When the eyes make a compensatory movement after the head is stopped to reacquire the target (a refixation saccade), the test results are abnormal. This test can indicate if the output of one or both labyrinths is depressed. This is usually not a typical finding in SCDS.

• Visual dynamic acuity test: Before and during vigorous vertical shaking, followed by horizontal head shaking, the patient is asked to read the smallest visible line on the Snellen eye chart. A normal result is the ability to maintain acuity within 2 lines of the acuity at rest. An abnormal visual acuity test suggests bilateral vestibulopathy, which is most commonly observed in ototoxicity. The dynamic visual acuity test results are normal in patients with SCDS.

• Fistula test: The fistula test is designed to elicit symptoms and signs of an abnormal connection between the labyrinth and the surrounding structures. This is usually performed while the patient wears Frenzel lenses. Pressure can be applied to the patient's ear by pushing the tragus over the ear canal or with the use of a Bruening otoscope. If vertiginous symptoms are elicited or if nystagmus is seen the patient has positive fistula test results. In SCDS, the superior canal can be thought of as a fistula with connection to the middle cranial fossa. With SCDS, the direction of the nystagmus, as a result of pressure applied to the ear, results in vertical-torsional nystagmus with slow waves directed away from the labyrinth suspected of being dehiscent. Other tests, such as the pinched nostril test, can demonstrate similar findings, whereas a Valsalva maneuver produces nystagmus in the opposite direction.

• Eye movements evoked by sound are found in most patients with SCDS. These sound-induced eye movements are typically found at frequencies of 500-2000 Hz, with an intensity of 100-110 dB. The eye movements are typically vertical and torsional, away from the side of the stimulus.

• A Barany noise box can also be used to help elicit the noise-induced vertigo (Tullio phenomenon). This commercially available box simply makes a loud (100 dB) noise. When the box is slowly moved towards the patient's symptomatic ear, the vertiginous symptoms may be re-created.

An embryological etiology of SCDS has been proposed; this theory involves a postnatal failure of bone formation over the superior semicircular canal. Tsunoda and Terasaki, with the use of a computer simulation model, determined that the cause of bony dehiscence of the superior semicircular canal was due to a malpositioned primitive otocyst.[5] When this otocyst lies too close to the developing brain, the migratory patterns of the loose reticular cells are altered. These mesenchymal cells are thought to be necessary for completion of the bony development of the labyrinthine structures. This region may be left with incomplete or thin bony development over the superior semicircular canal.

No specific laboratory studies confirm the diagnosis of superior canal dehiscence syndrome (SCDS). Laboratory tests may be obtained to rule out other pathological causes of vertigo (ie, multiple sclerosis, syphilis).

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. Views that are oblique to the temporal bone must be obtained with a HRCT scan in order to properly see the superior semicircular canal. The image below displays the typical findings in a patient with SCDS.

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.

Magnetic resonance imaging (MRI) cannot be used to confirm the diagnosis of SCDS; however, it may be of benefit in ruling out a retrocochlear process.

Audiometric testing

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.

Patients with SCDS may have conductive hearing loss. The air-bone gaps are typically greatest at frequencies below 1 kHz.

The conductive hearing loss is believed to be due to the third mobile window of the superior semicircular canal dehiscence, resulting in elevation of thresholds of air-conduction sounds and a reduction of thresholds for bone-conduction sounds.

Acoustic reflex testing is preserved in a patient with SCDS as a cause of conductive hearing loss, as opposed to a loss of the reflex with other conductive or mixed hearing loss conditions such as otosclerosis.

Electronystagmography testing

Routine electronystagmography (ENG) testing reveals no objective or pathognomonic signs of SCDS. All patients with suspicion of the disorder should undergo ENG to further eliminate other potential causes.

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.

Vestibular evoked myogenic potentials

Vestibular evoked myogenic potentials (VEMPs) have recently been suggested to help with the diagnosis of SCDS. The inferior vestibular nerve innervates the saccule, which has some sound sensitivity. The inferior vestibular nerve has its main input to the lateral vestibular nucleus (Deiter nucleus), where the 2 main postural tracts originate. The medial vestibulospinal tract is responsible for postural control of the neck, whereas the lateral vestibulospinal tract is dedicated to the lower trunk and limbs.

For the most part, sound-evoked VEMPs are considered completely unilateral. This test is performed by placing electrodes on the sternocleidomastoid neck muscle. Patients hold their head up unsupported, using only their anterior neck muscles. Patients are instructed to tense the muscle during acoustic stimulation and to relax after the stimulation stops. Loud clicks or tone bursts (95-100 dB nHL) are repeatedly presented to each ear.[6] If the neck muscles are activated at this level, a VEMP is produced.

In patients with SCDS, a response at very low thresholds (< 65 dB) can be noted to produce a VEMP on the affected side. This is thought to occur secondary to the hypercompliance of the vestibular system on the affected sided secondary to the third-window effect. A low-threshold VEMP raises the suspicion of SCDS.

A study by Verrecchia et al indicated that in ocular VEMP tests, an amplitude of greater than 33.8 μVolts has a sensitivity and specificity of 87% and 93%, respectively, for differentiating an ear with SCDS from a healthy one.[7]

Vibration-induced nystagmus

Most recently, patients with known SCDS underwent a series of cranial vibratory tests. A 100 Hz oscillator was placed against the cranium at different places for 10-15 seconds. Nystagmus was digitally recorded by infrared video oculography. All patients demonstrated distinct torsional vibration-induced nystagmus that was especially prominent with suboccipital vibration. Vibration of the suboccipital region and the demonstration of torsional nystagmus is an emerging technique for the diagnosis of SCDS.[8]

Surgical correction of superior canal dehiscence syndrome (SCDS) is reserved for patients with severe disabling symptoms.

Recently, albeit rare, it has been found that children also can have with superior canal dehiscence. They are almost always asymptomatic, and their management is no different from that of adults. Usually, they present with auditory complaints initially, and conservative management is recommended; however, if a child has complaints of vertigo, surgical management may be indicated.[9]

The treatment options for patients with SCDS are still expanding, with new innovative methods under development.

Traditional middle fossa craniotomy and repair of fistula

In this procedure, patients undergo a middle cranial fossa craniotomy on the affected side. The temporal lobe is gently retracted. Upon elevation of the dura, care is required to avoid stretching the greater superficial petrosal nerve, which could injure the facial nerve. The region of the superior semicircular canal is located with identification of the arcuate eminence. A dehiscence of the superior semicircular canal can be covered with bone wax, bone cement, or fascia, or the canal can be ablated with wax or bone cement.

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. This approach has an average hospital stay of 2 days and a smaller incision than the typical middle fossa approach originally described. Presented at the Virginia Society of Otolaryngology in 2010, Shaia and Peng discussed 10 patients who underwent this modified approached relief of preoperative symptoms of more than 90% with no adverse outcomes. One patient had continued vestibular complaints for 12 months after the procedure that eventually resolved. This approach allows the surgeon visualization of the dehiscence without the need for a larger craniotomy.

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. Brantberg et al performed a superior canal–plugging procedure via a transmastoid approach in 2 patients.[10] Postoperatively, sound- and pressure-induced symptoms and nystagmus were resolved in response to offending stimuli.

Similarly, a retrospective study by Banakis Hartl and Cass indicated that transmastoid canal plugging can be safely and effectively used in place of the traditional middle cranial fossa approach in patients with SCDS. Among 17 patients (19 ears) who underwent transmastoid canal plugging, statistically significant improvements were reported in autophony, vertigo, aural fullness, and pulsatile tinnitus. Additionally, a 10-dB improvement in the air-bone gap at 250 Hz was found. The disequilibrium rating did not significantly improve, however, and the pure-tone average and word recognition score did not change.[11]

Although transmastoid superior canal occlusion can be effective, the risk of sensorineural hearing loss increases with this procedure because of limited exposure versus middle fossa craniotomy.

Minimally invasive approach via transcanal oval and round window reinforcement

The optimal surgical treatment of superior canal dehiscence syndrome has yet to be determined. 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]

The concept is to reduce the effects of a third window at the superior semicircular canal by reinforcing the other 2 natural windows. By dampening the presumed hypercompliance of the inner ear at the round windows, rather than intracranial, the risks of craniotomy, which include death, stroke, cranial palsies, and cerebrospinal fluid leaks, are avoided.

This procedure can be performed under local anesthesia. A transcanal approach to the middle ear is performed with elevation of the tympanic membrane. Small amounts of fascia are harvested from a postauricular incision are used to reinforce both the oval and round windows. The risks of the procedure, performed on an outpatient basis, are extremely low.

The preliminary study by Kartush in 2002 propagated discussions with others in the field of otology/neurology. As a result, a combined multi-institutional study under the direction of Silverstein was initiated in 2011 and was presented at the 2012 American Academy of Otolaryngology Annual meeting.[12, 13]

Twenty-two patients from 4 centers who opted for the minimally invasive approach via transcanal oval and/or round window reinforcement were studied. In this study, most had only round window reinforcement. Symptoms improved in nearly all patients treated with this novel method. A total of 21 patients completed a postoperative survey, grading preoperative and postoperative symptom severity. The questionnaire measured the 9 symptoms, including autophony, sensitivity to bone conduction, pulsatile tinnitus, sensitivity to loud sounds, dizziness on straining, dizziness on increased middle ear pressure loss, aural fullness, and imbalance. Analysis revealed statistically significant improvement in nearly all symptoms. The only symptom that did not improve was hearing loss.

If future studies confirm that such a minimally invasive procedure is routinely effective, middle fossa craniotomy or transmastoid approaches with resurfacing or occlusion of the canal could then be reserved for patients with persistent symptoms.

Combination approach

Gianoli and Soileau have advocated a combination approach.[12] In this technique, the superior canal defect is repaired via the middle fossa approach with concomitant reinforcement of the oval and round windows. The authors claim improved outcomes over middle fossa repair alone, reporting a resolution of vertigo in 24 cases with a 5-year follow-up period.

Consultation with an otologist or neurotologist should be obtained in any patient with symptoms that coincide with superior canal dehiscence.

A study by Barber et al indicated that new-onset benign paroxysmal positional vertigo often follows repair of SCDS. The study found that 23.8% of patients who underwent the surgery developed this form of vertigo, compared with 6.2% of those who did not undergo the operation. Moreover, the vertigo was known to be lateralized to the side on which surgery was performed in all but one affected surgical patient (in whom the laterality was not known). The investigators suggested that the vertigo may result from otolith mobilization caused by inner ear pressure changes.[14]

Patients are monitored based on the intervention and the severity and complexity of the patient's symptoms. Patients who undergo a craniotomy for repair should expect to stay in the hospital for 1-7 days (median 2 d). They may experience imbalance for approximately one month postoperatively.

The success rate in the treatment of superior canal dehiscence is quite high. In 2005, a study looked at 20 patients with severe symptoms who underwent surgical repair of their superior canal dehiscence through a middle fossa approach.[15] Canal plugging was performed in 9 patients and a resurfacing technique was performed in 11 patients.

Complete resolution of all vestibular symptoms and signs was achieved in 8 of the 9 patients after the canal was plugged. A lower but still significant number of patients with a resurfacing procedure (7 of the 11) had resolution of their vestibular complaints. A total of 15 of the 20 patients (75%) of the patients had resolution of their symptoms after surgical plugging or resurfacing of their dehiscent canal.