Superior semicircular canal dehiscence syndrome

Otology & Neurotology 31:447Y454 Ó 2010, Otology & Neurotology, Inc.

A Superior Semicircular Canal Dehiscence Syndrome Multicenter Study: Is There an Association Between Size and Symptoms? *Alain Pfammatter, †Vincent Darrouzet, *Marcel Ga¨rtner, ‡Thomas Somers, ‡Joost Van Dinther, §Franco Trabalzini, kDenis Ayache, and *Thomas Linder *Department of OtorhinolaryngologyYHead & Neck Surgery, Kantonsspital Luzern, Luzern, Switzerland; ÞDepartment of Otorhinolaryngology and Skull Base Surgery, Pellegrin-Tripode University Hospital, Bordeaux, France; þDepartment of Otorhinolaryngology, Saint Augustine Hospital, Antwerp, Belgium; §Department of Otosurgery, Azienda Ospedaliera di Padova, Padova, Italy; and kOtology-Neurotology Unit, Fondation A. de Rothschild, Paris, France

Objective: The aim of this investigation was to determine if there is any association between the size of the canal dehiscences and the symptoms and signs of patients presenting with the superior semicircular canal dehiscence syndrome. Study Design: Prospective multicenter study. Setting: Tertiary referral center. Patients: Twenty-seven patients, 14 females and 13 males, aged 25 to 83 years, coming from Switzerland, France, Belgium, or Italy, with dehiscence of the superior semicircular canal diagnosed by high-resolution computed tomographic scans of the temporal bone. Interventions: Audiologic tests, a battery of vestibular tests (Tullio phenomenon, Hennebert sign, Valsalva maneuver), vestibular evoked myogenic potentials (VEMPs), and highresolution computed tomographic scans of the temporal bone. Main Outcome Measures: Association between the symptoms/ signs and the size of the superior canal dehiscence. Results: Clinically patients could be divided into three different groups: Superior canal dehiscences (Q2.5 mm) presented pre-

dominantly with cochleovestibular symptoms and/or signs (sensitivity, 91.7%; specificity, 70%), whereas smaller one_s showed either cochlear or vestibular dysfunction. Patients with larger dehiscences were significantly more associated with vestibulocochlear symptoms/signs, lower VEMP thresholds, and objective vestibular findings (e.g., Tullio phenomenon) than subjects with smaller bony defects. No significant association between the size of the dehiscence and the audiogram pattern or individual findings could be found. The location of the dehiscence seemed to have no influence on the clinical manifestation and findings. Conclusion: Patients with larger superior canal dehiscences show significantly more vestibulocochlear symptoms/signs, lower VEMP thresholds, and objective vestibular findings compared with smaller ones. Smaller dehiscences mainly present with either cochlear or vestibular dysfunction. Key Words: Conductive hearing lossVDizzinessVInner earVSuperior semicircular canal dehiscenceVVertigo. Otol Neurotol 31:447Y454, 2010.

In 1998, Minor et al. presented a new vestibular entity caused by a dehiscence of the bone overlying the superior semicircular canal (1). The syndrome is marked by sound- (Tullio phenomenon) and/or pressure-induced (Hennebert sign, Valsalva maneuver) vertigo and oscillopsia (1,2). Typical findings are vertical-torsional eye movements in the plane of the superior canal provoked by sound and or pressure changes (1,2). Audiometry tests often show low-frequency conductive hearing loss with normal or characteristically negative bone-conduction

thresholds (3Y6). The Weber tuning fork test typically lateralizes to the affected ear, and the acoustic reflexes are intact (3,7). Some patients with a Bbetter than normal[ bone conduction can hear their own eyes or joints moving (conductive hyperacusis), which can be very disturbing (8Y11). The symptoms and signs in superior semicircular canal dehiscence syndrome (SSCDS) can be understood from the effect of the dehiscence creating a Bthird mobile window[ and thereby making the inner ear more sensitive to sound and/or pressure changes (1,12,13). Studies of temporal bone histology hypothesize that SSCDS may arise from failure of postnatal bone development and that thin bone overlying the superior canal may later be disrupted by pressure of the temporal lobe or trauma (14,15). The diagnosis of superior canal

Address correspondence and reprint requests to Thomas Linder, M.D., Department of OtorhinolaryngologyYHead & Neck Surgery, Kantonsspital Luzern, Spitalstrasse, CH-6000 Luzern, Switzerland; E-mail: [email protected]

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dehiscence syndrome is based on temporal bone highresolution computed tomographic (HRCT) scans and the typical symptoms and signs previously described (1,15). Patients with SSCDS often show reduced vestibular evoked myogenic potential (VEMP) thresholds (3,6,7,16,17). This is probably because of increased effectiveness of the transmission of sound energy to the sacculus caused by the increased compliance of the inner ear (16). The clinical manifestation of SSCDS varies between individuals. Thorough counseling of the patient is usually sufficient. In severe cases, SSCDS can be treated by resurfacing orV more recentlyVplugging (18,19) the dehiscent superior canal, which considerably relieves the patient_s distress, but can also lead to sensorineural hearing loss and vestibular hypofunction (1,3,9,20,21). Since its first description, physicians and radiologists are increasingly aware of the lesion, and the diagnosis is made in a growing number of patients with quite a diversity of symptoms. The aim of this investigation was to determine if there is any association between the size of the bony dehiscence of the superior semicircular canal and the symptoms and signs of the individual patients. PATIENTS AND METHODS Members of the Club de Recherche et d_Enseignement en Neurotologie et Otologie (CRENO) decided in 2005 to perform a prospective study on newly diagnosed patients with SSCDS. A standardized clinical evaluation protocol was advocated in the different centers. A total of 27 patients (14 female and 13 male subjects), aged 25 to 83 years, were examined in 4 different European ENT clinics. The clinical data and the HRCT scans were collected in the Swiss center, further analyzed, and processed for statistical analysis. Thirteen patients were investigated at the Department of Otorhinolaryngology in Luzern (Switzerland), 7 patients in the center in Bordeaux (France), 6 patients in Antwerp (Belgium), and 1 subject in the Italian center. Initially, 4 more patients with a small (1Y2.5 mm) superior canal dehiscence were examined but later excluded from the study for the following reasons: three of these subjects presented other ear pathologies like malformations of the temporal bone, a vestibular schwannoma, and a profound hearing loss interfering with their symptoms. The forth patient did not appear for vestibular testing. These 4 subjects showed no typical SSCDS clinical pattern, and the superior canal dehiscence was just an incidental finding on the CT scan. The following neurootological and audiological investigations were recorded.

Audiologic Tests Pure tone audiometry was performed from 250 to 8000 Hz for air conduction and from 250Y500 to 4000 Hz for bone conduction. Bone conduction thresholds were measured starting at minus 10 dB nHL to identify negative bone conduction thresholds mimicking a possible air-bone gap. If required, masking was used for bone and for air conduction. Weber and Rinne tuning fork tests completed the audiological workup. Speech audiometry was optional and not specifically evaluated.

Vestibular Evoked Myogenic Potentials The click-evoked vestibular evoked myogenic potentials (VEMPs) are the motor response of the vestibulocollic reflex and a reproducible screening test of otolith function based on Otology & Neurotology, Vol. 31, No. 3, 2010

the acoustic sensitivity of the sacculus. Intense clicks of approximately 85 to 100 dB SPL stimulate the saccular receptors. The signal is then transmitted through the inferior branch of the vestibular nerve, the nuclear vestibular centers in the brainstem, the vestibulospinal tractus, and the spinal accessory nerve to the ipsilateral sternocleidomastoid muscle (SCM). Surface electromyographic (EMG) activity of the SCM can be subsequently obtained using disk electrodes. The active electrodes were placed over the SCM (2 active electrodes over the SCM on the stimulated side and 1 over the SCM on the other side), and the ground electrode was placed at the forehead. Recordings were made in a sitting position, and the subjects were instructed to turn their head to the opposite side of the stimulated ear to activate their SCM during measurements. The stimulation occurred monaurally via earphones with a stimulus rate of 5 Hz. One test period involved 250 click measurements. The VEMP response recorded from the SCM consists of an initial positivity (p13) followed by a negativity (n23) creating a characteristic biphasic wave. Patients with SSCDS often show lowered VEMP thresholds (e80dB SPL) and larger p13-n23 amplitudes compared with healthy people.

Tullio Phenomenon Measurements of sound-induced vertigo and/or nystagmus (Tullio phenomenon) were made as follows. High-level (Q90dB nHL) sounds over a frequency range of 500 to 4,000 Hz were presented to the patients via headphones. During the test period, the subjects were questioned about sensations of vertigo, and their eye movements were observed using Frenzel_s goggles. Direction of the nystagmus was recorded.

Hennebert Sign Vertigo and/or eye movements provoked by pressure changes in the external auditory canal (EAC) are known as the Hennebert sign. Politzer_s balloon was used to produce either positive or negative pressure in the patient_s EAC. During pressure appliance, Frenzel_s goggles allowed the observation of the subject_s eye movements, and the patients were subsequently asked about their judgment of vertigo or imbalance.

Valsalva Maneuver The Valsalva maneuvers can also lead to vertigo and/or nystagmus in patients with SSCDS. Subjects were instructed to take a deep breath, hold their nose, and blow forcefully against the pinched nostrils. In addition, the patients performed Valsalva maneuver against closed glottis by taking a deep breath and increasing their intrathoracic pressure.

Computed Tomography of the Temporal Bone The patients underwent helical high-resolution computed tomography of the temporal bone with a collimation of 64
0.6 mm and a reconstruction increment of 0.4 mm. Axial, coronal imaging and reconstructions in the dedicated best image plane of the superior semicircular canal were performed. Once verified in all planes, the size of the dehiscence was measured (in millimeters) on the high-resolution computed tomography reconstructions in the plane of the superior canal as a straight line between the bony ends of the dehiscence. In addition, the site of the dehiscence, whether anteriorly, centrally, or posteriorly placed was identified. All CT scans were analyzed in a standardized fashion by the same 2 investigators, unaware of the clinical symptoms of the patient on a PACS workstation. Real-time 3-dimensional multiplanar reconstruction reformats of the original 0.6-mm datasets were used. We estimate that a

12 S/54 L1 CHL + + -

22 9/66 R1 CHL + 85

Patient Sex/age Dehiscence (side and size)a Hearing loss Negative bone conduction Conductive hyperacusis Subjective vertigo Tullio phenomenon Hennebert sign Valsalva maneuver Vertigo during sport VEMP threshold (dB) Remarks

Patient Sex/age Dehiscence (side and size)a Hearing loss Negative bone conduction Conductive hyperacusis Subjective vertigo Tullio phenomenon Hennebert sign Valsalva maneuver Vertigo during sport VEMP threshold (dB) Remarks

3 S/44 R3.5 CHL + + + + 70

14 9/52 R5/L2 CHL/CHL + +/+ b 80/80 Right side operated Cochlear dysfunction 23 24 9/25 9/65 R1 L2 CHL MHL 85 80

13 S/72 L2 MHL + + +

2 9/43 R3 CHL + + + + 65

5 9/38 L2.5 CHL + + + 70

6 S/37 R4.5 CHL + + + + + 50

7 S/38 R4 MHL + + 70

15 9/58 R5/L3 CHL/CHL + +/+/+/+

Vestibulocochlear symptoms and/or signs 16 17 18 9/43 9/34 9/55 R5/L3 R3 R3 MHL MHL +/+ + + + +/+ + + +/+ + + +/+ + + + c 80/80 70 85 Right side Right side operated operated Vestibular disorders 25 26 27 S/48 S/71 9/54 R2 R1.5 R1 d d CHL + + + + + + b 90 100

4 S/49 R6/L7 CHL/+/+ + + +/+ + 75/75

Vestibulocochlear symptoms and/or signs

Patient’s overview

L indicates left side; R, right side; CHL, conductive hearing loss; MHL, mixed hearing loss; VEMP, vestibular evoked myogenic potential. a Size in mm (e.g., L6 = dehiscence of 6 mm on the left side). b Not tested. c Difficult to measure. d Bilateral hearing loss not caused by superior canal dehiscence.

b

1 S/68 L6 CHL + + + 65

Patient Sex/age Dehiscence (side and size)a Hearing loss Negative bone conduction Conductive hyperacusis Subjective vertigo Tullio phenomenon Hennebert sign Valsalva maneuver Vertigo during sport VEMP threshold (dB) Remarks

TABLE 1.

19 9/83 R3/L7 MHL/MHL + -/+ -/+ 80/80

8 S/36 L5 + + + + + 65 20 9/39 R4 CHL + + + 80

b

9 S/80 R3 MHL + + -

b

21 9/41 R5/L2.5 CHL/CHL + +/-

b

10 S/72 R3 MHL + + +

11 S/27 R4/L4 CHL/CHL +/+ + + +/+ +/+ + 980

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A. PFAMMATTER ET AL. Typical vestibular disorders were vertigo and/or eye movements associated with Tullio phenomenon, Hennebert sign, and Valsalva maneuver, or a subjective constant sense of vertigo or disequilibrium. Most of the patients demonstrated a nystagmus beating downwards and torsionally in the direction of the affected ear (clockwise on the right side, counterclockwise on the left side), provoked by one of the maneuvers previously described. VEMP thresholds were decreased (50Y80 dB SPL) in 17 of 20 ears belonging to Group I. Sixteen (57%) of 28 ears presented a conductive hearing loss and/or negative (G0 dB nHL) bone conduction thresholds (Fig. 1). Eight subjects exemplified the phenomenon of conductive hyperacusis. High-resolution computed tomography showed superior canal dehiscences of 1 to 7 mm length (mean, 3.9 mm; Fig. 2). All patients with bilateral dehiscences had cochleovestibular symptoms attributed mainly to the larger size. Only one of these 7 bilateral cases had a minimal unilateral size of 2 mm, whereas the other 6 patients presented with their smaller size between 2.5 and 6 mm.

FIG. 1. Audiogram of Patient 1, as a typical example for low frequency conductive hearing loss with negative bone conduction thresholds up to 1 kHz.

minimal lesion size of 1 mm could be established using this technique.

Statistics Statistical analyses were performed by the Department of Biostatistics at the University of Zurich. Independent samples t test for probability (p) and crosstabs for sensitivity and specificity with Fisher_s exact test were calculated. Differences were taken as significant if the p value was below 0.05. For all statistical procedures, SPSS statistical software was used.

Group II: Cochlear Symptoms and Signs Four patients (15%) presented with only cochlear disturbances. The cochlear dysfunctions in these subjects consisted of conductive and mixed hearing losses. The high-resolution computed tomography displayed dehiscence of the superior semicircular canals in the range of 1 to 2 mm. Group III: Vestibular Symptoms and Signs Two subjects showed only vestibular disorders. Patient 26 fell to the right side during forced Valsalva maneuver. Tullio phenomenon was associated with vertigo and nystagmus beating to the right side and torsionally clockwise to the affected ear. Patient 27 complained of a paroxysmal sensation of vertigo pulling her to the right side. The Valsalva maneuver against pinched nostrils and closed glottis both elicited the vertigo. The SSCD seemed to have no

RESULTS The main results of the clinical tests and the patient_s symptoms have been analyzed and are summarized in Table 1. Of the 27 patients, 7 had bilateral dehiscences. In cases of bilateral dehiscences, both ears have been used for calculations. Hearing tests were available in all of the patients, VEMP measurements could not be evaluated in 7 subjects. Clinical Classification Based on their symptoms and signs, the patients could be divided into 3 different groups: patients with cochleovestibular signs and/or symptoms, patients with cochlear symptoms only, and patients with vestibular symptoms only (Table 1). Group I: Cochleovestibular Symptoms and Signs Twenty-one patients (78%) showed vestibular as well as cochlear symptoms and signs. Only two of them had small dehiscences of 1 and 2 mm (patients 12 and 13). Otology & Neurotology, Vol. 31, No. 3, 2010

FIG. 2. High-resolution CT reconstruction in the plane of the left superior canal showing a dehiscence of 7 mm (Patient 4).

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FIG. 3. Box plot graphic comparing the relationship of dehiscence size between patients with cochleovestibular symptoms and the other 2 groups (p G 0.001).

FIG. 5. Box plot graphic comparing the relationship of dehiscence size between patients with pathologically lowered VEMP thresholds (e80 dB SPL) and normal VEMP thresholds (980 dB SPL) (p 0.009).

influence on the cochlear function in both individuals. High-resolution computed tomography showed dehiscence of the right superior canals of 1.5 and 1 mm. Both patients presented VEMP thresholds in a normal range.

70% with a Fisher_s exact test of less than 0.001 for the ability to predict from the size of the dehiscence the cochleovestibular symptom group versus the other groups.

Analysis of Symptoms and Findings Relationship Between Size and Signs/Symptoms Statistical analysis revealed a significant relationship between the size of the dehiscence and the patient_s clinical classification previously described. The patients of the cochleovestibular group showed significantly (p G 0.001) larger dehiscences of the superior semicircular canal (mean, 4.1 mm) compared with the other 3 groups. Subjects with only vestibular or cochlear symptoms presented with a mean dehiscence size of 1.9 mm (Fig. 3). The impact of the size difference became also apparent by analyzing the 3 clinical groups in reference to a dehiscence size of larger or smaller than 2.5 mm (Fig. 4). Larger dehiscences of the superior canal (Q2.5 mm) mainly go together with cochleovestibular symptoms, whereas smaller bony defects (G2.5 mm) either with cochlear or vestibular symptoms. The sensitivity was 91.7%, and the specificity was

FIG. 4. Graphical overview illustrating the association between the size of the superior canal dehiscence and the 3 clinical groups.

Relationship Between VEMPs and Size of Dehiscence A negative linear relationship between the VEMPs and the size of the superior canal dehiscence was identified in this study. Subjects with larger bone defects showed significantly (p 0.009) more often pathologically lowered VEMP thresholds (e80 dB SPL; Fig. 5). Patients with a dehiscence size of 2.5 mm or greater presented characteristically lowered VEMP thresholds (e80dB SPL) than subjects with dehiscence sizes below 2.5 mm (Fig. 6). The sensitivity was 88.9%, but the specificity was only 57.1% with a Fisher_s exact test of 0.032. Relationship Between Size of Dehiscence and Objective Vestibular Tests The otoneurological examinations revealed a significant ( p = 0.001) association with the size of dehiscence (Fig. 7). The variable Bobjective vestibular test[ was

FIG. 6. Graphic illustrating the association between size of dehiscence and VEMP threshold. Otology & Neurotology, Vol. 31, No. 3, 2010

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FIG. 7. Box plot graphic showing the relationship between dehiscence size and the existence of objective vestibular signs (Tullio phenomenon, Hennebert sign, Valsalva maneuver) (p 0.001).

called positive when at least one of the following findings were positive: Tullio-phenomenon, Hennebert sign, and/or Valsalva maneuver. The mean dehiscence size of subjects with at least one objective vestibular sign was 3.9 mm, whereas patients without any sign had, on average, a size of 2.2 mm. Subjects with dehiscence sizes of 2.5 mm or greater had more often objective vestibular findings compared with patients with dehiscence sizes below 2.5 mm (Fig. 8). The sensitivity was 83.3%, but the specificity only reached 55.6% with a Fisher_s exact test of 0.073. DISCUSSION It seems tempting to assume that the size of the dehiscence correlates with the severity of the disease or the patient_s distress. In daily practice, however, occasional findings of SSCD may not always be associated with the patient_s symptoms, and previous reports did not indicate a strong correlation. In analyzing 27 patients (34 ears) from different European centers, 3 different groups could be distinguished: patients with cochleovestibular symptoms and cochlear or vestibular symptoms/signs only (Table 1). In this cohort of patients, a significant relationship was detected between the size of the dehiscence and the symptoms and signs of patients with SSCDS. Patients with larger dehiscences were significantly more associated with cochleovestibular symptoms, lower VEMP thresholds, and objective vestibular findings (e.g., Tullio phenomenon) than subjects with a small bony defect. The theory of a Bthird mobile window[ (13) may explain the increased sensibility of the inner ear for sound pressure changes through bone conduction or for cerebrospinal fluidYtransmitted pressure gradients in the presence of larger mobile window sizes. All patients with bilateral findings had cochleovestibular symptoms, most commonly attributed to the larger side. It is generally assumed Otology & Neurotology, Vol. 31, No. 3, 2010

that the Bshunting[ of sound pressure from the stapes footplate to a large third window enhances the probability of a conductive hearing impairment (worsening air conduction thresholds) and increases the likelihood of conductive hyperacusis (improving bone conduction thresholds). In the chinchilla model (22), very small dehiscences had little effect on auditory sensitivity, whereas sizes approaching the cross-sectional area of the canal did have an impact. On the other hand, sizes exceeding the cross-sectional area revealed little additional change in sound pressure, and small changes were observed in relation to the location of the dehiscence. The model could not predict any effect on the vestibular symptoms. Our group with small dehiscences showed clinically either cochlear or vestibular symptoms. With a sensitivity of 91.7% and specificity of 70%, patients with dehiscence sizes of 2.5 mm or greater presented with cochleovestibular symptoms and/or signs. However, a low VEMP threshold (e80 dB SPL) as well as objective vestibular signs showed a high sensitivity but a relatively low specificity in relation with superior canal dehiscence sizes of 2.5 mm or greater. In Subject 15, VEMPs could not be tested properly because she experienced a strong sensation of vertigo. Around 90dB SPL, the amplitude of the VEMP on the side with the larger dehiscence was much higher compared with the other. Thus, most likely the VEMP threshold would have been pathologically lowered on the side with the larger SSCD. There was no apparent correlation between the size of the dehiscence and the audiogram patterns or single individual vestibular findings (e.g., Tullio phenomenon, Hennebert sign). The reason why patients with an almost identical size of dehiscence present different audiogram patterns, as well as pressure and sound-induced signs, remains unclear. Furthermore, the location of the dehiscence seemed to have no significant influence on the symptoms and signs. Four patients with a small superior canal dehiscence were initially included in the study but later removed because of concomitant pathologies of the temporal bone or cerebellopontine angle. They did not reveal typical symptoms of an SSCD. Another patient was excluded because he refused vestibular testings. The statistical test results showed no marked difference whether these 4 patients were included in the calculations or not. Patient

FIG. 8. Graphic illustrating the association between size of dehiscence and existence of objective vestibular signs (Tullio phenomenon, Hennebert sign, Valsalva maneuver).

SUPERIOR SEMICIRCULAR DEHISCENCE SYNDROME 7 presented as an emergency with an interesting history: While driving by car over a Swiss mountain pass, the patient experienced on his way down slight ear pain and performed a forced Valsalva maneuver. He immediately felt vertigo, a loud sound, and noisy tinnitus in his right ear. Further workup revealed no nystagmus but a sensorineural hearing loss of 70 to 100 dB nHL. Emergency middle ear exploration showed faint hematomas within the middle ear mucosa, no perilymph leak, but interestingly no round window reflex upon palpation of the stapes. The oval and round window were plugged with fat, and the bone conduction improved by 10 to 15 dB over time. A typical low-tone conductive hearing loss persisted, and the CT scan demonstrated the right SSCD of 4 mm. The phenomenon of conductive hyperacusis mainly appeared in subjects with larger dehiscences. Patients with this disorder may hear their eyeball movements, their heart beat at night, their joints moving while jogging, or their own voice too loudly in the ear with the superior semicircular canal dehiscence. This symptom can be very disturbing and a hindrance to the patient_s life. The cause of the conductive hyperacusis is the negative bone conduction in certain subjects with SSCDs which creates a higher sensitivity to bone-conducted sound at frequencies below 2 kHz (11). The autophony should be distinguished from that found in patients with a patulous Eustachian tube. Poe recently discussed these 2 different disease entities (23). Both groups of patients may experience relief of their autophony with the head in a dependent position or when supine. However, patients with SSCDS do not feel or hear their breathing, and their tympanic membrane does not move during forced nasal breathing. Placing a Bone-anchored Hearing Aid test device on some patients with a dehiscence of greater than 3 mm revealed unpleasant loud sound experience even on the lowest loudness setting. The duration and the progression of the disease is still a mystery. Does the size enlarge slowly and progressively over time? Using the transtemporal-supralabyrinthine approach (modified middle cranial fossa approach), the superior semicircular canal is always blue lined as a consistent surgical landmark along the floor of the middle cranial fossa (24,25). The senior authors_ experience is that these patients do not report similar symptoms as the ones described above. Long-term radiological and clinical studies documenting any progression of the dehiscences over time are still lacking. Operative Treatment of SSCDS Patients 14, 15, and 16 were operated on by a middle cranial fossa approach on the side of the larger dehiscence with plugging of the canal. Postoperatively, Patient 14 showed hearing improvement, experienced no longer unsteadiness, and revealed no more Tullio phenomenon. Patient 15 improved his hearing around 30 dB, and the objective vestibular tests were negative after the surgery. However, the patient still experienced a sensation of unsteadiness either because of the dehiscent contralateral

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side or lack of central compensation. In Patient 16, the objective vestibular tests turned negative on the operated side but remained positive on the other side, the patient still being unsteady at certain occasions. Recently, patients at the Antwerp Center have been operated through the mastoid, and the superior semicircular canal has been plugged at both ends of the dehiscence. Further surgical outcomes will be addressed in a separate study. When to Think About SSCDS In patients who display low-frequency conductive hearing loss (especially with negative bone conduction thresholds), normal otoscopy, paroxysmal sensations of vertigo, and normal acoustic (stapes) reflexes, the possibility of SSCDS as the likely cause should be reconsidered. Particularly helpful to the diagnosis are the pathologically lowered VEMP thresholds and the identification of the dehiscence on HRCT scan with reconstruction in the plane of the superior semicircular canal. The sensitive objective vestibular tests, such as Tullio phenomenon, Hennebert sign, or nystagmus during Valsalva maneuver, are further tools to support the diagnosis and to explain some of the patient_s signs and symptoms. Acknowledgments: This study has been conducted under the auspices of the Club de Recherche ed d_Enseignement en Neurotologie et Otologie (CRENO). The authors thank the other members of the CRENO for reviewing the article before submission and the lively discussions: F. Denoyelle (Paris, France), B. Godey (Rennes, France), M. Mondain (Montpellier, France), A. Chays (Reims, France), Ph. Bordure (Nantes, France), B. Gratacap (Colombier, France), S. Schmerber (Grenoble, France), J-P. Lavieille (Marseille, France), and E. Lescannes (Tours, France).

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