Superior Canal Dehiscence Patients Research Articles

Superior Canal Dehiscence and the Risk of Additional Dehiscences: A Retrospective CT Cohort Study.

Determine if superior canal dehiscence (SCD) found on flat-panel CT increases the risk for other defects in the otic capsule. Retrospective cohort study. Tertiary care center. One hundred ears (50 with SCD and 50 matched controls without SCD). Flat-panel CT imaging. (1) Prevalence of other dehiscences in SCD ears, (2) dehiscences in controls, and (3) otic capsule thickness in other reported dehiscence locations (cochlea-carotid, lateral semicircular canal [SCC] and mastoid, facial nerve-lateral SCC, vestibular aqueduct, posterior SCC-jugular bulb, posterior SCC-posterior fossa). Between-group comparisons were considered significant at p < 0.007 after applying the Bonferroni correction for multiple comparisons. Not including the SCD, there was a mean of 0.04 additional dehiscences in the SCD group (n = 2/50, 4%) and 0.04 non-SCD dehiscences in the controls (n = 2/50, 4%, p > 0.007). In the SCD group, there was one dehiscence between the cochlea and carotid artery and one between the posterior SCC and posterior fossa. The control group had one enlarged vestibular aqueduct and one dehiscence between the facial nerve and lateral SCC. As a group, SCD ears had wider vestibular aqueducts (0.68 ± 0.20 vs 0.51 ± 0.30 mm, p < 0.007) and thinner bone between the posterior SCC and posterior fossa (3.12 ± 1.43 vs 4.34 ± 1.67 mm, p < 0.007). The bone between the facial nerve and lateral SCC was thicker in SCD ears (0.77 ± 0.23 vs 0.55 ± 0.27 mm, p < 0.007) and no different for cochlea-carotid, and lateral SCC and mastoid (p > 0.007). SCD does not increase the likelihood of a second dehiscence in the same otic capsule. SCD patients may have congenitally thinner otic capsule bones compared to controls, particularly near the posterior SCC, where the vestibular aqueduct may be enlarged.

The effect of superior canal dehiscence size and location on audiometric measurements, vestibular-evoked myogenic potentials and video-head impulse testing.

To correlate objective measures of audio-vestibular function with superiorcanaldehiscence (SCD) size and location in ears with SCD and compare results with literature. We retrospectively evaluated 242 patients exhibiting SCD and/or extremely thinned bone overlying superior canals (SC) on CT scans and selected 73 SCD patients (95 ears with SCD). Data concerning audiometry, impedance audiometry, video-head impulse test (vHIT), cervical vestibular-evoked myogenic potentials (cVEMPs) and ocular VEMPs (oVEMPs) to air- (AC) and bone-conducted (BC) stimuli were collected for each pathologic ear and correlated with dehiscence size and location. AC pure-tone average (PTA) (p = 0.013), low-frequency air-bone gap (ABG) (p < 0.001), AC cVEMPs amplitude (p = 0.002), BC cVEMPs amplitude (p < 0.001) and both AC and BC oVEMPs amplitude (p < 0.001) positively correlated with increasing SCD size. An inverse relationship between dehiscence length and both AC cVEMPs and oVEMPs thresholds (p < 0.001) and SC vestibulo-ocular reflex (VOR) gain (p < 0.001) was observed. Dehiscences at the arcuate eminence (AE) exhibited lower SC VOR gains compared to SCD along the ampullary arm (p = 0.008) and less impaired BC thresholds than dehiscences at the superior petrosal sinus (p = 0.04). We confirmed that SCD size affects AC PTA, ABG and both amplitudes and thresholds of cVEMPs and oVEMPs. We also described a tendency for SC function to impair with increasing SCD size and when dehiscence is located at the AE. The latter data may be explained either by a spontaneous canal plugging exerted by middle fossa dura or by a dissipation through the dehiscence of mechanical energy conveyed to the endolymph during high-frequency impulses.

Superior Canal Dehiscence Surgery Outcomes Following Failed Round Window Surgery.

Round window (RW) occlusion or reinforcement is a less-invasive option compared with direct repair approaches to improve symptoms of superior canal dehiscence (SCD) syndrome. However, RW surgery is associated with variable outcomes. Middle fossa craniotomy or transmastoid repair is an option for SCD patients who fail RW surgery, but it is unknown whether sequential repair following RW plugging improves SCD symptoms or increases complications. The objective of this study is to evaluate outcomes of SCD repair via middle fossa craniotomy following failed RW surgery. Retrospective review. Academic tertiary care center. Adult patients with SCD syndrome who underwent failed RW surgery followed by sequential middle fossa craniotomy and plugging of the arcuate eminence defect. Patients with SCD associated with the superior petrosal sinus were excluded. None. Prospectively collected pre- and postoperative symptom questionnaires, threshold audiograms, and cervical vestibular evoked myogenic potentials (cVEMP). Seven SCD patients (out of a total of 194 surgical cases at our institution) underwent sequential middle-fossa SCD repair following failed RW surgery. Resolution of symptoms and reversal of diagnostic indicators were observed in the majority of subjects following sequential repair. Two of seven patients underwent a third procedure with plugging of the superior semicircular canal by a transmastoid approach due to the presence of residual symptoms. Middle fossa craniotomy and SCD occlusion is a safe and reasonable option for patients who fail RW surgery. Our cohort did not show increased risks of auditory or vestibular dysfunction.

Enhanced Auditory Sensitivity to Body Vibrations in Superior Canal Dehiscence Syndrome

A key feature of superior canal dehiscence (SCD) syndrome is supranormal hearing of body sounds. The aim of the present study was to quantify this phenomenon and to ascertain whether auditory sensitivity to body vibrations can distinguish SCD patients. Hearing thresholds in response to vibration at the vertex, at the spinous process of the 7th cervical vertebra, and at the medial malleolus were tested in 10 SCD patients and 10 controls. Both patients and controls had insert earphones in both ears. The insert in the test ear was blocked while masking was presented to the other ear. Vibration in the frequency range of 125-1,000 Hz was presented to each of the 3 stimulation sites. The SCD patients were found to have significantly lower hearing thresholds compared with controls. The two study groups reacted differently with respect to frequency. The SCD patients showed an enhanced sensitivity for the lower stimulus frequencies. The difference was, however, rather independent of stimulus presentation site. The findings suggest that hearing thresholds in response to low-frequency body vibration at sites distant from the ears can distinguish SCD patients. The present findings may also support the idea that auditory sensation to body vibrations is a response related to soft tissue conduction.

Ocular vestibular evoked myogenic potentials to vertex low frequency vibration as a diagnostic test for superior canal dehiscence

To explore ocular vestibular evoked myogenic potentials (oVEMP) to low-frequency vertex vibration (125 Hz) as a diagnostic test for superior canal dehiscence (SCD) syndrome. The oVEMP using 125 Hz single cycle bone-conducted vertex vibration were tested in 15 patients with unilateral superior canal dehiscence (SCD) syndrome, 15 healthy controls and in 20 patients with unilateral vestibular loss due to vestibular neuritis. Amplitude, amplitude asymmetry ratio, latency and interaural latency difference were parameters of interest. The oVEMP amplitude was significantly larger in SCD patients when affected sides (53 μVolts) were compared to non-affected (17.2 μVolts) or compared to healthy controls (13.6 μVolts). Amplitude larger than 33.8 μVolts separates effectively the SCD ears from the healthy ones with sensitivity of 87% and specificity of 93%. The other three parameters showed an overlap between affected SCD ears and non-affected as well as between SCD ears and those in the two control groups. oVEMP amplitude distinguishes SCD ears from healthy ones using low-frequency vibration stimuli at vertex. Amplitude analysis of oVEMP evoked by low-frequency vertex bone vibration stimulation is an additional indicator of SCD syndrome and might serve for diagnosing SCD patients with coexistent conductive middle ear problems.

Thickness of the Bony Otic Capsule: Etiopathogenetic Perspectives on Superior Canal Dehiscence Syndrome

The etiology of superior canal dehiscence (SCD) is controversial. An embryological perspective suggests that SCD may occur through the failure of postnatal bone formation over the superior semicircular canal (SC), whereas an acquired theory suggests that trauma or pressure from the overlying temporal lobe could break or gradually thin the SC. We infer the etiology of SCD by comparing the thickness of the bony otic capsule of the unaffected side of SCD patients with that of non-SCD participants. Twelve SCD patients (13 SCD ears and 11 normal ears) and 34 age-matched controls (68 ears) were included. The control group was subdivided into an aerated group (49 ears) and a nonaerated group (19 ears), as defined by the presence of air cells above the SC. A high-resolution temporal bone CT was performed in all participants. The thicknesses of the SC, horizontal canal (HC) and posterior canal (PC) of the unaffected ears of SCD patients were compared with those of the controls. The SC of the unaffected side in the SCD group (n = 11, 0.41 ± 0.23 mm) was significantly thinner than the one in the control group (n = 68, 0.64 ± 0.21 mm, p = 0.002). The HC and PC were also thinner in the SCD group (n = 24, 0.58 ± 0.11 and 1.39 ± 0.31 mm, respectively) than in the controls (0.70 ± 0.08 and 1.61 ± 0.32 mm; p < 0.0001 and p = 0.005, respectively). The SC, HC and PC thicknesses were also compared between the aerated and nonaerated ears within the control group. The SC was significantly thicker in the aerated group (0.73 ± 0.14 mm) than in the nonaerated group (0.60 ± 0.23 mm; p = 0.046); however, no significant difference was observed for the HC and PC thickness (aerated group, n = 49, 0.72 ± 0.07 and 1.67 ± 0.34 mm; nonaerated group, n = 19, 0.70 ± 0.09 and 1.59 ± 0.34 mm; p = 0.350 and p = 0.428, respectively). The bony otic capsule was significantly thinner in the SCD patients than in the controls. However, even within unaffected individuals, SCs lacking overlying air cells were also thinner than those with overlying air cells. These results suggest that both embryological and acquired factors affect the occurrence of SCD.

Electrocochleographic findings in superior canal dehiscence syndrome

This study evaluated the electrocochleographic findings of patients with superior canal dehiscence (SCD) syndrome and determined their diagnostic values and relationships with audiometric parameters. Thirteen symptomatic SCD patients (1 bilateral) confirmed by temporal bone computed tomography (TBCT) and cervical vestibular evoked myogenic potentials (cVEMP) were recruited. SCD sizes were measured on reformatted images in the plane of the superior canal (SC). Results of audiologic tests (audiometry, cVEMP, electrocochleography (ECoG)) for 14 affected and 12 contralateral unaffected ears were evaluated. Relationships between summating potential (SP) to action potential (AP) ratios, as measured by ECoG, and other audiometric parameters were evaluated. Sensitivity analysis of SP/AP ratios was performed by plotting receiver operating characteristic (ROC) curves for SCD syndrome patients and 19 age-matched healthy controls. Mean SP/AP ratio of SCD ears was significantly higher than that of unaffected ears (0.52 versus 0.25, p < 0.001) and SPs were significantly elevated in affected ears (p = 0.011), whereas APs were similar for affected and unaffected ears. SP/AP ratio showed a sensitivity of 92.3% and a specificity of 94.0% for distinguishing SCD syndrome patients given the inclusion criteria applied (symptoms, TBCT, cVEMP threshold) at a cutoff value of 0.34 (p < 0.001). SP/AP ratio was not correlated with SCD size or cVEMP threshold in affected ears. Negative absolute values of bone conduction at low frequency tended to increase with SP/AP ratio. Five out of 13 patients underwent surgical repair experienced symptomatic improvement with normalization of SP/AP ratios. ECoG appears to be a valuable diagnostic adjunct for functional demonstration of the third window in the otic capsule with high sensitivity and specificity, and thus, can support a clinical diagnosis of SCD when used in conjunction with clinical and radiological findings.

Costimulation of the Horizontal Semicircular Canal during Skull Vibrations in Superior Canal Dehiscence Syndrome

A sound- and pressure-induced vestibuloocular reflex (VOR) has been described as vertical and torsional in superior canal dehiscence (SCD), and the rotational axes of induced VOR have been assumed to fit with the axis of the affected superior semicircular canal (SC). However, it has been difficult to characterize the pattern of vibration-induced VOR (ViVOR). We aimed to characterize the pattern of ViVOR by comparing the intensity and the axis of ViVOR with several clinical parameters of SCD. Ten symptomatic SCD patients were recruited. SCD size and location were measured on a reformatted image in the plane of the SC. Unilateral vibratory stimulation (100 Hz) was applied to the mastoid surface. ViVOR were recorded using 3D videooculography. The median 3D velocity of ViVOR was measured and the 3D vector trajectory plotted for reference against the axes of the human semicircular canals. A correlation between the magnitude of ViVOR and the size of SCD was evaluated. We also compared the location of SCD with the vertical-to-torsional component ratio of the ViVOR. ViVOR were present in 7 patients; 6 patients showed a substantial horizontal component in the excitatory direction in addition to strong torsional and weak vertical components. The computed rotational axes of ViVOR were located mostly between the axes of the ipsilateral SC and horizontal canal (HC) with a variable deviation to the axis of the ipsilateral posterior canal (PC). The magnitude of ViVOR was not related to the size of the SCD. The vertical-to-torsional component ratio of ViVOR tended to decline as the dehiscence was closer to the common crus. In SCD, mastoid vibration may stimulate the affected-side HC and PC as well as the SC. SCD can be suspected when excitatory horizontal torsional ViVOR direct to the side of the auditory symptoms.

Superior Canal Dehiscence Length and Location Influences Clinical Presentation and Audiometric and Cervical Vestibular-Evoked Myogenic Potential Testing

Superior canal dehiscence (SCD) is caused by an absence of bony covering of the arcuate eminence or posteromedial aspect of the superior semicircular canal. However, the clinical presentation of SCD syndrome varies considerably, as some SCD patients are asymptomatic and others have auditory and/or vestibular complaints. In order to determine the basis for these observations, we examined the association between SCD length and location with: (1) auditory and vestibular signs and symptoms; (2) air conduction (AC) loss and air-bone gap (ABG) measured by pure-tone audiometric testing, and (3) cervical vestibular-evoked myogenic potential (cVEMP) thresholds. 104 patients (147 ears) underwent SCD length and location measurements using a novel method of measuring bone density along 0.2-mm radial CT sections. We found that patients with auditory symptoms have a larger dehiscence (median length: 4.5 vs. 2.7 mm) with a beginning closer to the ampulla (median location: 4.8 vs. 6.4 mm from ampulla) than patients with no auditory symptoms (only vestibular symptoms). An increase in AC threshold was found as the SCD length increased at 250 Hz (95% CI: 1.7-4.7), 500 Hz (95% CI: 0.7-3.5) and 1,000 Hz (95% CI: 0.0-2.5), and an increase in ABG as the SCD length increased at 250 Hz (95% CI: 2.0-5.3), 500 Hz (95% CI: 1.6-4.6) and 1,000 Hz (95% CI: 1.3-3.3) was also seen. Finally, a larger dehiscence was associated with lowered cVEMP thresholds at 250 Hz (95% CI: -4.4 to -0.3), 500 Hz (95% CI: -4.1 to -1.0), 750 Hz (95% CI: -4.2 to -0.7) and 1,000 Hz (95% CI: -3.6 to -0.5) and a starting location closer to the ampulla at 250 Hz (95% CI: 1.3-5.1), 750 Hz (95% CI: 0.2-3.3) and 1,000 Hz (95% CI: 0.6-3.5). These findings may help to explain the variation of signs and symptoms seen in patients with SCD syndrome.

Outcomes following Semicircular Canal Plugging

Objectives: 1) Examine audiometric outcomes following semicircular canal plugging for either superior semicircular canal dehiscence syndrome or refractory positional vertigo. 2) Review symptom control rates and complication rates for canal plugging procedures. Methods: Retrospective chart review at a private neurotologic tertiary referral center. Patients undergoing semicircular canal plugging for superior semicircular canal dehiscence syndrome or refractory positional vertigo from January 1, 2007, to December 31, 2012, were included. Pre- and postoperative audiometry, operative findings, and clinical symptoms were assessed through chart review. Acoustic reflex testing, vestibular testing, and canal dehiscence size were examined in superior canal dehiscence patients. Results: A total of 28 patients underwent a canal plugging procedure during the study period: 24 superior canal dehiscence syndrome patients and 4 posterior canal occlusion patients. Postoperative air conduction pure-tone averages were 22.5 and 13.0 dB for these two groups, respectively, compared to their preoperative values of 21.1 and 11.0 dB. The average pre- and postoperative word-recognition scores for the entire cohort were 96.2% and 94.0%. Vestibular evoked myogenic potential data showed reduced thresholds in 7 canal dehiscence patients. The average size of the canal dehiscence, based on preoperative radiological measurements, was 3.4 mm. Surgical complications were limited to one temporary facial weakness (Grade 2). Complete symptom improvement was noted in 35.7% of all patients, while 16.7% of canal dehiscence patients reported no change in symptoms. Conclusions: Semicircular canal plugging procedures are associated with excellent hearing outcomes and may reduce preoperative symptoms in patients with canal dehiscence and refractory positional vertigo.

Rapid cVEMP and oVEMP Responses Elicited by a Novel Head Striker and Recording Device

To develop a reliable, easy to use bedside, office, or field system that allows the rapid measurement of cervical and ocular vestibular evoked myogenic potentials (cVEMP and oVEMP) using a bone-conduction stimulus. Prospective bioengineering design and proof of concept of the test system with saccular and utricular otolith response studies in human subjects. Private practice, tertiary neurotology referral center. Twenty healthy adult controls without history of auditory or vestibular dysfunction and 5 preoperative and postoperative patients with confirmed superior canal dehiscence (SCD) participated. The subjects underwent auditory stimuli-based cVEMP and oVEMP studies using a commercially available system as well as testing with a novel bone-conduction cVEMP and oVEMP head striker system. Duration of each study, healthy subject and patient comfort, reproducibility, latency, and amplitude of auditory and striker evoked cVEMP and oVEMP responses. The mean age of the healthy controls was 43.8, with a range of 19 to 69 years (10 male and 10 female subjects). The mean age of the SCD patient group was 46, with a range of 25 to 54 years; all female subjects. Although the cVEMP responses were similar using either the auditory or head strike stimuli, the healthy subjects preferred the latter, but the SCD patients became more symptomatic. The oVEMP data showed more consistent responses using the striker system. A statistically significant reduction in latency for the striker-evoked cVEMP occurred compared with the auditory cVEMP evoked response in the 5 SCD preoperative patients. All normalized postoperatively. Recording the cVEMP and oVEMP responses using the striker system was much more rapid than with auditory stimuli and was more comfortable for the healthy subjects. The striker system and the acoustic method elicited strong otolithic receptor dysfunction symptoms in all SCD patients; however, they preferred the shorter striker studies. The striker system, because of the statistically shorter latency of p13 during the striker evoked cVEMP, which normalized after SCD closure, suggests that this method may be useful in identifying SCD patients before imaging studies. In addition, based on our biomechanical data, the striker was able to reliably produce a consistent and defined head striker impact.

Tuning of the ocular vestibular evoked myogenic potential (oVEMP) to air- and bone-conducted sound stimulation in superior canal dehiscence

Recent studies have demonstrated the frequency selectivity of air-conducted (AC) and bone-conducted (BC) stimuli in eliciting ocular vestibular evoked myogenic potentials (oVEMPs). In this study, frequency tuning of the oVEMP was assessed in patients with superior canal dehiscence (SCD) and compared with responses previously reported for healthy subjects. Six (five unilateral) SCD patients were stimulated using AC sound (50-1,200Hz) and BC transmastoid vibration (50-1,000Hz). Stimuli were delivered at two standardized intensities: one the same as previously used for healthy controls and the other at 10dB above vestibular threshold (a similar relative intensity to that used in controls). For AC stimulation, SCD patients had larger oVEMP amplitudes across all frequencies tested for both stimulus intensities. Normalized tuning curves demonstrated greater high-frequency responses with the stronger stimulus. For BC stimulation, larger oVEMP amplitudes were produced at frequencies at and above 100Hz using standard intensity stimuli. For the matched intensity above vestibular threshold, enhancement of the oVEMP response was present in SCD patients for 500-800Hz only. We conclude that SCD causes greater facilitation for AC than BC stimuli. The high-frequency response is likely to originate from the superior (anterior) canal and is consistent with models of inner ear changes occurring in SCD.

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Effectiveness of different click stimuli in diagnosing superior canal dehiscence using cervical vestibular evoked myogenic potentials

Conclusion: Testing cervical vestibular evoked myogenic potential (cVEMP) in response to 90 dB nHL clicks can, in contrast to high-intensity 500 Hz tone bursts, be used as a screening test for superior canal dehiscence (SCD) syndrome. Objectives: cVEMP testing has its key clinical significance for evaluating saccular and inferior vestibular nerve function, but also for assessment of vestibular hypersensitivity to sounds in patients with SCD syndrome. The routine stimulus used in cVEMP testing is high-intensity 500 Hz tone bursts. The aim of the present study was to compare the high-intensity tone burst stimulation with less intense click stimulations for the diagnosis of SCD syndrome. Methods: cVEMP amplitudes in response to 500 Hz tone bursts and clicks were studied in 38 patients with SCD syndrome unilaterally. Results: cVEMP testing using high-intensity 500 Hz tone bursts did not consistently distinguish SCD patients. This nonfunctioning of high-intensity 500 Hz stimulation is most likely due to saturation. With 90 and 80 dB nHL clicks there is low risk for saturation and both these click stimulations were effective. Testing with both 80 and 90 dB nHL clicks did not have any significant advantage over just using 90 nHL dB clicks.

Superior Canal Dehiscence Patients Have Smaller Mastoid Volume than Age- and Sex-Matched Otosclerosis and Temporal Bone Fracture Patients

Background and ObjectivesThe purpose of the study was to compare the mastoid air-cell volume of the patients with superior semicircular canal dehiscence syndrome (SCDS) and that of the control patients with otosclerosis and temporal bone (TB) fracture.Subjects and MethodsTen patients with SCDS were enrolled and 10 patients with bilateral otosclerosis and TB fracture were selected as control groups by age and sex matching. To measure the mastoid air-cell volume, 3D reconstruction software was used.ResultsIn 10 patients with SCDS, the mean age was 44.5 years, ranging from 16 to 79 years (M : F=4 : 6). Mean mastoid air-cell volume in the SCDS side was 3319.9 mm3, whereas 4177.2 mm3 in the normal side (p=0.022). Mean mastoid air-cell volume in the right side of otosclerosis patients was 6594.3 mm3 and it was not different from 6380.5 mm3 in the left side (p=0.445). Mean mastoid air-cell volume in normal side of TB fracture was 6477.2 mm3. The mastoid air-cell volume in the SCDS side was significantly smaller than that of otosclerosis and TB fracture patients (p=0.009, p=0.002, respectively). The mastoid air-cell volume in the normal side of SCDS was significantly smaller than that of TB fracture (p=0.019), but not significant with that of otosclerosis (p=0.063).ConclusionsOur findings revealed that the mastoid air-cell volume in the SCDS side was significantly smaller than control group, which suggest that the decreased mastoid pneumatization is closely related to the generation of SCDS.

Three-Dimensional Vibration-Induced Vestibulo-ocular Reflex Identifies Vertical Semicircular Canal Dehiscence

Vertical semicircular canal dehiscence (VSCD) due to superior canal dehiscence (SCD) or posterior canal dehiscence (PCD) of the temporal bone causes vestibular and cochlear hypersensitivity to sound. This study aimed to characterize the vibration-induced vestibulo-ocular reflex (ViVOR) in VSCD. ViVORs in one PCD and 17 SCD patients, confirmed by CT imaging reformatted in semicircular canal planes, were measured with dual-search coils as binocular three-dimensional eye rotations induced by skull vibrations from a bone oscillator (B71-10 ohms) at 7 ms, 500 Hz, 135-dB peak-force level (re: 1 μN). The ViVOR eye rotation axes were computed by vector analysis and referenced to known semicircular canal planes. Onset latency of the ViVOR was 11 ms. ViVOR from VSCD was up to nine times greater than normal. The ViVOR's torsional rotation was always contraversive-torsional (the eye's upper pole rotated away from the stimulated ear), i.e. its direction was clockwise from a left and counterclockwise from a right VSCD, thereby lateralizing the side of the VSCD. The ViVORs vertical component distinguishes PCD from SCD, being downwards in PCD and upwards in SCD. In unilateral VSCD, the ViVOR eye rotation axis aligned closest to the dehiscent vertical semicircular canal axis from either ipsilateral or contralateral mastoid vibrations. However, in bilateral VSCDs, the ViVOR eye rotation axis lateralized to the ipsilateral dehiscent vertical semicircular canal axis. ViVOR was evoked in ossicular chain dysfunction, even when air-conducted click vestibulo-ocular reflex (VOR) was absent or markedly reduced. Hence, ViVOR could be a useful measurement to identify unilateral or bilateral VSCD even in the presence of ossicular chain dysfunction.

Vestibular hypersensitivity to sound in superior canal dehiscence: Large evoked responses in the legs produce little postural sway

Objective Patients with superior canal dehiscence (SCD) typically have enhanced sound-evoked vestibular reflexes, such as vestibulo-collic and vestibulo-ocular reflexes. We wished to investigate whether sound-evoked lower limb EMG responses and postural sway are also enhanced in this condition. Methods Eight patients with CT confirmed SCD (11 affected ears) and 8 age-matched normal controls participated. Three sound-evoked responses were measured; vestibulo-collic reflexes (i.e. vestibular-evoked myogenic potentials, VEMPs), lower limb vestibulo-spinal reflexes and body sway (centre of pressure in mm). Sound stimuli were 500 Hz air-conducted tone bursts of varying lengths (VEMPs: 2 ms; vestibulo-spinal: 20 ms; sway: 1 s and 200 ms) set at fixed levels above each subject’s VEMP threshold. Results SCD patients had very large VEMP and vestibulo-spinal responses following high intensity stimulation, but at the matched intensity of 15 dB above threshold amplitudes were similar in both SCD patients and controls. The amplitude of both responses increased linearly with increasing stimulus intensity in both groups. Large (∼20 mm), stereotyped sway responses were present in only one (atypical) patient with high intensity stimulation. Small (∼2 mm) sway responses were present in the remaining patients, and began immediately following the vestibulo-spinal responses. Conclusions Despite the presence of large vestibular reflexes, there is usually very little body sway in response to loud sounds in SCD patients. Significance Large short-latency vestibulo-spinal reflexes in SCD do not necessarily evoke large sway responses.

Click-evoked vestibulo-ocular reflex

An enlarged, low-threshold click-evoked vestibulo-ocular reflex (VOR) can be averaged from the vertical electro-oculogram in a superior canal dehiscence (SCD), a temporal bone defect between the superior semicircular canal and middle cranial fossa. To determine the origin and quantitative stimulus-response properties of the click-evoked VOR. Three-dimensional, binocular eye movements evoked by air-conducted 100-microsecond clicks (110 dB normal hearing level, 145 dB sound pressure level, 2 Hz) were measured with dual-search coils in 11 healthy subjects and 19 patients with SCD confirmed by CT imaging. Thresholds were established by decrementing loudness from 110 dB to 70 dB in 10-dB steps. Eye rotation axis of click-evoked VOR computed by vector analysis was referenced to known semicircular canal planes. Response characteristics were investigated with regard to enhancement using trains of three to seven clicks with 1-millisecond interclick intervals, visual fixation, head orientation, click polarity, and stimulation frequency (2 to 15 Hz). In subjects and SCD patients, click-evoked VOR comprised upward, contraversive-torsional eye rotations with onset latency of approximately 9 milliseconds. Its eye rotation axis aligned with the superior canal axis, suggesting activation of superior canal receptors. In subjects, the amplitude was less than 0.01 degrees, and the magnitude was less than 3 degrees/second; in SCD, the amplitude was up to 60 times larger at 0.66 degrees, and its magnitude was between 5 and 92 degrees/second, with a threshold 10 to 40 dB below normal (110 dB). The click-evoked VOR magnitude was enhanced approximately 2.5 times with trains of five clicks but was unaffected by head orientation, visual fixation, click polarity, and stimulation frequency up to 10 Hz; it was also present on the surface electro-oculogram. In superior canal dehiscence, clicks evoked a high-magnitude, low-threshold, 9-millisecond-latency vestibulo-ocular reflex that aligns with the superior canal, suggesting superior canal receptor hypersensitivity to sound.

Cranial Thickness in Superior Canal Dehiscence Syndrome

To use morphometric analyses of cranial thickness to investigate 2 cases of unanticipated calvarial bone resorption in superior canal dehiscence (SCD) resurfacing surgery. Retrospective morphometric analysis of high-resolution computed tomography (CT) temporal bone scans in normal and control subjects with accompanying case reports. Tertiary care referral center. Two patients with SCD and failed resurfacing because of bone resorption. Temporal bone CT scans from 30 sex-matched controls. Resurfacing of SCD via a middle fossa approach using a split thickness calvarial graft from the craniotomy site. Mean cross-sectional area of the middle fossa craniotomy bone flap and mean cranial thickness at 30 and 45 degrees above the middle fossa floor. Two patients had delayed failure of SCD resurfacing surgery as manifested by return of symptoms. High-resolution CT scans in both, and intraoperative confirmation in one, confirmed resorption of the bone graft. Measurements of cross-sectional area of the middle fossa craniotomy on high-resolution CT scans demonstrated significantly reduced values in the two SCD patients as compared with normal controls (Mann-Whitney U test, p<0.05). Cranial thickness outside the squamous temporal bone was reduced but did not reach statistical significance. Morphometric measurements of the calvarium have demonstrated that the squamous temporal bone is thinner in patients with SCD as compared with controls. Thus, the process leading to defects in the tegmen extends beyond the petrous pyramid. This suggests that there may be extratemporal factors leading to the development of a dehiscence. These findings also have implications for the surgical treatment of this disorder. Resurfacing methods may have a higher failure rate as the bone graft has reduced mass and maybe prone to resorption. Canal plugging methods may provide a more definitive means of addressing the dehiscent labyrinth than resurfacing.