Published online before print January 19, 2006, 10.1148/radiol.2382042098
(Radiology 2006;238:938-942.)
© RSNA, 2006
Superior Semicircular Canal Dehiscence: Oblique Reformatted CT Images for Diagnosis1
Barton F. Branstetter, IV, MD,
Chivonne Harrigal, BA,
Edward J. Escott, MD and
Barry E. Hirsch, MD
1 From the Departments of Radiology (B.F.B., C.H., E.J.E.) and Otolaryngology (B.F.B., B.E.H.), University of Pittsburgh School of Medicine, 200 Lothrop St, PUH Room D132, Pittsburgh, PA 15215. Received December 10, 2004; revision requested February 4, 2005; revision received February 11; accepted March 3.
Address correspondence to B.F.B. (e-mail: bfb1{at}pitt.edu).
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ABSTRACT
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Purpose: To retrospectively determine, by using thin-section multi–detector row computed tomography (CT), whether additional reformations in the planes of Stenver and Pöschl change the diagnostic interpretation for superior semicircular canal dehiscence (SSCD) when compared with the diagnostic interpretation of standard coronal reformations for SSCD.
Materials and Methods: Institutional review board approval was obtained, patient anonymity was maintained, and the study was HIPAA compliant. Twenty-seven patients (17 men, 10 women; average age, 45 years; range, 19–72 years) suspected of having SSCD who underwent temporal bone multi–detector row CT were retrospectively identified from electronic medical records. An additional 27 control subjects (nine men, 18 women; average age, 50 years; range, 18–87 years), who underwent temporal bone multi–detector row CT for other reasons, were retrospectively selected from the same period. Two neuroradiologists with certificates of added qualification, one with 5 years and one with 9 years of experience interpreting temporal bone CT images, independently reviewed the 108 temporal bones twice. One review was restricted to transverse images and coronal reformations. The other review used transverse images, coronal reformations, and oblique reformations in the planes of Stenver and Pöschl. The observers were blinded to clinical history, and the two reviews took place 3 months apart to avoid recall bias. The primary outcome measure was the intraobserver discordance rate between the two reviews. 
Statistics were used to evaluate both intraobserver and interobserver variability.
Results: Observer 1 diagnosed SSCD in 25 of 108 (23%) temporal bones and had no discordances between the two reviews. Observer 2 diagnosed SSCD in 21 of 108 (19%) temporal bones and had one intraobserver discordance. After a post hoc consensus review of this one discordance, the radiologic diagnosis remained equivocal. The discordance involved the right temporal bone of a patient suspected of having SSCD in the left temporal bone, so no clinical follow-up was available.
Conclusion: Coronal reformations from multi–detector row CT of the temporal bone are sufficient for the evaluation of SSCD. Additional reformations in the planes of Stenver and Pöschl do not change the radiologic diagnosis and may be reserved for equivocal or confusing cases.
© RSNA, 2006
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INTRODUCTION
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Superior semicircular canal dehiscence (SSCD) is an unusual abnormality of the temporal bone. Patients most frequently experience dizziness induced by loud noises (Tullio phenomenon) (1). Since its initial description in 1998 (2), SSCD has rapidly become an accepted diagnosis in the evaluation of vertigo.
Radiologic assessment of SSCD consists of thin-section multi–detector row computed tomography (CT) of the temporal bone in the transverse plane with multiplanar reformations (3). The normal bony covering over the apex of the superior semicircular canal is absent in patients with SSCD (Fig 1). This finding, however, can be seen in up to 13% of typical people, and thus it does not necessarily imply a pathologic condition (4).
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Figure 1: SSCD. Coronal reformatted image from thin-section transverse CT examination shows bony covering of superior semicircular canal is intact on right (arrowhead) but absent on left (arrow), which indicates left SSCD.
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Reformatted images in the coronal plane are routinely obtained at multi–detector row CT of the temporal bone, but for the diagnosis of SSCD, acquisition of additional reformations in the planes of Stenver (perpendicular to the semicircular canal) and Pöschl (parallel to the semicircular canal) has been recommended (5). Recent studies have documented improved radiologic specificity by using thin-section multiplanar reformations based on 0.5-mm transverse reconstructions, instead of using direct transverse and coronal 1-mm-thick images. It is unclear, however, whether improved specificity is the result of the improved resolution or of the oblique planes of reconstructions (3).
The purpose of this study, therefore, was to retrospectively determine, by using thin-section multi–detector row CT, whether additional reformations in the planes of Stenver and Pöschl change the diagnostic interpretation for SSCD when compared with the diagnostic interpretation of standard coronal reformations for SSCD.
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MATERIALS AND METHODS
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Patients and Control Subjects
The study was performed with the approval of our institutional review board; direct patient consent was not required. Patient anonymity was maintained, and the study was Health Insurance Portability and Accountability Act compliant.
We retrospectively reviewed the electronic medical record at the University of Pittsburgh (Pa) from January 2001 to June 2004 to identify patients who were suspected of having SSCD, who were referred to the radiology department, and who underwent thin-section multi–detector row CT of the temporal bone. Twenty-seven such patients were identified (17 men, 10 women; average age, 45 years; range, 19–72 years). No exclusion criteria were applied.
To prevent prevalence bias, records of 27 control subjects (nine men, 18 women; average age, 50 years; range, 18–87 years) who had undergone multi–detector row CT of the temporal bone for other reasons were randomly selected from the same period. No exclusion criteria were applied. Because left and right ears were evaluated on each patient, 108 temporal bones were evaluated.
CT Protocol
All imaging was performed with a 16-section multi–detector row CT scanner (LightSpeed; GE Healthcare, Waukesha, Wis). Helical transverse images were acquired with a section thickness of 0.625 mm and a spacing of 0.3 mm (kV, 120; mA, 360; helical pitch, 0.562:1; rotation time, 0.8 second; field of view, head). The raw data were reconstructed by using a bone algorithm and a display field of view of 16 cm.
For each ear, reformatted images were created in the coronal plane, in the plane of Stenver (perpendicular to the course of the superior semicircular canal) (Fig 2), and in the plane of Pöschl (parallel to the plane of the superior semicircular canal) (Fig 3). All reformations had a section thickness of 1 mm and a spacing of 1 mm and were constructed by using average voxel density. Although the planes of Stenver and Pöschl are approximately 45° from coronal and sagittal planes, the reformations were specifically angled to align with the superior semicircular canal in each ear. Similarly, the coronal reformations were angled to show both ears symmetrically, even if the patient was tilted in the scanner. One neuroradiologist (B.F.B.) retrospectively performed all of the reformations used in this study. This neuroradiologist has performed or overseen all temporal bone CT reformation examinations performed at our institution since the installation of the 16-section multi–detector row CT scanner in 2001.
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Figure 2a: Plane of Stenver. (a) Angle of reformation demonstrated on transverse scout image. (b) Intact superior semicircular canal (arrowhead). (c) Dehiscent superior semicircular canal (arrow).
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Figure 2b: Plane of Stenver. (a) Angle of reformation demonstrated on transverse scout image. (b) Intact superior semicircular canal (arrowhead). (c) Dehiscent superior semicircular canal (arrow).
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Figure 2c: Plane of Stenver. (a) Angle of reformation demonstrated on transverse scout image. (b) Intact superior semicircular canal (arrowhead). (c) Dehiscent superior semicircular canal (arrow).
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Figure 3a: Plane of Pöschl. (a) Angle of reformation demonstrated on transverse scout image. (b) Intact superior semicircular canal (arrowhead). (c) Dehiscent superior semicircular canal (arrow).
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Figure 3b: Plane of Pöschl. (a) Angle of reformation demonstrated on transverse scout image. (b) Intact superior semicircular canal (arrowhead). (c) Dehiscent superior semicircular canal (arrow).
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Figure 3c: Plane of Pöschl. (a) Angle of reformation demonstrated on transverse scout image. (b) Intact superior semicircular canal (arrowhead). (c) Dehiscent superior semicircular canal (arrow).
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Image Review
The two observers were neuroradiologists with certificates of added qualification and devote most of their clinical time to head and neck imaging. Observer 1 (B.F.B.) has 5 years of experience with temporal bone CT; observer 2 (E.J.E.) has 9 years of experience with temporal bone CT. The observers were blinded to the clinical histories of the patients and control subjects.
Each of the 108 temporal bones was reviewed twice by each observer (432 reviews). In one review, the observer was presented with only transverse images and coronal reformations ("coronal only"). In the other review, the observer was presented with transverse images, coronal reformations, and reformations in the planes of Stenver and Pöschl ("all reformations").
Each observer participated in two review sessions, which were spaced 3 months apart to avoid recall bias. For half of the patients and control subjects (randomly selected), the coronal-only images were interpreted in the first session and the all-reformations images were interpreted in the second session. If observer 1 started with the coronal-only review, observer 2 started with the all-reformations review. When the reviewers were presented with the all-reformations images, the oblique reformations were presented first to avoid bias from the findings on the coronal images. Presentation order was randomized for each session and each observer.
On each review, the observer assigned a value of "dehiscent" (positive) or "intact" (negative) to the bony covering of the superior semicircular canal. Any instance of an intraobserver discrepancy between coronal-only and all-reformations readings was subjected to a post hoc consensus review with both neuroradiologists to determine the cause of the discrepancy.
Statistical Analysis
The primary outcome measure was the intraobserver discrepancy rate between the coronal-only and all-reformations readings. For each observer, the number of patients and control subjects with a radiologic diagnosis of SSCD, as well as the number of temporal bones affected, was tabulated, and the intraobserver discrepancy rate was calculated. Interobserver discrepancy rates were also calculated on the basis of the all-reformations assessment. Statistics were performed to evaluate both intraobserver and interobserver variability. A 95% confidence interval was calculated for the overall intraobserver discrepancy rate.
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RESULTS
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Observer 1 identified SSCD in 25 of 108 (23%) temporal bones and had no intraobserver discrepancies between the coronal-only and all-reformations readings. Observer 2 identified SSCD in 21 of 108 (19%) temporal bones and had one intraobserver discrepancy between the coronal-only and the all-reformations readings (Table). Observer 1 identified SSCD in four temporal bones that observer 2 considered normal, one of which was the case in which observer 2 had an intraobserver discrepancy.
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Comparison of the Use of Coronal-only Images and the Use of All-Reformations Images to Identify SSCD in 27 Patients Suspected of Having SSCD and in 27 Control Subjects
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Observer 1 had a discrepancy rate of zero per 108 (0%) ( = 1.00). Observer 2 had a discrepancy rate of one per 108 (0.93%) ( = 0.97). Thus, the overall intraobserver discrepancy rate was one per 216 (0.46%) (95% confidence interval: 0.01%, 2.6%; = 0.99). The interobserver statistic was 0.89.
The one case that elicited an intraobserver discrepancy was subjected to a post hoc analysis, but no consensus could be reached between the two neuroradiologists with regard to the presence of SSCD. Observer 1 maintained that SSCD was present, whereas observer 2 maintained that the bone was intact. The patient's clinical history was pertinent for Tullio phenomenon in the contralateral ear, so no clinical or surgical follow-up was available on the side of interest.
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DISCUSSION
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The results of this study suggest that thin-section multi–detector row CT images with coronal reformations are sufficient for the radiologic evaluation of SSCD. Oblique reformatted images do not change the diagnostic interpretation of the evaluation. Thus, we believe that the added workload for technologists and radiologists is not warranted.
Interobserver discrepancies exceeded intraobserver discrepancies in this study. That is, the choice of neuroradiologist was more important than the inclusion of oblique reformatted images. In fact, in the one case that elicited an intraobserver discrepancy, the two neuroradiologists could not agree on a diagnosis.
Observer 1 consistently interpreted more scans as positive; there were no scans that observer 2 considered positive and observer 1 considered negative. This suggests that the two observers have chosen different thresholds for positivity, rather than that this study has high inherent interobserver variability.
Eventually, radiologists will likely interpret all cross-sectional images by using volumetric displays of data (6). Most radiologists, however, do not currently have the technology or software interfaces to allow volumetric interpretation in everyday practice. Thus, radiologists rely on technologists to produce standardized reformatted images from thin-section transverse CT acquisitions.
Performing reformations in several planes is attractive because it produces additional data without additional radiation or scanning time. Unfortunately, multiplanar reformations produced on a CT scanner take valuable time away from scanning and burden technologists with additional time-consuming workflow steps. Radiologists, too, are slowed by the expansion of data, because they are required to review every image that is produced. Thus, it is advantageous to produce the minimum number of reformatted images necessary to make a diagnosis.
The results of this study suggest that reformatted images in the traditional coronal plane, when produced from a 0.6-mm-thick, 50% overlapped helical scan, are as reliable as oblique reformations for the diagnosis of SSCD. Thus, the additional images in the planes of Stenver and Pöschl are not needed for this diagnosis. Occasionally, information on the coronal reformations will be equivocal, and additional oblique reformations may be obtained at the discretion of the interpreting radiologist. Our study indicates, however, that these additional reformatted images are unlikely to change the interpretation.
SSCD can be diagnosed clinically according to history and evoked eye movements. Although the Tullio phenomenon is the most common complaint at presentation, conductive and mixed hearing loss have also been described (7,8). Certain forms of nystagmus are also associated with SSCD.
Radiology is used to confirm the diagnosis and to localize the portion (anterior, apex, or posterior) of the superior semicircular canal that is dehiscent (9). Although 1-mm-thick direct coronal images have a high sensitivity for the detection of SSCD, they have poor specificity when they are compared with submillimeter helical acquisition with reformations (3). Treatment of SSCD consists of surgically plugging the defect in the superior canal by using either a transmastoid or middle fossa approach (1,9).
The Tullio phenomenon has been traditionally associated with otosyphilis. However, tertiary syphilis is now rare in Western countries, and the syndrome of vestibular dysfunction induced by loud noises is more frequently associated with SSCD. Other conditions can manifest as the Tullio phenomenon. These include endolymphatic hydrops (Ménière disease), perilymphatic fistula, previous fenestration surgery and stapedectomy, Paget disease of the temporal bone, granulomatous diseases such as Wegener granulomatosis, infectious diseases such as tuberculosis, and primary or secondary malignancies that erode the otic capsule.
Previous literature suggests that radiologic evidence of SSCD can be seen in 13% of asymptomatic individuals (4). (This is likely an overestimation because older scanning protocols were used.) Although our study was not designed to establish the rate of SSCD in the normal population, our rate of 7%, when given the sample sizes and differences in protocols, is similar to that of previous investigators.
Our study has several limitations. We did not attempt to surgically confirm our radiologic diagnoses for three reasons: (a) We would have been unable to perform surgery on patients with negative radiologic findings, (b) not all patients with a radiologic diagnosis of SSCD have ipsilateral symptoms, and (c) some patients with radiologic and clinical evidence of SSCD choose not to undergo surgery. We did not include an "oblique reformation only" arm in our study because it is unlikely that radiologists would be comfortable performing only oblique reformations when evaluating the temporal bone in actual practice. Additional reformations may carry an advantage too small to identify with our sample size. Such a small effect is unlikely to be clinically important, however, because additional reformations can still be performed in equivocal cases, and interobserver variability is greater than intraobserver variability in our study.
The results of this study support the hypothesis that coronal reformatted images based on submillimeter helical CT acquisition data are sufficient for the radiologic evaluation of SSCD. To improve the efficiency of technologists and radiologists, additional reformatted images in the planes of Stenver and Pöschl may be reserved for difficult or equivocal cases.
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ACKNOWLEDGMENTS
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The authors thank Rose Jarosz and Claudine Martin for their assistance in organizing the review sessions and maintaining the research server.
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FOOTNOTES
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Abbreviations: SSCD = superior semicircular canal dehiscence
Author contributions: Guarantor of integrity of entire study, B.F.B.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, B.F.B., C.H., B.E.H.; experimental studies, B.F.B., C.H., E.J.E.; statistical analysis, B.F.B.; and manuscript editing, all authors
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References
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ABSTRACT
INTRODUCTION
