Orientation of Human Semicircular Canals Measured

Authors: Charles C Della Santina Valeria Potyagaylo Americo A Migliaccio Lloyd B Minor John P Carey

Publish Date: 2005/08/09

Volume: 6, Issue: 3, Pages: 191-206

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Abstract

Analysis of vestibuloocular reflex experiments requires knowledge of the absolute orientations with respect to skull landmarks of semicircular canals SCC Data relating SCC orientations to accessible skull landmarks in humans are sparse apart from a classic study of 10 skulls which concluded that the horizontal and anterior SCC are not mutually orthogonal 111 ± 76° Multiple studies of isolated labyrinths have shown the interSCC angles are close to 90° We hypothesized that a larger sample would yield mean absolute SCC orientations closer to the mutual orthogonality demonstrated for isolated labyrinths We measured canal orientations with respect to accessible skull landmarks using 3D multiplanar reconstructions of computerized tomography scans of the temporal bones of 22 human subjects Images were acquired with 05mm thickness and reconstructed with inplane resolution of 234 μm There was no significant difference between the left and a mirror image of the right p 057 on multiway ANOVA of orientation vector coefficients so data were pooled for the 44 labyrinths The angle between the anterior and posterior SCC was 940 ± 40° mean ± SD The angle between the anterior and horizontal SCC was 906 ± 62° The angle between the horizontal and posterior SCC was 904 ± 49° The direction angles between a vector normal to the left horizontal SCC and the positive Reids stereotaxic X +nasal Y +left and Z +superior axes were 1087 ± 75° 922 ± 57° and 199 ± 70° respectively The angles between a vector normal to the left anterior SCC and the positive Reids stereotaxic X Y and Z axes were 1259 ± 52° 384 ± 51° and 1001 ± 62° respectively The angles between a vector normal to the left posterior SCC and the positive Reids stereotaxic X Y and Z axes were 1336 ± 53° 1315 ± 51° and 1056 ± 66° respectively The mean anterior SCC–contralateral posterior SCC angle was 153 ± 72° The absolute orientations of human SCC are more nearly orthogonal than previously reportedThe semicircular canals SCC of the vestibular labyrinth encode head rotational velocity and provide input to the vestibuloocular reflex VOR vestibulocollic reflex vestibulospinal system vestibuloreticular system cerebellum and cortex Ewald 1892 Cohen et al 1964 Suzuki and Cohen 1964 Wilson et al 1995 Maeda et al 1975 Shinoda et al 1994 1997 Peterson and Abzug 1975 Wilson et al 1976 Ebata et al 2004 Accurate knowledge of the absolute orientation with respect to skull landmarks and position of SCC is essential for design of experiments studying the vestibular system and for understanding exam findings during clinical evaluation of patients with vestibular disorders Apart from a widely cited classic study of 10 human skulls by Blanks et al Blanks et al 1975 Curthoys et al 1977 few of the many studies of human labyrinth morphology have related SCC orientations to accessible skull landmarks that can guide an investigator in orienting a subjects head for canalplanespecific vestibular stimulation Blanks et al reported that the angles between ipsilateral SCC of one labyrinth depart significantly from mutual orthogonality with the angle between the anterior SCC and horizontal SCC equal to 1118 ± 76° mean ± SD Multiple studies of the interSCC angles of isolated human labyrinths using highresolution histologic or radiographic reconstructions suggest that SCC orientations are much more nearly orthogonal to one another Archer et al 1988 Harada et al 1990 Hashimoto 2003 Ifediba et al 2004 Takagi et al 1989 Spoor and Zonneveld 1995 1998 This disparity suggests that one or more of the absolute SCC orientations reported by Blanks et al may have been up to ∼20° off of the true population mean a deviation that would be significant for many experimental designsWe hypothesized that the departure from canal orthogonality observed by Blanks et al does not represent the true population mean and that a larger sample would yield mean canal orientations that are closer to mutual orthogonality To test our hypothesis we measured SCC orientations with respect to accessible skull landmarks using threedimensional multiplanar reconstructions of highresolution computed tomography CT scans of 44 labyrinths in 22 human subjectsComputed tomography scans were acquired for both ears of 7 male and 15 female subjects aged 48 ± 15 years CT scans were performed for accepted clinical indications in all patients The indication for CT imaging was headache with disequilibrium in nine subjects motion sickness fatigue lightheadedness or anxiety associated with dizziness in six subjects evaluation of middle ear or mastoid inflammatory disease in three subjects conductive hearing loss in two subjects benign paroxysmal positional vertigo BPPV in one subject and sensorineural hearing loss in one subject Diagnosis after 15 years of followup was migraine in 10 subjects middle ear or mastoid inflammatory disease in three subjects otosclerosis in two subjects depression or anxiety in two subjects and obstructive sleep apnea enlarged vestibular aqueduct orthostatic hypotension BPPV and Eustachian tube dysfunction in one subject each No subject had a history of temporal bone fracture Except for the one subject whose CT scan revealed mildly enlarged vestibular aqueducts without abnormal SCC configuration or significant deviation in SCC orientation from the rest of the dataset no patient had cochlear or labyrinthine dysmorphism abnormal location patency caliber shape size or orientation as judged by the neuroradiologists reading the CT for purposes of routine clinical care and by two of the authors CCDS JPC This study was a review of existing clinical data with patient identifiers removed The Joint Committee on Clinical Investigation of the Johns Hopkins University School of Medicine determined that the study was exempt from a protocol requirement and that it qualified for exemption from an institutional review board protocol based on Department of Health and Human Services criteria 45 CFR 46101b4The CT scans were acquired on a 16detector Toshiba Aquilion CT scanner using a bone algorithm To avoid loss of inplane resolution that can occur in helical scanning modes scans were performed in stepscan mode Each axial slice was scanned with the subject stationary and the table was axially stepped 05 mm before each subsequent acquisition The xray beam was generated with a 300 mA/100 kVP source and collimated to 05mm slice thickness at fullwidth halfmaximum Each temporal bone was separately encompassed by 120mm diameter region of interest using a 512 × 512 voxel matrix creating voxels of 0234 × 0234 × 05 mm Axial scans of the whole head were acquired using a 240mm region of interest yielding 0468 × 0468 × 05 mm voxels The 3D datasets were exported to a workstation for multiplanar analysis using Vitrea® 2331 software Vital Images Inc Plymouth MN and Matlab Mathworks Cambridge MA The higherresolution temporal bone reconstructions were used to measure distances and angles reported for the semicircular canals and wholehead reconstructions were used to define Reids planes with respect to skull landmarks as defined belowThe Vitrea® software system allows one to view a CT dataset simultaneously using a 3D rendered image and a set of three 2D images that represent mutually orthogonal “slices” through the dataset We measured a SCCs orientation by optimally aligning one slice plane with the SCC then extracting the coordinate transformation matrix Vitrea® used to reach that alignment from a spacefixed coordinate frame defined by the CT dataset volumes boundaries Kuipers 1999 Measurements were made with 1voxel resolution ∼022° angular 0234 mm inplane translationalMeasuring semicircular canal SCC orientations and centroid positions using 3D multiplanar reconstructions of left temporal bone CT of a 56yearold woman Within each column images show three mutually orthogonal slices through the volume data in the orientations shown by the colorcoded planes in the 3D rendering at bottom All three planes are shown in each image but two planes appear as lines in each slice image ABC EFG IJK because they are viewed on edge In all images red arrows = horizontal SCC HSCC green arrows = posterior SCC PSCC blue arrows = anterior SCC ASCC iac = internal auditory canal sig = sigmoid sinus va = vestibular aqueduct mc = mandibular condyle A/P/R/L/S/I = anterior/posterior/right/left/superior/inferior In the first set A–D the red plane is aligned with the HSCC the blue and green planes are arbitrary In the second set E–H the green plane is aligned with the PSCC the red and blue planes are arbitrary In the third set I–L the blue plane is aligned with the ASCC the red and green planes are arbitraryConfirming optimal alignment of image plane with a semicircular canal plane A Image plane tentatively aligned with canal lumen of the anterior SCC of Figure 1K Stepping one B then two C voxel lengths ∼234 μm each along the image plane normal causes uniform disappearance of canal lumen

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