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Is the Foramen Magnum Line Reliable for Measuring Cerebellar Tonsillar Descent? An Interobserver Study of Cerebellar Tonsillar Tip Localization Using Three Different Imaging Landmarks 2014

Category General Spine Andreea G. Moore, MD Diana Gomez Hassan, MD, PhD Douglas J. Quint, MD Hugh Garton, MD Cormac Maher, MD Ashok Srinivasan, MD Purpose The aim of our study was to evaluate the interobserver agreement in assessment of cerebellar tonsillar descent using three different bony landmarks on MRI (foramen magnum, C1 arch and C2 arch). Materials & Methods Our IRB approved, HIPAA-compliant retrospective study consisted of consecutive brain MRIs performed at our institution between January 2010 and December 2012. Exclusion criteria included skull base osseous abnormalities (e.g., basilar invagination/impression, platybasia, Paget’s disease etc.), poor quality sagittal T1-weighted images, significant motion artifacts, and congenital or acquired osseous abnormalities of C1 and C2 vertebrae. Sagittal T1-weighted images in these patients were interpreted by two board certified neuroradiologists (blinded to underlying clinical diagnoses) with measurement of bilateral tonsillar descent based on lines drawn perpendicular from the tonsillar tip to the: (i) Foramen magnum (FM) line (joining the anterior and posterior margins) (ii) C1 arch line (joining superior cortical margins of the anterior and posterior arches) (iii) C2 line (joining the anterior and posterior margins of the inferior endplate) The readers evaluated the entire stack of sagittal images on each scan and measured the tonsillar descent on the image that showed the most caudal position of a tonsil. Pertinent medical records including surgical and pathology reports were also reviewed. Non-parametric Spearman correlation coefficients between the readers were calculated for each of the three techniques. Interobserver agreement between the readers was assessed using Bland-Altman analysis. Results A total of 130 cerebellar tonsils on 65 patients (mean age 22 +/- 13 yrs; 41 females, 24 males) were studied – 13 with proven Chiari I malformation and 52 without Chiari I malformation. The Spearman correlation coefficients for the three techniques were 0.864, 0.939 and 0.899 respectively for FM, C1 and C2 landmarks. Bland-Altman analysis showed the best interobserver agreement for C1 line (0.25mm bias) and the least for C2 line (3.2 mm bias), implying that the differences in measurements between the 2 readers were the least when the C1 line was employed.

Conclusion Our results demonstrate better interobserver agreement in localization of the cerebellar tonsillar tip when osseous landmarks based on C1 vertebra are used compared to the FM line (which is currently widely used). This may be due to better delineation of the bony cortices at C1 compared to the margins of the FM on sagittal T1-weighted images. We plan to expand this study in the future to include a larger sample group and to evaluate the differences in “misclassification” rates of patients with respect to the presence or absence of Chiari I malformations using these three different techniques. References 1. Geerdink N, van der Vliet T, Rotteveel JJ, et al. Interobserver reliability and diagnostic performance of Chiari II malformation measures in MR imaging—part 2. Childs Nerv Syst. 2012:28(7):987-95. 2. Dufton JA, Habeeb SY, Heran MK, et al. Posterior fossa measurements in patients with and without Chiari I malformation. Can J Neurol Sci. 2011:38(3):452-5. 3. Furtado SV, Reddy K, Hegde AS. Posterior fossa morphometry in symptomatic pediatric and adult Chiari I malformation. J Clin Neurosci. 2009:16(11):1449-54. 4. Dagtekin A, Avci E, Kara E, et al. Posterior cranial fossa morphometry in symptomatic adult Chiari I malformation patients: comparative clinical and anatomical study. Clin Neurol Neurosurg. 2011:113(5):399-403. 5. Noudel R, Jovenin N, Eap C, et al. Incidence of basioccipital hypoplasia in Chiari malformation type I: comparative morphometric study of the posterior cranial fossa. Clinical article. J Neurosurg. 2009:111(5):1046-52. 6. Sun X, Qiu Y, Zhu Z, et al. Variations of the position of the cerebellar tonsil in idiopathic scoliotic adolescents with cobb angle >40 degrees: a magnetic resonance study. Spine (Phila Pa 1976). 2007:32(15):1680-6. 7. Sgouros S, Kountouri M, Natarajan K. Skull base growth in children with Chiari malformation type I. J Neurosurg. 2007:107(3 Suppl):188-92.