title: Posterior odontoid process angulation in pediatric Chiari I malformation: an MRI morphometric external validation study.
abstract: OBJECT Osseous anomalies of the craniocervical junction are hypothesized to precipitate the hindbrain herniation observed in Chiari I malformation (CM-I). Previous work by Tubbs et al. showed that posterior angulation of the odontoid process is more prevalent in children with CM-I than in healthy controls. The present study is an external validation of that report. The goals of our study were 3-fold: 1) to externally validate the results of Tubbs et al. in a different patient population; 2) to compare how morphometric parameters vary with age, sex, and symptomatology; and 3) to develop a correlative model for tonsillar ectopia in CM-I based on these measurements. METHODS The authors performed a retrospective review of 119 patients who underwent posterior fossa decompression with duraplasty at the Monroe Carell Jr. Children's Hospital at Vanderbilt University; 78 of these patients had imaging available for review. Demographic and clinical variables were collected. A neuroradiologist retrospectively evaluated preoperative MRI examinations in these 78 patients and recorded the following measurements: McRae line length; obex displacement length; odontoid process parameters (height, angle of retroflexion, and angle of retroversion); perpendicular distance to the basion-C2 line (pB-C2 line); length of cerebellar tonsillar ectopia; caudal extent of the cerebellar tonsils; and presence, location, and size of syringomyelia. Odontoid retroflexion grade was classified as Grade 0, > 90°; Grade I,85°-89°; Grade II, 80°-84°; and Grade III, < 80°. Age groups were defined as 0-6 years, 7-12 years, and 13-17 years at the time of surgery. Univariate and multivariate linear regression analyses, Kruskal-Wallis 1-way ANOVA, and Fisher's exact test were performed to assess the relationship between age, sex, and symptomatology with these craniometric variables. RESULTS The prevalence of posterior odontoid angulation was 81%, which is almost identical to that in the previous report (84%). With increasing age, the odontoid height (p < 0.001) and pB-C2 length (p < 0.001) increased, while the odontoid process became more posteriorly inclined (p = 0.010). The pB-C2 line was significantly longer in girls (p = 0.006). These measurements did not significantly correlate with symptomatology. Length of tonsillar ectopia in pediatric CM-I correlated with an enlarged foramen magnum (p = 0.023), increasing obex displacement (p = 0.020), and increasing odontoid retroflexion (p < 0.001). CONCLUSIONS Anomalous bony development of the craniocervical junction is a consistent feature of CM-I in children. The authors found that the population at their center was characterized by posterior angulation of the odontoid process in 81% of cases, similar to findings by Tubbs et al. (84%). The odontoid process appeared to lengthen and become more posteriorly inclined with age. Increased tonsillar ectopia was associated with more posterior odontoid angulation, a widened foramen magnum, and an inferiorly displaced obex.
authors.affiliation: Department of 1 Neurological Surgery and.
authors.forename: Travis R
authors.initials: TR
authors.lastname: Ladner
authors.affiliation: Department of 1 Neurological Surgery and.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Dewan
authors.affiliation: Radiology, Vanderbilt University School of Medicine, Nashville, Tennessee; and.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Day
authors.affiliation: Department of 1 Neurological Surgery and.
authors.forename: Chevis N
authors.initials: CN
authors.lastname: Shannon
authors.affiliation: Department of Neurological Surgery, Dell Children's Medical Center of Central Texas, Austin, Texas.
authors.forename: Luke
authors.initials: L
authors.lastname: Tomycz
authors.affiliation: Department of 1 Neurological Surgery and.
authors.forename: Noel
authors.initials: N
authors.lastname: Tulipan
authors.affiliation: Department of 1 Neurological Surgery and.
authors.forename: John C
authors.initials: JC
authors.lastname: Wellons
mesh_terms: D001139:Arnold-Chiari Malformation / Q000175:diagnosis* / Q000601:surgery; D001837:Body Weights and Measures; D002508:Cephalometry; D002648:Child; D002675:Child, Preschool; D019299:Decompression, Surgical; D004388:Dura Mater / Q000601:surgery; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009809:Odontoid Process / Q000473:pathology*; D012189:Retrospective Studies
keywords: CCJ = craniocervical junction; CM-I = Chiari I malformation; Chiari I malformation; craniocervical junction; dens; morphometry; syringomyelia