title: Occipitalization of the atlas in children. Morphologic classification, associations, and clinical relevance.
abstract: BACKGROUND
  Occipitalization is defined as a congenital fusion of the atlas to the base of the occiput. We are not aware of any previous studies addressing the morphologic patterns of occipitalization or the implications of occipitalization in children. We present data on what we believe is the largest reported series of children with occipitalization studied with computed tomography and/or magnetic resonance imaging, and we provide a description of their clinical characteristics.
  
  
  METHODS
  We retrospectively reviewed all cases of occipitalization in children included in our spine database. Patient charts and imaging studies were reviewed. A new morphologic classification of occipitalization was developed from the two-dimensional sagittal and coronal reformatted computed tomographic reconstructions and/or magnetic resonance images. The classification includes four patterns according to the anatomic site of occipitalization (Zones 1, 2, and 3 and a combination of those zones), and it was applied to this group of patients. Imaging studies were also reviewed for evidence of cervical instability and for other anomalies of the craniovertebral junction.
  
  
  RESULTS
  Thirty patients with occipitalization were identified. There were twenty-four boys and six girls with a mean age of 6.5 years. The morphologic categorization was Zone 1 (a fused anterior arch) in six patients, Zone 2 (fused lateral masses) in five, Zone 3 (a fused posterior arch) in four, and a combination of fused zones in fifteen. Seventeen patients (57%) had atlantoaxial instability, and eight of them had an associated C2-C3 fusion. Eleven patients (37%) had spinal canal encroachment, and five of them had clinical findings of myelopathy. The highest prevalence of spinal canal encroachment (63%) was noted in patients with occipitalization in Zone 2.
  
  
  CONCLUSIONS
  Occipitalization is associated with abnormalities that lead to narrowing of the space available for the spinal cord or brainstem. The risk of atlantoaxial instability developing is particularly high when there is an associated congenital C2-C3 fusion. Two-dimensional sagittal and coronal reformatted computed tomographic reconstructions and/or magnetic resonance images can help to establish the diagnosis and permit categorization of occipitalization in three zones, each of which may have a different prognostic implication.
authors.affiliation: Division of Orthopaedic Surgery, Children's Hospital of Philadelphia, 2nd floor, Wood Building, 34th Street and Civic Center Boulevard, Philadelphia, PA 19104, USA.
authors.forename: Purushottam A
authors.initials: PA
authors.lastname: Gholve
authors.forename: Harish S
authors.initials: HS
authors.lastname: Hosalkar
authors.forename: Eric T
authors.initials: ET
authors.lastname: Ricchetti
authors.forename: Avrum N
authors.initials: AN
authors.lastname: Pollock
authors.forename: John P
authors.initials: JP
authors.lastname: Dormans
authors.forename: Denis S
authors.initials: DS
authors.lastname: Drummond
mesh_terms: D000293:Adolescent; D001269:Atlanto-Occipital Joint / Q000002:abnormalities* / Q000000981:diagnostic imaging / Q000473:pathology; D001270:Cervical Atlas / Q000002:abnormalities* / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D002648:Child; D002675:Child, Preschool; D016208:Databases, Factual; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009777:Occipital Bone / Q000002:abnormalities* / Q000000981:diagnostic imaging / Q000473:pathology; D012189:Retrospective Studies; D013123:Spinal Fusion; D013580:Synostosis / Q000145:classification* / Q000175:diagnosis; D014057:Tomography, X-Ray Computed