title: Atlantal hemi-rings and craniocervical instability: identification, clinical characteristics, and management.
abstract: OBJECTIVE
  Congenital craniovertebral anomalies are relatively common, but anomalies leading to overt craniocervical instability may be difficult to recognize and treat. The authors present a series of patients with atlantal hemi-rings, a disorder resulting in congenital craniovertebral instability. Presentation, treatment, imaging, and follow-up data obtained in patients with atlantal hemi-rings were assessed to identify factors relevant to craniocervical instability.
  
  
  METHODS
  Nineteen patients were identified with atlantal hemi-rings, defined as a bony discontinuity of the C-1 ring in conjunction with lateral displacement of the C-1 lateral masses (as seen on coronal CT scans). Clinical and radiological characteristics were analyzed, including patient age at presentation, extent of occipitocervical motion, amount of C-1 lateral mass displacement, associated craniocervical anomalies, integrity of the transverse ligament, and neurological status.
  
  
  RESULTS
  The mean patient age at presentation was 22 months (range birth to 9 years). The mean amount of occipitocervical translation seen on dynamic imaging was 9 mm (range 2-20 mm). Four patients required occipitocervical fusion at presentation. The remaining 15 patients were monitored for a mean of 20 months, and 9 ultimately underwent fusion. Surgery was also recommended for 4 of the remaining 6 children.
  
  
  CONCLUSIONS
  This report describes the radiological and clinical characteristics of patients with atlantal hemirings and craniocervical instability. The authors believe that this anomaly is the underlying cause of progressive instability in a significant proportion of patients with craniocervical abnormalities. The presence of atlantal hemi-rings should prompt immediate and thorough neurosurgical evaluation.
authors.affiliation: Department of Neurosurgery, University of Utah, Salt Lake City, Utah 84132, USA. douglas.brockmeyer@hsc.utah.edu
authors.forename: Douglas L
authors.initials: DL
authors.lastname: Brockmeyer
authors.forename: Meghan M
authors.initials: MM
authors.lastname: Brockmeyer
authors.forename: Taryn
authors.initials: T
authors.lastname: Bragg
mesh_terms: D001268:Atlanto-Axial Joint / Q000002:abnormalities* / Q000000981:diagnostic imaging / Q000601:surgery; D001269:Atlanto-Occipital Joint / Q000002:abnormalities* / Q000000981:diagnostic imaging / Q000601:surgery; D002574:Cervical Vertebrae / Q000002:abnormalities* / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007593:Joint Instability / Q000000981:diagnostic imaging* / Q000209:etiology* / Q000473:pathology / Q000601:surgery; D008279:Magnetic Resonance Imaging; D008297:Male; D019635:Neurosurgical Procedures; D009989:Orthotic Devices; D013123:Spinal Fusion; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome