title: Congenital partial aplasia of the posterior arch of the atlas causing myelopathy: case report and review of the literature.
abstract: METHODS
  A case report of a patient with a congenital anomaly of the posterior arch of the atlas and review of the literature are reported.
  
  
  OBJECTIVE
  To describe a unique presentation of cervical myelopathy caused by a mobile isolated bone fragment in an otherwise healthy young male. The description is supplemented by a video showing the repeated trauma the bone fragment caused to the dorsal spinal cord with neck extension.
  
  
  BACKGROUND
  Congenital anomalies of the posterior arch of C1 are well described and are almost always asymptomatic and found incidentally. Neurologic symptoms, either of a chronic nature or developing acutely following head or neck trauma, have been described in patients with isolated posterior arch tubercles (Types C or D).
  
  
  METHODS
  A 17-year-old male developed sensory deficit in his distal lower extremities acutely that progressed over several weeks proximally and into his left upper extremity. He also described a Lhermitte sign, only with extension of his neck and an episode of temporary quadriparesis with a minor fall. Plain radiographs and a computed tomography scan of his neck revealed a congenital deformity of the posterior arch of the atlas. Magnetic resonance imaging showed T1 and T2 signal abnormality within the posterior spinal cord at the level of C1 to C2.
  
  
  RESULTS
  The patient underwent flexion and extension movements under fluoroscopy, which showed the posterior tubercle moving anteriorly and compressing the spinal cord every time he extended his neck. The fragment was resected, and, intraoperatively, dense fibrous tissue was found to be bridging the bony defect. After surgery, his sensory deficits almost completely resolved, and he no longer had a Lhermitte phenomenon.
  
  
  CONCLUSIONS
  This case illustrates a symptomatic congenital deformity of the posterior arch of the atlas due to a mobile, isolated fragment. These anomalies are exceedingly rare. To date, only 17 patients, including ours, have been described in the literature as having myelopathy related to an isolated posterior tubercle. Of these, only two patients had documented movement of this tubercle before our report. Recognizing the anomaly is crucial because treatment is relatively simple, produces resolution of symptoms, and prevents major neurologic deficits from occurring after trauma.
authors.affiliation: Department of Neurosurgery, University of Utah, Salt Lake City 84132-2303, USA.
authors.forename: Paul
authors.initials: P
authors.lastname: Klimo
authors.forename: Deborah T
authors.initials: DT
authors.lastname: Blumenthal
authors.forename: William T
authors.initials: WT
authors.lastname: Couldwell
mesh_terms: D000293:Adolescent; D001847:Bone Diseases / Q000151:congenital* / Q000000981:diagnostic imaging / Q000601:surgery; D001270:Cervical Atlas / Q000002:abnormalities* / Q000000981:diagnostic imaging / Q000601:surgery; D006801:Humans; D008297:Male; D013117:Spinal Cord Compression / Q000209:etiology* / Q000503:physiopathology; D014057:Tomography, X-Ray Computed