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Atlantooccipital disassociation

- See:
     - Occipital-Atlanto-Axial Anomalies:
     - Anatomy of C1 / C2

- Discussion:
    - more common children, since the pediatric occipital condyles are small and almost horizontal & lack inherent stability;
    - this injury is usually but not invariably fatal 2nd to respiratory arrest caused by injury to the lower brain stem;
    - caused by severe hyperextension Injury w/ distraction;
    - ligaments opposing occipital condyles to superior articulating facets of atlas (tectorial ligaments) are disrupted, resulting in either anterior translation (hyperflexion),
           posterior translation (hyperextension), or longitudinal distraction;
           - along w/ joint capsules, tectorial membrane is torn;
    - dissociation may be complete (dislocation) or incomplete (subluxation);
           - detection of this injury is difficult in cases of partial disruption or if reduction occurs after the initial subluxation;
    - non traumatic atlanooccipital subluxation may occur, most frequently in Down's syndrome & rheumatoid arthritis;
    - in the report by Kenter et al, the authors report on 5 children with traumatic AOD;
           - average distance from the dens to the basion was 9.8 mm; and average ratio of Powers was 1.38
           - there were three survivors, two having a concomitant spinal cord injury;
           - all survivors underwent a posterior occipitovertebral fusion;
           - 3 cases initially went undiagnosed;
           - ref: Pediatric Traumatic Atlanto-Occipital Dislocation: Five Cases and a Review

- Exam:
    - cranial nerve disfunction is common;
    - myelopathic changes may be present;

- Radiographs:
    - increased distance between clivus & dens, w/ disruption of basilar line of Wackenheim;
    - Wachenheim's line
          - usded to determine anterior / posterior subluxation
          - this line is drawn down the posterior surface of the clivus and its inferior extension should barely touch the posterior aspect 
                   of the odontoid tip;
          - this relationship does not change in flexion and extension;

          - thus if this line runs behind the odontoid, posterior subluxation has occured and vice versa;
          - in children that atlantooccipital distraction has occurred if distance between the occiput & atlas is > 5 mm at any point in joint;
    - Atlanto-Occipital Condyle Distance:
          - should be less than 5 mm regardless of age;
    - Powers Ratio:
          - identifies anterior subluxation & is described as ratio of BC/OA;
          - BC is the distance from the basion to the midvertical portion of posterior laminar line of the atlas;
          - OA is distance from opisthion to midvertical portion of posterior surface of anterior ring of Atlas;
          - if this ratio is greater than 1, anterior subluxation exists;

- Treatment:
    - initially it is essential to avoid flexion of C-spine, which can occur on standard adult trauma boards;
    - ensure that the matress allows the child's head to remain in an anatomic position;
    - head is immobilized w/ sandbags (or equivolent);
    - traction is contraindicated because of this severe instability & potential for injury to vertebral arteries & spinal cord;
    - rigid immobilization of the patient in halo allows adjustment to obtain reduction, & maintains position during and after operation;
    - fusion involves occiput to C2 fusion


Traumatic Atlanto-Occipital Dislocation in Children.