The Chiari I malformation is associated with a variety of congenital osseous anomalies of the skull and spine. In occipitalization of the atlas, also know as assimilation of the atlas, C1 fails to segment from the cranium and remains completely or partially assimilated to the foramen magnum (image above). Association with the Chiari I malformation has been described. Fusion can occur at the anterior arch, the lateral masses, the posterior arch, or a combination of the three. Patients with associated congenital C2-C3 fusion have a higher incidence of instability are treated with craniocervical fusion.
retrospective study of congenital osseous anomalies at the craniocervical
junction treated by occipitocervical plate-rod systems.
Ding X, Abumi K, Ito M, Sudo H, Takahata M, Nagahama K, Iwata A.
Eur Spine J. 2012 Aug;21(8):1580-9.
junction anomaly with atlas assimilation and reducible atlantoaxial
dislocation: a rare constellation of bony abnormalities.
Kumar R, Kalra SK, Vaid VK, Sahu RN, Mahapatra AK.
Pediatr Neurosurg. 2008;44(5):402-5.
Occipitalization of the atlas in children. Morphologic
classification, associations, and clinical relevance.
Gholve PA, Hosalkar HS, Ricchetti ET, Pollock AN, Dormans JP, Drummond DS.
J Bone Joint Surg Am. 2007 Mar;89(3):571-8.
malformation accompanied by assimilation of the atlas, Klippel-Feil syndrome,
and syringomyelia: case report.
Kagawa M, Jinnai T, Matsumoto Y, Kawai N, Kunishio K, Tamiya T, Nagao S.
Surg Neurol. 2006 May;65(5):497-502; discussion 502.
craniovertebral anomalies and the hindbrain herniation syndrome (Chiari I):
data base analysis.
Pediatr Neurosurg. 1995;23(5):260-9.