Table 1. Congenital conditions associated with atlantoaxial dislocation.

Congenital condition	Class	Etiology	Incidence
Down syndrome16 17	Chromosomal disorder	Inflammatory process or an intrinsic defect in collagen fibers that form ligaments20	15–20%19
Goldenhar syndrome	Skeletal dysplasia	Hypoplasia of the dens with atlantoaxial instability16	3 of 8 in one case series21
Spondyloepiphyseal dysplasia	Skeletal dysplasia	Abnormal growth of the spinal vertebrae and epiphysis,16 hypoplasia of the dens or ligamentous laxity22	35%22
Morquio syndrome (mucopolysaccharidosis type IV)	Skeletal dysplasia	Autosomal recessive lysosomal storage disease, resulting in odontoid dysplasia	42–90%23 24
Occipitalization of the atlas	Congenital osseous abnormalities	Abnormal motility in the joint region26	–