Atlanto-occipital assimilation occurs in approximately 0.5% (range 0.08-3%) of the population . It is thought to affect males and females equally.
It is typically asymptomatic but symptoms from nerve or vascular compression can occur.
Fusion of C1 to the occiput can be either :
- complete: C1 not identifiable
- incomplete: C1 partially identifiable
It can also be classified as
- posterior arch assimilation
- anterior arch assimilation or
- assimilation of both the anterior and posterior arches
Atlanto-occipital assimilation is associated with :
- fusion of C2 and C3 (occurs in 50% of cases)
- basilar invagination: type 1
- cleft palate
- cervical ribs
- urinary tract anomalies
- cranio-cervical instability