Atlanto-occipital assimilation

Atlanto-occipital assimilation is the fusion of the atlas (C1) to the occiput and is one of the transitional vertebrae. 

Epidemiology

Atlanto-occipital assimilation occurs in approximately 0.5% (range 0.08-3%) of the population ^2-5,. It is thought to affect males and females equally.

Clinical presentation

It is typically asymptomatic but symptoms from nerve or vascular compression can occur.  

Gross anatomy

Fusion of C1 to the occiput can be either ³:

• 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

Associations

Atlanto-occipital assimilation is associated with ²:

• fusion of C2 and C3 (occurs in 50% of cases)
• basilar invagination: type 1
• cleft palate
• cervical ribs
• urinary tract anomalies
• cranio-cervical instability ⁵