Atlanto-occipital assimilation

Last revised by Dr Vicci du Plessis on 30 Jul 2019

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

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.

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

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

Atlanto-occipital assimilation is associated with 2:

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Cases and figures

  • Case 1
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  • Case 2
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  • Case 3: with C2-C3 fusion
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  • Case 4
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