Atlanto-occipital assimilation

Case contributed by Shailaja Muniraj


Occipital headache. Clinically suspicious chairi malformation

Patient Data

Age: 40 years old
Gender: Female

There is fusion of the anterior arch of the atlas with clivus and posterior arch with occiput on the right side, identical with complete atlanto-occiptal assimilation on the right side. The posterior arch shows incomplete fusion centrally, failing to form posterior tubercle.

This assimilation causes secondary widening of the atlantodental space contralaterally.

3 D images


Right sided complete atlanto-occiptal assimilation and incomplete fusion of atlas posterior arch is seen. 

The right sided atlanto-occiptal assimilation is well seen again.

There is downward herniation of the cerebellar tonsils over a length of 11.5 mm from the foramen magnum level, in keeping with Chiari type I malformation. No syringomyelia/hydrocephalus.

There is a moderate sized well defined, thin walled cystic lesion noted in the right cerebello-pontine angle. This lesion follows CSF signal on all sequences. Mass effect noted in the form of compression of adjacent cerebellar hemisphere. No internal solid components.  No restriction on ADC/DWI images. Features suggestive of arachnoid cyst. No features of bone remodeling.

Special thanks: Dr. Anand H.K

Case Discussion

This is a case of atlanto-occipital assimilation with associated Chiari I malformation and right cerebellopontine angle arachnoid cyst.

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