The craniovertebral junction (CVJ) is composed of the occiput, the foramen magnum, and the first two cervical vertebrae, encompassing the medulla oblongata and the upper cervical spinal cord. Anomalies of the CVJ may be congenital or acquired.

CVJ anomalies that decrease the volume of the posterior cranial fossa (e.g., platybasia) cause Chiari malformations, while erosion of the cervical vertebrae causes basilar invagination (cranial migration of the odontoid process), and anomalous fusion of the cervical vertebrae causes Klippel-Feil syndrome.

Arnold-Chiari malformations

the most frequent congenital anomalies of the CNS

There are two main kinds of Chiari malformations.Type 1 Chiari malformation symptoms and signs can show up in infants, children, teens or adults.Type 2 Chiari malformation is associated with spina bifida and is present at birth. Arnold-Chiari type 2 is more common in infants and children.Surgery can address symptoms such as headache, hydrocephalus, sleep apnea and others.

The small posterior fossa is a crucial diagnostic feature of Arnold-Chiari type 2.

This anatomical defect is responsible for downward displacement of the cerebellar vermis and medulla through the foramen magnum

This leads to obstruction of the CSF flow and hydrocephalus.

Important associated abnormalities include lumbar myelomeningocele and syringomyelia

Arnold-Chiari type 1

definitely more frequent in adolescents and adults than the type 2 malformation but is usually asymptomatic

It is characterized by downward displacement of the cerebellar tonsils through the foramen magnum

Dandy-Walker syndrome:

1. abnormally large posterior fossa; & the tentorium and torcula are elevated
2. absence of cerebellar vermis
3. development of a large, ependyma-lined cyst that represents an expanded 4th ventricle

Dx

Best Evaluated On Midline Sagittal Images Of The Brain.