Basilar invagination

Basilar invagination is a craniocervical junction abnormality where the tip of the odontoid process projects above the foramen magnum. It may be congenital or acquired (also termed basilar impression) and is often associated with platybasia. There is stenosis of the foramen magnum and compression of the medulla oblongata resulting in neurological symptoms, obstructive hydrocephalus, syringomyelia or even death.

The terms basilar invagination and basilar impression are often used interchangeably because in both cases there is upwards migration of the upper cervical spine, but precisely, basilar impression is defined as upward displacement of vertebral elements into the normal foramen magnum with normal bone, while basilar invagination is a similar displacement due to softening of bones at the base of the skull. 

A classification system based on the absence (group I) and presence (group II) of chiari malformation can be of use in planning surgical management. Brainstem compression relates to odontoid process indentation in group I, while reduced posterior cranial fossa volume is the cause in group II.¹

Causes

Common causes can be recalled using the mnemonic PF ROACH. Another approach is to divide them into congenital and acquired causes:

Congenital

• osteogenesis imperfecta
• Klippel-Feil syndrome
• achondroplasia
• Chiari I malformation / Chiari II malformation
• cleidocranial dysostosis
• Schwartz-Jampel syndrome ²

Acquired

• rheumatoid arthritis
• Paget's disease
• hyperparathyroidism
• osteomalacia / rickets

Basilar invagination occurs in around 5 - 10% of patients with cervical rheumatoid arthritis due to loss of axial supporting structures in the upper cervical spine. ^3,4

See also

• platybasia