Basilar invagination

Dr Owen Kang and Dr Andrew Dixon et al.

Basilar invagination, also called basilar impression, is a congenital or acquired craniocervical junction abnormality where the tip of the odontoid process projects above the foramen magnum

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 they are not synonyms.

Basilar invagination is defined as the upward displacement of vertebral elements into a normal foramen magnum with normal bone.

In contrast, basilar impression is a similar upward displacement of the dens due to, however, softening of bones at the base of skull

It may be congenital or acquired 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.

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

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.

A classification system has been proposed based on the absence (group I) and presence (group II) of Chiari malformation, and 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.

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Article information

rID: 10024
Section: Gamuts
Synonyms or Alternate Spellings:
  • Basilar impression

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Cases and figures

  • Figure 1: lateral
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  • MRI
    Case 1: osteogenesis imperfecta
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  • Figure 2: frontal
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  • Basilar invaginat...
    Case 2
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  • Case 3: osteogenesis imperfecta
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