Basilar Invagination-MRI

Basilar invagination refers to a condition in which the odontoid process protrudes upward into the intracranial space.

Basilar invagination may be classified as primary (congenital) or secondary (acquired). 

1. Congential Causes: Down syndrome, Klippel-Feil syndrome and Chiari malformations
2. Acquired causes: basilar invagination, also known as basilar impression, is associated with softening of the skull base and is often due to rheumatoid arthritis, Paget disease, osteomalacia, hyperparathyroidism and osteogenesis imperfecta.

Two craniovertebral junction lines are particularly useful in defining basilar invagination.

• Chamberlain’s line extends between the posterior pole of the hard palate and the posterior edge of the foramen magnum (opisthion). If the dens is >3.0 mm above this line basilar invagination is present.
• McGregor’s line, a modification of Chamberlain’s line was developed because the opisthion could not always be seen on plain radiographs. This line extends from the posterior pole of the hard palate to the undersurface of the occiput. If the dens extends >4.5 mm above this line basilar invagination is present.