Basilar invagination refers to cephalad migration of the c-spine in relation to the foramen magnum. This is usually seen in patients with rheumatoid arthritis. The rheumatoid process destroys the articular surfaces between C1 and C2, causing the skull and C1 to settle on C2. This results in cephalad displacement of the odontoid process. In advanced stages, this process can lead to brain stem compression and potential pulmonary arrest and death.