Basilar Invagination and Atlantoaxial Subluxation

Timothy C. Hain, MD

Last updated: 2/16/01

Basilar Invagination

Basilar invagination is an uncommon syndrome that occurs when the superior part of the odontoid (part of the C2 vertebrae) migrates upward. It is uncommon but somewhat dangerous. It occurs both congenitally and in persons with bone diseases, such as rheumatoid arthritis. It may lead to static or dynamic stenosis of the foramen magnum, and compression of the medulla oblongata (lower brainstem) which is manifested clinically as sudden death due to fatal brainstem compression. A closely related condition, the Chiari malformation, mainly occurs congenitally.

Basilar invagination from rheumatoid arthritis is due to loss of axial supporting structures in the upper cervical spine. It is estimated that about 10% of patients with rheumatoid arthritis are at risk for sudden death. Obstructive hydrocephalus or syringomyelia may also be seen because of direct mechanical blockage of normal CSF flow.

Symptoms become apparent when there is a great deal of flexion. It can present as posterior skull pain. A C2 sensory deficit should be looked for. Patients may also present with a "pseudo-ulnar hand," with tingling and numbness in the 4th and 5th digit, and tingling and numbness in the medial forearm. Patients will go into a pool and notice that below the umbilicus the water is not as cold as it above (this suggests central cord disease). Lhermitte's sign (a tingling on neck movement, flexion in this case) can be demonstrated at any stage.

A plain lateral x-ray, with odontoid views, is a good place to start. Flexion extension MRIs have a higher yield. A plain CT scan can also document this, but MRI provides more information. Somatosensory evoked potentials (SSEPs) may have false positives.

Neurosurgery is recommended when neurologic symptoms and signs are present, and cord compression is confirmed by MRI. When these features are absent, a conservative approach may be pursued, such as a collar, nonsteroidal anitinflammatory drugs (NSAIDs), and simple neck traction. In patients who are considered poor surgical risks, neurologic progression is likely and the one-year prognosis is poor.

Atlantoaxial Subluxation (AAS)

Atlantoaxial subluxation (AAS) is a related problem to basilar invagination. AAS can occur anteriorly, posteriorly, vertically, laterally, or in combinations. Movement of the axis anterior to the atlas (greater than 3mm from the arch of C1 and the odontoid) results from transverse ligament laxity/destruction, or odontoid process erosion/fracture caused by invasive rheumatoid arthritis. Cord compression is more likely when the subluxation exceeds 9mm. In most cases, subluxation less than this is asymptomatic. Lateral x-rays taken in extension and flexion are used to demonstrate AAS. Indications for surgery include neurologic abnormality with instability, intractable neck and head pain, vertebral artery compromise, and cord compression on MRI (even without symptoms). The most common surgical procedure for anterior C1-C2 subluxation is posterior fusion by internal fixation of C1-C2.