Basilar Invagination

Basilar Invagination and Basilar impression

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Basilar invagination and Basilar impression (we will use the term BI for both) are uncommon syndrome that occurs when the superior part of the odontoid (part of the C2 vertebrae) migrates upward. For the most part, the terms basilar invagination and basilar impression are often used interchangably 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. Thus different terms are used according to whether bone is normal or not.

BI is uncommon but somewhat dangerous. It occurs both congenitally (i.e. basilar impression due to Down's syndrome, Klippel Feil syndrome, Chiari malformation, ) and in persons with bone diseases (basilar invagination), such as rheumatoid arthritis, hyperparathyroidism, Paget's disease, Osteogenesis imperfecta, Rickets, Hurler's syndrome, and Hadju-Cheney syndrome. 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.

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.

Diagnosis

Symptoms of BI generally become apparent when there is a great deal of flexion. It can present as posterior skull pain. According to Sawin and Menezes series in persons with osteogenesis imperfecta (1997), symptoms and signs included headache (76%), lower cranial nerve dysfunction (68%), hyperreflexia (56%), quadriparesis (48%), ataxia (32%), nystagmus (28%), and scoliosis (20%). Downbeating nystagmus and postural hypotension has been reported (Pratiparnawatr, Tiamkao et al. 2000)
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, but is not 100% sensitive (Riew, Hilibrand et al. 2001). Chamberlain's line is drawn between the posterior hard palate to the posterior edge of the foramen magnum. If the dens is more than 3 mm above this line, the patient has basilar invagination.

McGregor's line is the line drawn from the posterior hard palate to the base of the occiput. If the dens is more than 4.5 mm above this line, again basilar invagination is diagnosed.

Flexion extension MRIs have a higher yield. A plain CT scan with saggital reconstructions can also document this, but MRI provides more information. Somatosensory evoked potentials (SSEPs) may have false positives. VEMP testing has not yet been explored in BI but seems promising.