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January 5, 2008
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Timothy C. Hain, MD Page last modified:
January 5, 2008
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.
Platybasia, is a related term, indicating an increase in the basal angle of the skull -- basically a flattening of the base of the skull. Platybasia is diagnosed when, on a lateral skull film or saggitally formatted CT, the angle of the line drawn along the plane of the sphenoid sinus and along the clivus is greater than 145 degrees. Platybasia, per se, is not a disease.
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.
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.
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.
Neurosurgical treatment is quite difficult. In the series of Sawin and Menezes (1997), they suggest that ventral brainstem compression should be treated with ventral decompression, followed by occipitocervical fusion with contoured loop instrumentation to prevent further squamooccipital infolding. In persons with Chiari malformation, there may also be occipital decompression surgery. Ventral decompression means operating through the front of the head, through the mouth. Both the transoral and posterior surgery can be done in one sitting (Zileli and Cagli 2002). Another approach is atlantoaxial joint distraction and direct lateral mass fixation using screws (Goel, 2004).
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.
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