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Neuroradiology Case of the Week
Case 440
September 2009
Lisa Siripun, MD, Richard White, DO and P-L Westesson, MD, PhD, DDS
Clinical
Presentation: Clinical history is unknown.
Imaging Findings: See below
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Figure 1: Sagittal T1-weighted image demonstrates vertical subluxation of the odontoid process.
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| Figure 2: Sagittal T2-weighted image shows severe compression of the spinal cord with atrophy and signal changes. The bright T2 signal represents myelopathy. There is remodeling of the dens, C1 and the clivus. |
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Figure 3: Axial CT image of the head shows the odontoid process is above the foramen magnum and result in narrowing the foramen magnum.
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Diagnosis: Basilar invagination
Discussion: Basilar invagination (BI) is an uncommon syndrome that occurs when the superior part of the odontoid migrates upward due to softening of bones at the base of the skull. It may result from erosion and/or bone loss between occiput and dens or may result from erosion and settling of the C1/C2 articulation. Lateral masses of atlas may collapse secondary to erosive changes in atlantooccipital and atlantoaxial synovial joints. This leads to vertical subluxation of the odontoid process which may be expanded by surrounding pannus and is brought into contact with the cervicomedullary junction.
Basilar invagination is often associated with platybasia which is referred to an increase in the basal angle of the skull -- basically a flattening of the base of the skull. Platybasia is diagnosed when the angle of the line drawn along the plane of the sphenoid sinus and along the clivus is greater than 145 degrees.
Basilar invagination 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. Symptoms and signs included headache, lower cranial nerve dysfunction, hyperreflexia, quadriparesis, ataxia, nystagmus, dysphagia and loss of pain/touch sensation over the trigeminal nerve distribution. Downbeating nystagmus and postural hypotension has been reported [2].
There are several reference lines, developed to assess possible basilar invagination on plain films that can be applied easily to midline sagittal MR imaging. But two the most common lines are:
- McGregor's line, extending from the posterior margin of the hard palate to the most inferior point of the cortex of the occipital bone posterior to the foramen magnum. If the odontoid extends more than 7 mm (or one third of the height of the odontoid process) above this line, basilar invagination present.
- Chamberlain's line; from the hard palate to the posterior margin of the foramen magnum. If the odontoid extends more than 5 mm above this line, basilar invagination is present.
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 anti-inflammatory 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. When symptoms progress and threaten disability, treatment is by surgical decompression or by transoral odontoidectomy with reduction of the basilar impression and craniovertebral junction bony realignment, and atlanto-axial fixation.
References:
- Pearce JMS. Platybasia and basilar invagination. Eur Neurol. 2007;58(1):62-4. Epub 2007 May 4. PMID: 17483591 [PubMed]
- Pratiparnawatr P, Tiamkao S, Tanapaisal C, Kanpittaya J, Jitpimolmard S. Downbeating nystagmus and postural hypotension due to basilar invagination. J Med Assoc Thai. 2000 Dec;83(12):1530-4. PMID: 11253895 [PubMed]
- Som PM, Curtin HD. Head and Neck Imaging. 4th ed, Mosby, 2002:853-854.
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