Basilar Impression

• Invagination of the upper cervical spine into the skull base
• Two types – congenital and developmental
• Symptoms typically develop over 2^nd and 3^rd decades
• Motor weakness, spasticity, and sensory deficits are common
• Brainstem compression by the dens can lead to lower cranial nerve dysfunction
• Diagnosis:
• Plainfilms
• McGregor's line – hard palate to caudal posterior occiput curve
• Suspect if tip of dens more than 4.5 mm above this line
• Chamberlain's line – hard palate to the posterior foramen magnum
• Suspect if tip of dens more than  6 mm above this line
• Ranawat's line – center of C2 pedicle to a line connecting the anterior and posterior C1 arches
• normal measurement is 17 mm in men, 15 mm in women
• 13 mm indicates likely impaction, 7 mm indicates likely medullary compression
• McRae's line – defines the opening of the foramen magnum;
• the tip of the dens may protrude slightly above this line, but if the dens is below this line then impaction is not present

• Can directly visualize on MRI of cervical spine
• Treatment: Anterior decompression with fusion