Basilar invagination means invagination of upper part of the vertebral column into the posterior fossa along with the rim of the foramen magnum.
As a result the upper part of cervical cord is stretched over the protecting odontoid peg. Clinically patient has low hairline, short neck and signs and symptoms of lower cranial nerve and cervico medullary compression.
Clinical signs are due to medullo spinal compression by the invaginated odontoid process. There are attacks of episodic headache, vertigo, diplopia and drop attacks. Motor weakness of the limbs may progress gradually.
Ataxia, nystagmus and intention tremors indicate cerrbellar involvement while pressure on the cervico medullary region results in pyramidal tract involvement.
In contrast to various neurological defects it may be asymptomatic in some. Most often it is a developmental anomaly while it may be an acquired condition as in osteo malacia, pagets disease etc.
On lateral X-ray of the skull, odontoid process and the anterior arch of atlas project above a line drawn from the dorsal lip of the formamen magnum to the dorsal margin of the hard palate.
In an anterior posterior tomograph of the skull, the tip of the odontoid process will be seen projecting into the posterior fossa. CT/MRI are important in making the diagnosis.
Basilar invagination alone or in association with other defects may produce neurological signs (weakness and pyramidal signs, cerebellar signs, ataxia) and here surgical excision of posterior arch of atlas and posterior margin of foramen magnum has been attempted and is known to be beneficial.