Basilar Impression (or Basilar Invagination) is the deformity
of the bones of the Base of the Skull at the margin of the Foramen
Magnum. The "floor of the Skull" appears to be
"indented" by the upper (superior) portion of the Cervical
Spine; therefore, the "tip" of the Odontoid is abnormally
located in an upwards (more cephalad) position. The unfortunate
consequences of this condition are that this increases the risk of
neurological damage from:
There are two types of Basilar Impression:
- Injury to the Cervico-medullary junction neural elements
- Injury to the Vertebral & Basilar Arteries resulting in
circulatory embarrassment to the Brain Stem
- Impairment of Cerebrospinal Fluid (CSF) flow due to a
relative obstruction of the normal subarachnoid space at the
With Basilar Impression, the upper Cervical Spine encroaches
on the Brain Stem and Spinal Cord as the Base of the Skull is
displaced toward the Cranial Vault.
Most affected patients remain asymptomatic until their 20's or
30's. By far the majority of patients who become
symptomatic will manifest neurological deficits involving
motor and sensory disturbances as well as headache and neck
Basilar Impression is difficult to assess radiographically, and
many measurement schemes have been proposed.
- Primary. This is a congenital
abnormality often associated with other anomalies such as
Atlanto-occipital Fusion, Hypoplasia of
the Atlas, Bifid Posterior Arch of Atlas, Odontoid
Klippel-Feil Syndrome, and
- Secondary. This is a "developmental
condition" (meaning it is not "inherited" or a consequence
of some "in utero" problem) which usually develops later in
life and is caused by "softening" of the bone
in this region.
CT and MRI scans are very helpful techniques for the evaluation of
these patients. CT Scans which incorporate "reconstruction
views, in particular, are the most accurate methods
and are highly recommended for patients in whom the clinical
findings may suggest an Occipitocervical anomaly.
Those most commonly used screening method is based on a measurement
scheme (McGregor's line) that are evaluated on the lateral X-ray images
of the Skull and upper Cervical Spine. McGregor's line is drawn
from the upper surface of the posterior edge of the hard palate to the
lowest point of the Occipital curve of the Skull. McGregor's line is the
best method for screening because the bony landmarks can be clearly
defined in persons of all ages on a routine lateral x-ray image. The
position of the tip of the Odontoid is measured in relation to this base
line and a distance of 4.5 mm above McGregor's line is considered to be
on the extreme edge of reference ranges.
Treatment depends on the cause of the symptoms and often requires an
extensive Neurosurgical procedure.
Anterior impingement upon the Brain Stem from a "hypermobile"
Odontoid may require fusion in a position of extension if the Odontoid
can be reduced towards a more normal position. If the Odontoid cannot be
reduced, an anterior excision of the Odontoid and stabilization in
extension may be required.
Posterior impingement upon the Spinal Cord and Medulla may
require Suboccipital Craniectomy and "decompression" of the posterior
ring of C1 and possibly C2 with the release of tight dural bands. This
is followed by fusion of the Occiput to C2 or C3.
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This page last edited on 2/19