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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:
  1. Injury to the Cervico-medullary junction neural elements
  2. Injury to the Vertebral & Basilar Arteries resulting in circulatory embarrassment to the Brain Stem
  3. Impairment of Cerebrospinal Fluid (CSF) flow due to a relative obstruction of the normal subarachnoid space at the Foramen Magnum.
There are two types of Basilar Impression:
  1. 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 abnormalities, Klippel-Feil Syndrome, and Goldenhar Syndrome
  2. 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.
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 pain.


Basilar Impression is difficult to assess radiographically, and many measurement schemes have been proposed.

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

All content ©2022 by Neurosurgical Consultants, P.A.
Author, Martin L. Lazar, MD, FACS
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