Diagnostic criteria of basilar impression.

Abstract

For twenty years it has been apparent that the diagnosis of basilar impression depends primarily on radiological evaluation. The anomaly is often missed clinically either because it is asymptomatic or because it presents deceptive signs and symptoms. The fact that numerous methods of measurement have been suggested for the radiological diagnosis indicates the complexity of the problem. The purpose of this article is to discuss various diagnostic criteria in the light of information now available. Definition Briefly, basilar impression (or basilar invagination) is a deformity of the osseous structures at the base of the skull in the region of the foramen magnum. The periforaminal components of the occipital bone, and later the petrous portions of the temporal bones, are invaginated upward in such a manner as to diminish the volume of the posterior cranial fossa. Because of the relative immobility of the tentorium cerebelli above, there may result compression of the contents of the posterior fossa and the high cervical spinal cord, embarrassment of circulation, or impairment of the flow of cerebrospinal fluid. Basilar impression may be congenital or it may be acquired as a result of diseases which cause malacic changes of the osseous structures around the foramen magnum. Paget's disease and osteogenesis imperfecta are relatively common causes of the acquired variety. Abnormalities of the central nervous system and adjacent osseous or soft-tissue structures may be associated, and signs and symptoms relating to these may result in a misleading symptom complex. The clinical findings have led too frequently to the incorrect diagnosis of multiple sclerosis, syringomyelia, or other degenerative disease of the central nervous system, with the result that all thoughts of possible surgical amelioration are abandoned and the patient is considered incurable. Review of Literature and Critique McGregor, in 1948, reviewed the historical development of measurements for diagnosing basilar impression (12). Most of the earlier methods which he described have fallen into disuse. Chamberlain's line, on the other hand, is still widely used despite the fact that subsequent investigations have shown it to be relatively unreliable. There are, at present, seven diagnostic methods which can be considered worthy of comment. They are summarized in Figures 1–3. McRae's line and the digastric line seem to be the only two criteria which have weathered the storm of subsequent critical analysis. McGregor's base line and Bull's angle use facial structures to define their lines of reference. In so doing they ignore the fact that the facial and neural components of the skull can develop quite differently and that, consequently, variations in the position and plane of the hard palate can lead to false readings. Bull's angle, in addition, has been shown to vary significantly with flexion and extension (1).