Basilar Impression: The Position of the Normal Odontoid

Abstract

In 1939 W. E. Chamberlain (1) called attention to the syndrome of basilar impression, which had been described in the European literature (2–4) years before. The syndrome includes symptoms mimicking syringomyelia, progressive spastic paralysis, or multiple sclerosis in the region of the upper cervical cord and medulla, and a shortening of the neck. The roentgenogram shows deformities of the foramen magnum and anomalies of the base of the skull, the atlas, and axis, sometimes including fusion of the bodies of some of the upper cervical vertebrae, and often fusion of the anterior arch of the atlas to the occipital bone. Since 1939 enough additional cases have been reported to indicate that the condition is not excessively rare. The symptoms can often be relieved by surgery. It becomes a matter of some importance, therefore, to set up proper diagnostic criteria. Chamberlain points out that the final diagnosis must usually be made by the radiologist on the basis of changes in the relations at the base of the skull, including displacement of the atlas and the tip of the odontoid above their normal position, which he states is entirely below a line joining the posterior margin of the hard palate with the dorsal margin of the foramen magnum. This dictum has been used as an important criterion, notably by Walsh, Camp, and Craig (5) and by Laube and Turner (6). The former authors have christened the determining line the “Chamberlain line.” Before this criterion is finally accepted, however, the range of position of the tip of the odontoid in the lateral roentgenogram of normal persons needs to be established. Since basilar impression is so rare, the general run of patients having skull examinations may be considered normal in this respect and can properly be used as controls. We have chosen from among our latest examinations 100 lateral skull films on which no roentgen diagnosis of disease could be made. In none of these cases were there symptoms at all suggestive of the syndrome of basilar impression. The position of the tip of the odontoid process was carefully measured with respect to a line joining the dorsal margin of the hard palate with the dorsal margin of the foramen magnum. Table I gives the results arranged by 1-mm. intervals, the minus sign indicating that the tip of the odontoid lies below the reference line and the plus sign above. The arithmetical mean position of the top of the odontoid in the normal roentgenogram is, therefore, about 1 mm. (0.94) below the reference line. The standard deviation is 3.6 mm. The normal probability curve corresponding to these coordinates is seen to lie close to the plot of the crude data. One may then use the normal curve to calculate the probability that the tip of the odontoid will lie above the reference line by varying amounts, as follows: 1 in 5 will lie more than 2 mm. above the line. 1 in 19 will lie more than 5 mm. above the line. 1 in 64 will lie more than 7 mm. above the line.