Unusual bone formation in the anterior rim of foramen magnum: cause, effect and treatment

Abstract

A rare case of proatlas segmental abnormality resulting in a bony mass in the anterior rim of the foramen magnum is studied. Case report of a 19-year-old female showed a progressive weakness of all four limbs for about 3 years. When admitted she could not perform any useful activities by herself. Investigations revealed an unusual bone growth in the region of the anterior rim of foramen magnum that resulted in severe cord compression. The abnormal bone formation involved the lower end of clivus, the tip of the odontoid process and the posterior arch of the atlas. Dynamic imaging did not reveal any clear evidence of instability. Following transoral decompression and posterior fixation, the patient showed dramatic and lasting clinical recovery. Conclusions were drawn as follows. Anomalies of the most caudal part of the occipital sclerotomes due to the failure of proatlas segmentation can be the cause of an abnormal bone mass in the anterior rim of foramen magnum. Transoral decompression, followed by posterior atlantoaxial fixation, results in neurological recovery and provides lasting cure from the problem.

Fig. 1

a CT scan reconstruction image showing the abnormal bone arising from the inferior end of the clivus and tip of the odontoid process. The posterior arch of the atlas is not identified. b Sagittal image of T1-weighted MRI showing the compression of the cord by abnormal bone growth. c T2-weighted image showing the abnormal bone and ligamentous mass and neural compression. d Coronal image of MRI showing the transverse extension of the abnormal bone as hypo-intense ridge at the level of arch of atlas

Fig. 2

a Postoperative CT scan showing the anterior decompression. Artifacts related to stainless steel implant are seen. b Sagittal image through the lateral mass showing the plate and screw fixation