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. identified. The anomaly leads to abnormal bone formation in the region of anterior rim of foramen magnum, either in the midline and/or laterally. The anomaly has been occasionally referred to as 'third condyle', 'remnants of occipital vertebra', 'un-formed bone masses on the anterior rim of foramen magnum' and 'prebasi-occipital arch in combination with an odontoid bone'. This anomaly has been grouped under the term 'proatlas segmentation abnormalities'. Due to the extreme rarity of such cases, the criterion for identification of the anomaly and the management protocol have not been elaborately discussed in the literature. We describe a case and discuss the available literature on the subject. © Springer-Verlag 2009.