The midline occipital fissure: diagnosis of fracture versus anatomic variants.

Abstract

ROENTGEN DIAGNOSIS of skull fracture in the child is complicated by the numerous accessory sutures which may be present. Previous authors (2–4) have documented many normal variations simulating fracture about the foramen magnum, an area unusually rich in accessory sutures and ossicles. Recently, several cases have come to my attention in which a midline occipital fissure extending to the foramen magnum was seen on roentgenograms taken for head trauma. Because of lack of information in the literature on the differentiation of fracture versus suture in these patients, the present study was initiated. A brief review of the pertinent embryology of the occipital bone (1, 3) shows that six ossification centers are present (Fig. 1). The interparietal bone is of membranous origin with the remainder preformed in cartilage. Augier (1) established that the supraoccipital bone forms from a single center, which is arranged around the dorsal surface of the original foramen magnum, the occipital fissure. The occipital fissure closes in a complicated fashion, with descent of a median process (Kerkring's process) and apposition of the lateral margins of the fissure. Anomalous sutures are found in the infant corresponding to this area, but they should extend no further dorsally than the original occipital fissure (1 to 2 em). The embryological evidence would not predict a fissure extending the entire length of the supraoccipital bone. In evaluation of approximately 900 skull roentgenograms, several midline occipital fissures were encountered, but in none of these was there any serious difficulty of ascertaining their nontraumatic nature. Several patients, one of whom is illustrated, exhibited a tripartite defect of the posterior lip of the foramen magnum (Fig. 2). This could be explained embryologically by failure of appositional growth of the lateral borders of the occipital fissure combined with lack of fusion of the innominate sutures (between the supraoccipital and exoccipitals) bilaterally. One case of a “keyhole” foramen magnum was seen (Fig. 3). This unusual variation was noted in an infant with oxycephaly and could be considered due to lack of downward migration of the Kerkring process. Examination of the skull in a cretin revealed the persistence of Kerkring process (Fig. 4). No cases of a midline occipital fissure (median cerebellar suture) resembling fracture were seen. One patient with cleidocranial dysostosis did show a midline fissure of the interparietal bone associated with deficient ossification elsewhere (Fig. 5). During this same period 2 patients with midline occipital fracture were encountered, and search of previous material revealed an additional case (Figs. 6–8). All 3 showed midline fissures extending from the foramen magnum dorsally beyond the superior margin of the supraoccipital bone.