Developmental abnormalities of the occipital bone in human chondrodystrophies (achondroplasia and thanatophoric dwarfism).

Specific developmental malformations have been demonstrated in the occipital bone of two chondrodysplastic disorders (achondroplasia and thanatophoric dwarfism). Analysis of these malformations indicates that the occipital bone is primary affected in these disorders. In both cases, the endochondral-derived components of the occipital bone (the basioccipital, the two lateral parts, and the planum nuchale of the squama occipitalis) have failed to grow properly and are smaller and shorter than normal. On the other hand, the planum occipitalis of the squama, which derives from intramembranous ossification, is unaffected. In addition, the nature of these abnormalities indicates that the occipital synchondroses, together with the epiphyseal plates of other bones, are primarily affected in these two chondrodysplasias. The components of the occipital bone formed between the affected synchondroses failed to grow normally. The resulting malformation of the occipital bone is undoubtedly the cause of the shortening of the posterior cerebral fossa and of the considerable narrowing of the foramen magnum often described in these chondrodysplasias. It is postulated that growth disturbances between the affected occipital bone and the unaffected central nervous system results in the inadequacy of the posterior cerebral fossa and the foramen magnum to accommodate the growing brain. Consequently, compression of the brain at the posterior cerebral fossa or the foramen magnum levels could occur and thus lead to neurologic complications such as hydrocephalus and compression of the brain stem. It is suggested that the surgical removal of the fused posterior border of the lateral parts of the occipital bone (partial nuchalectomy) for the purpose of enlarging the narrow foramen magnum may be indicated in those chondrodysplastic children who develop these types of neurologic complications.