Bone dysplasia series. Achondroplasia, hypochondroplasia and thanatophoric dysplasia: review and update.

The authors summarize the clinical, genetic and histopathologic features, as well as the complications, and radiological diagnosis of 3 related generalized short-limb skeletal dysplasias: achondroplasia, hypochondroplasia and thanatophoric dysplasia. In all of these dysplasias, there is abnormal endochondral ossification, but periosteal ossification is not affected. These 3 relatively common entities are known to be allelic to the same gene: the fibroblast growth factor receptor 3 gene on chromosome 4p. Heterozygous achondroplasia is the most common nonlethal skeletal dysplasia. The distinctive clinical and radiological features allow a precise diagnosis, as there is little variability in the appearance of affected patients. There is also a very evident molecular homogeneity. On histopathology of the growth plate, there is a quantitative decrease in endochondral ossification. Precise prenatal ultrasonographic diagnosis is possible in the third trimester, and sometimes even in the second. Hypochondroplasia is a relatively common, milder form of achondroplasia, which varies within and between families and lacks the neurological complications often seen in achondroplasia of this group. An accurate prenatal ultrasonographic diagnosis is rare. There are milder changes on histology of the growth plate. Thanatophoric dysplasia is the lethal and most severe dysplasia. It has distinct features--mainly short tubular bones and short ribs with platyspondyly--allowing a precise radiologic and prenatal ultrasonographic diagnosis. On histopathology of the growth plate, there is disruption of endochondral ossification.