Skeletal dysplasias, or chondrodystrophies, are a heterogeneous group of disorders resulting in short-limb or short-trunk types of disproportionate short stature. In the types of dwarfism that primarily affect the limbs, the shortening may predominate in the proximal segments (rhizomelia), the middle segments (mesomelia), or the distal segments (acromelia). The term dwarf has traditionally been applied to persons of disproportionate short stature, whereas the term midget referred to those of proportionate short stature.
Disproportionate dwarfism is caused by a hereditary intrinsic skeletal dysplasia, whereas proportionate dwarfism results from chromosomal, endocrine, nutritional, or nonosseous abnormalities. Over the past several years, we have gained further understanding regarding the mode of inheritance, the genetic defect, and the fundamental biochemical and/or molecular fault that causes the dysplasia. Many cases of dwarfism are the result of a rare genetic event, the spontaneous mutation. Unaffected parents of a child with a mutation are essentially at no risk of having another affected child, and unaffected siblings are not at risk of having children with the disorder. Affected parents may pass the trait on to their children, depending on the mode of inheritance—autosomal dominant, autosomal recessive, or X-linked.
Genetic counseling must be based on an accurate diagnosis and on familiarity with the natural history, range of manifestations, severity, and associated findings of the specific disorder.
Prenatal Testing. Prenatal diagnosis of certain skeletal dysplasias without biochemical markers can be established by radiography (less commonly used), ultrasonography (most widely used), fetoscopy, amniography, three-dimensional ultrasonography, fetal magnetic resonance imaging (MRI), and intrauterine computed tomography (CT). Knowledge of the natural history of intrauterine growth in dwarfing conditions is incomplete. Ossification of the fetal skeleton is not well established until 16 weeks, and it is not known when limb-length discrepancy becomes apparent in the fetus. Serial sonograms are necessary to recognize the decreased growth rate of the femur or to monitor the fetal biparietal diameter, polydactyly, clubfoot, and other skeletal abnormalities.
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