Skeletal dysplasias, also called osteochondral dysplasias, result from abnormal formation and remodeling of bone, which lead to diffuse skeletal deformity and shortening of long bones. Most are caused by genetic defects that affect cartilage and bone development. Skeletal dysplasias are often characterized prenatally based on the severity of the deformities and the ability of an affected infant to survive. Forms of skeletal dysplasia that are lethal include thanatophoric dysplasia, osteogenesis imperfecta type II, achondrogenesis, and hypophosphatasia. With lethal skeletal dysplasias, the neonate typically dies shortly after birth, most often because the thorax fails to develop large enough for adequate lung growth to support respiration. The entire skeleton is affected in these cases, with multiple deformities, including bowing, fractures, and marked shortening of the long bones as well as poor ossification, particularly of the cranium and spine.

Less severe skeletal dysplasias such as heterozygous achondroplasia and osteogenesis imperfecta types I, III, and IV, may be compatible with life. Affected infants may have moderate shortening of the long bones and poor mineralization of some osseous structures.

The diagnosis of a skeletal dysplasia is made when the long bones are markedly short, measuring more than four standard deviations below the mean for gestational age. Additional findings include poor mineralization of bones, particularly the skull, long-bone fractures and bowing, and a narrow thorax. Determination of the specific type of skeletal dysplasia may be possible with careful sonographic assessment of the degree of long-bone shortening, degree of cranial ossification, and the shape of the cranium.

Thanatophoric dysplasia is one of the more common lethal skeletal dysplasias. This dysplasia is characterized by marked shortening of the long bones, bowing of the long bones, a narrow thorax, and cloverleaf deformity of the skull (Figure 16.1.1).

Osteogenesis imperfecta type II, is an autosomal recessive condition characterized by marked shortening with fractures and deformities of the long bones and ribs, a small thorax, poor mineralization of the cranium, and a soft skull (Figure 16.1.2). The cranium may be easily compressible with gentle pressure from the ultrasound transducer.

Achondrogenesis is an autosomal recessive lethal skeletal dysplasia in which there is almost complete absence of ossification except for the calvarium. The vertebral bodies fail to ossify, and the long bones are markedly shortened and poorly ossified (Figure 16.1.3). The thorax is very small.

Hypophosphatasia is an autosomal dominant metabolic disorder that causes poor ossification and shortening of the long bones. The skull is typically poorly ossified.

The less severe, nonlethal skeletal dysplasias are often not detectable prenatally during the second trimester because the long bones are not abnormally shortened or deformed at
that stage of pregnancy. In the third trimester, measurements of the long bones with these skeletal dysplasias may begin to lag behind the expected size for gestational age, but typically they do not fall more than four standard deviations below the mean. Sometimes, in the third trimester, bowing or fractures (Figures 16.1.4 and 16.1.5) may become apparent, particularly with the nonlethal forms of osteogenesis imperfecta.

Defective ossification of one bone or a group of bones is called a skeletal dysostosis. Some skeletal dysostoses have a recognizable pattern of deformities and are part of a known syndrome, such as Nager acrofacial dysostosis, Poland syndrome, and proximal focal femoral deficiency. Nonskeletal anomalies are common in many of these syndromes. Other dysostoses are isolated bony deformities with no other fetal anomalies. The prognosis for skeletal dysostoses is related to the degree of severity of other anomalies and the extent of osseous deformities. Deformities of the head, spine, and thorax carry a worse prognosis than deformities isolated to the extremities.

The sonographic appearance of a skeletal dysostosis depends on the bony structures involved. For example, with Nager acrofacial dysostosis, the upper extremities are markedly shortened, with absence of one or more of the long bones. The hands are present, but they may be incompletely formed. A severely hypoplastic mandible and external ear deformities are also characteristics of this type of dysostosis (Figure 16.2.1). Proximal focal femoral deficiency is characterized by the absence of the proximal femur. The finding is unilateral in 90% of cases. The diagnosis is made when there is extreme shortening of one femur with a normal contralateral femur. Other skeletal anomalies may be present, most commonly in the affected lower extremity (Figure 16.2.2).