Thanatophoric dysplasia (TD) is characterized by micromelia, large head, a long, narrow trunk and extremely short ribs. Most infants die in the neonatal period, often due to pulmonary hypoplasia secondary to the small thoracic cage. The incidence is quoted as being from 1:6,400 to 1: 100,000. Two types have been described. Type I has bowed femurs while type II has straight femurs and cloverleaf skull. The platyspondylic varieties are a separate subgroup that includes the San Diego, Torrance, Lutton and Shiraz types. Differential diagnosis among these entities is based on radiologic and histologic characteristics. Most cases probably represent a new sporadic, lethal autosomal dominant mutation although familial cases of TD type II have been reported. It is also known as thanatophoric dwarfism.
Diagnosis can be made at routine mid trimester scan. In general, there is a large calvarium with frontal bossing and a small face. Premature closure of the coronal and lambdoid suture result in the cloverleaf appearance of the skull. Early signs include absence of the normal echolucent line that corresponds to these sutures with bulging of the surrounding bone, creating an indentation in the contour of the calvarium. The chest is narrow, with short ribs and small scapulae. There are varying degrees of platyspondyly and a narrow spinal canal has been documented in some cases. The pelvic bones are small. The femurs may be straight or bowed (‘telephone receiver’) with irregularity and flaring of the metaphyses. The bones of the hands and feet are very short and broad. The precise sonographic findings depend on the type encountered. The association of cloverleaf skull and severe micromelia is specific for TD type II, while ‘telephone receiver’ femurs are typical of type I. Severe platyspondyly suggests one of the platyspondylic varieties.
Achondrogenesis may be differentiated by abnormal spinal and/or skull ossification. In homozygous achondroplasia both parents will be affected. In the Antley-Bixler syndrome there may be a cloverleaf skull but there is no micromelia and radiohumeral synostosis is frequently seen. Atelosteogenesis type II features hitchhiker thumbs, vertebral body coronal clefts and a horizontal sacrum. Fibrochondrogenesis has an undermineralized skull, coronal clefts and the metaphyses are dumbbell-shaped rather than irregular with flaring. Osteogenesis imperfecta type II has multiple fractures and, frequently ,abnormal calvarial ossification. Schneckenbecken dysplasia can be very difficult to differentiate prenatally, the only potential distinguishing feature being the absence of ossification of the posterior vertebral body. The short rib-polydactyly syndromes have severe rib shortening and polydactyly.
Cloverleaf skull (type II)
Straight or bowed femur
Irregularity and flaring of metaphyses
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