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Thanatophoric Dysplasia

(Cloverleaf Skull)

Kleeblattschadel (Cloverleaf Skull)
Shortened, bowed femur (Telephone-receiver)

Platyspondyly
Markedly shortened ribs

Findings:

Marked deformity of the fetal calvarium due to premature closure of the cranial sutures is seen. Ossification of the calvarium over the protruding areas excludes an encephalocele as a consideration. In addition, hydrocephalus and agenesis of the corpus callosum is present. All extremities are markedly shortened, with bowing deformity and flaring of the metaphyses resembling a "French telephone receiver". The vertebral bodies were flattened and the ribs were also markedly shortened, extending only 1/3 to 1/2 of the way around the thorax. Soft-tissue redundancy and polyhydramnios were also noted.

Discussion:

This fetus manifests nearly all of the classic descripitons of the sonographic findings of thanatophoric dwarfism. Thanatophoric dysplasia is frequently sited as the most common lethal skeletal dysplasia with incidences in the literature varying from one in 4,000 to 3.8 per 100,000 births. Two types of this dysplasia have been described: Type I, a sporadic form in which there are short and bowed long bones without a cloverleaf skull and Type II which is autosomal recessive, characterized by short, straight limbs and a cloverleaf skull. In fact, some cases do not adhere to this division and there is disagreement about the inheritance.

Most fetuses with thanatophoric dysplasia will present with polyhydramnios, which has been described as occuring in 50-71% of cases. In addition to the severe limb shortening other skeletal features of this disease include a large head with frontal bossing as well as hyperteleorism and a saddle nose, flaring of the metaphyses, a small and contracted thorax and pelvis, flat spiculated acetabula, flattening of the vertebral bodies and soft tissue redundancy. A normal examination early in pregnancy may not exclude the diagnosis. The diagnosis may not be evident until the late second or third trimester with a normal study, early in gestation.

As was mentioned above, cloverleaf skull may be associated with severe limb shortening and when this association exists, the diagnosis of thanatophoric dysplasia may be made with confidence. It should be noted however, that cloverleaf skull may occur as an isolated entity. It is often associated with hydrocephalus, agenesis of the corpus callosum and mental retardation.

Differential Diagnosis:

There are a number of conditions which may cause marked limb shortening and a small thorax including: homozygous achondroplasia, achondrogenesis, congenital hypophosphatasia, asphyxiating thoracic dystrophy, short-rib polydactyly syndrome and type II osteogenesis imperfecta. The severe form of homozygous achondroplasia occurs only from two heterozygous achondroplastic dwarfs. Achondrogenesis and congenital hypophosphatasia are both marked by abnormal mineralization, not evident in this case. In addition, there was no evidence of polydactyly. The amniotic band syndrome may result in non-embryologic encephaloceles which may simulate cloverleaf skull.

References:

Fink IJ, Filly RA, Callen PW and Fiske CC. Sonographic diagnosis of thanatophoric dwarfism in utero. J Ultrasound Med 1:337-339, 1982

Macken MB, Grantmyre EB, Rimoin DL and Lachman RS. Normal sonographic appearance of a thanatophoric dwarf variant fetus at 13 weeks gestation. Amer J Roentgenol 156:149-150, 1991

Issacson G, Blakemore KJ, Chervenak FA. Thantophoric dysplasia with cloverleaf skull. Am J Dis Child 137:896898, 1983

Tabyi H and Lachman RS Ed., Radiology of syndromes, metabolic disorders, and skeletal dysplasias. 4th Ed., Mosby , St Louis

Sherer DM, Ghezzi F, Cohen J and Romero R. Fetal skeletal abnormalities In Sonography in Obstetrics and Gynecology: Principles and Practice, Ed by Arthur C. Fleischer, Frank A. Manning, Philippe Jeanty and Roberto Romero, 5th Ed., Appleton and Lange, Stamford

Spirt BA, Oliphant M, Gottlieb RH, Gordon LP. Prenatal sonographic evaluation of short-limbed dwarfism: An algorithmic approach. Radiographics 10:217-236, 1990

Mahony BS. Ultrasound evaluation of the fetal musculoskeletal system. In Ultrasonograpy in Obstetrics and Gynecology, Ed Peter W, Callen, M.D., 3rd Ed., WB Saunders and Co., Philadelphia

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Peter W. Callen, M.D.
Professor of Radiology, Obstetrics, Gynecology and Reproductive Science
University of California Medical Center, San Francisco, California