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Hypophosphatasia

Hypophosphatasia - Non-ossified vertebra
Hypophosphatasia - Non-ossified vertebra

Findings:

Coronal and sagittal sonographic images of a fetus with hypophosphatasia. The spine demonstrates numerous unossified segments, particularly in the thoracic area In addition, the fetus also demonstrated marked limb shortening as well as demineralization of the calvarium.

Discussion:

Hypophosphatasia may present as an autosomal recessive (severe lethal) or an autosomal dominant form of the disease. It is characterized as a short-limbed dwarfism with abnormal (poor) bone mineralization, low or deficient alkaline phosphatase and high urine phosphoethanolamine. The incidence of the lethal recessive form of the disease is said to be 1 in 100,000 births. The sonographic findings of this condition were described by DeLange and Rouse to be: "1) general underossification of the bones of the fetus, 2) limb shortening, 3) lack of ossification of groups of vertebral bodies, 4) lack of ossification of the neural arches of the spine and 5) lack of ossification of the hands." In addition, the pelvic bones are small, and there may be major portions of the calvaria, base of skull and facial bones which are not calcified. Of the various sonographic findings the most notable in addition to the limb shortening are the marked demineralization of the fetal calvarium and absent segments of the spine. Patients with the heterozygous form of the disease are often normal or may be mildly affected.

Differential Diagnosis:

This condition may be indistinguishable from osteogenesis imperfecta (Type II). The sporadic segmental vertebral lack of ossification described by Delange may be a helpful differentiating feature, if present. Likewise, in contrast to the thickened bones of osteogenesis type II, the long bones in hypophosphatasia tend to be thin or may be absent. Campomelic dwarfism may also involve an absent spinal segment. Achondrogenesis may also cause lack of ossification of the spine however it is the bodies of the spine that are not ossified as opposed to the neural arches in hypophosphatasia. In addition, the calvarium will be ossified in achondrogenesis as opposed to hypophatasia where it will be absent.

References:

DeLange M and Rouse GA. Prenatal diagnosis of hypophosphatasia. J Ultrasound Med 9:115-117, 1990

Wladimiroff JW, Niermeijer MF, Van der Harten JJ, Stewart PA et al. Early prenatal diagnosis of congenital hypophosphatasia: Case report. Prenatal diagnosis 5:47-52, 1985

Bowerman RA. Anomalies of the fetal skeleton: Sonographic findings. Amer J Roentgenol 164:973-979, 1995

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. 1994, WB Saunders and Co., Philadelphia

Tongsong T, Pongsatha S. Early prenatal sonographic diagnosis of congenital hypophosphatasia. Ultrasound Obstet Gynecol 15:252-255, 2000

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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