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Findings: On the AP pelvis view, there is a narrowing of the interpediculate distance in the lumbar spine. There are squared iliac wings, bilateral narrowed sacrosciatic notches, bilaterally flattened acetabulate, and shortened proximal femoral metaphyses. On the lateral thoracolumbar spine view, there are bullet shaped vertebral bodies in the lumbar spine. There is narrowing of the spinal canal, posterior scalloping of the vertebral bodies in the lumbar spine, and widened intervertebral disc spaces. There is slight kyphosis in the lumbar spine.

Diagnosis: Achondroplasia.

Discussion: Of the three types of short-limbed dwarfism (rhizomelia, mesomelia, acromelia), achondroplasia causes rhizomelic dwarfism. In rhizomelic dwarfism, the proximal segments of the extremities are predominantly affected. This includes the humeri and femora.

The mode of inheritance for achondroplasia is autosomal dominant, and this disorder is the most common nonlethal skeletal dysplasia. It occurs in approximately 1 per 26,000 live births.

The clinical manifestations of achondroplasia include short-limbed, short-trunk dwarfism, large head with prominent forehead, trident hands, gibbus in the thoracolumbar region in infants, and normal mentality.

The radiologic manifestations include a large skull with relatively small base and a narrow foramen magnum, megaencephaly, dilatation of the lateral ventricles, short flat vertebral bodies (bullet shaped in early life), lack of normal increase in interpediculate distance from upper lumbar vertebrae caudally, and narrow vertebral canal. Other findings include champagne appearance of the pelvis, squared iliac wings, short narrow sciatic notch, and notched growth plates.

Prenatal diagnosis can be made with sonography. Heterozygous achondroplasia cannot be diagnosed during the second trimester as growth falls off during the third trimester. Homozygous achondroplasia can be diagnosed in the second trimester, with findings including short long bones, flat vertebral body, and a large skull.

References:
Blickman JG. Pediatric Radiology: The Requisites. 1994. Mosby.

Taybi TL and Lachman RS. Radiology of Syndromes, Metabolic Disorders and
Skeletal Dysplasias. 1990. Mosby.

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