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8.0 INTRODUCTION

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DIAGNOSTIC APPROACH TO SUSPECTED SKELETAL DYSPLASIAS
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The overall incidence of a lethal skeletal dysplasia is 1/10,000. A detailed fetal anatomic survey is required to satisfactorily evaluate a fetus for a suspected skeletal dysplasia. However, specific key measurements, ratios, and anatomic landmarks will aid the sonographer/sonologist in determining not only if a skeletal dysplasia is present, but also if it is lethal.

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  • Because of the rarity of lethal skeletal dysplasias, a femur length less than 2 SD (standard deviations) of the mean is not necessarily diagnostic. However, when the femur length is 5 mm below 2 SD, the diagnosis of a skeletal dysplasia can be confirmed.

  • When a skeletal dysplasia is suspected, all of the long bones should be measured bilaterally to exclude a focal skeletal abnormality.

  • The appearance of the long bones (i.e., bowing, fractures, or degree of mineralization) may provide important clues in classifying the type of skeletal dysplasia.

  • Pre- or postaxial polydactyly and clubbed feet are associated with specific skeletal dysplasias.

  • The femur length/abdominal circumference ratio is normally between 0.20 and 0.24. A ratio of less than 0.16 in a population at risk is consistent with a severe skeletal dysplasia.

  • In a normal fetus, the femur and foot are of comparable length. A femur length/foot ratio of less than 0.87 discriminates between intrauterine growth restriction and a severe skeletal dysplasia.

  • A thoracic circumference at the level of the four-chamber view below fifth centile or a thoracic/abdominal circumference ratio less than 0.83 is consistent with severe skeletal dysplasia. A narrow chest and protruding abdomen subjectively suggest a diagnosis of severe skeletal dysplasia.

  • The presence of severe femur length shortening, in isolation, is insufficient to diagnose a severe skeletal dysplasia. Utilizing the previous criteria, the sonographic detection of lethal skeletal dysplasias is greater than 90%. However, the ability to accurately diagnosis the type of severe skeletal dysplasia is only 50%. A thorough postdelivery evaluation of the fetus is therefore required to confirm a diagnosis and to provide appropriate counseling to the family.

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SUGGESTED READINGS

1. +
Brons  JTJ, van der Harten  JJ, van Geijn  HP,  et al: Ratios between growth parameters for the prenatal ultrasonographic diagnosis of skeletal dysplasias. Eur J Obstet Gynecol Reprod Biol 1990; 34:37–46.  [PubMed: 2406168]
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Hadlock  FP, Harris  RB, Shah  Y, Park  SK: The femur length/head circumference relation in obstetric sonography. J Ultrasound Med 1984; 3:439–442.  [PubMed: 6387166]
3. +
Hersh  JH, Angle  B, Pietrantoni  M,  et al: Predictive value of fetal ultrasonography in the diagnosis of a lethal skeletal dysplasia. Southern Med J 1998; 91:1137–1142.  [PubMed: 9853726]
4. +
Krakow  D, Lachman  LS, Rimoin  DL: Guidelines for the prenatal diagnosis of fetal skeletal dysplasia. Genet Med 2009; 11:127–133.  [PubMed: 19265753]
5. +
Kurtz  AB, Needleman  L, Wapner  RJ,  et al: Usefulness of a short femur in the in-utero detection of ...

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