CHAPTER 8: SKELETAL ABNORMALITIES

8.0 INTRODUCTION

DIAGNOSTIC APPROACH TO SUSPECTED SKELETAL DYSPLASIAS

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.

• 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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