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Skeletal dysplasias

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    Skeletal     dysplasia
    Polydactyly and     syndactyly


Introduction Antenatal Postnatal West Midlands Data


Skeletal dysplasias are a heterogeneous group of congenital anomalies which affect cartilage and/or bone growth. There are many specific syndromes which are often very precisely characterised, down to knowledge of the specific gene defect in many cases, with only subtle differences between classifications. Individually these syndromes have a low birth prevalence.

Skeletal dysplasias are generally caused by a single gene defect and most are autosomal recessive, however some types are autosomal dominant (e.g. achondroplasia) or caused by teratogens (e.g. warfarin). The syndromes in this group have certain features in common, and allow for a general overview of these conditions. The long bones of the limbs and the ribs are the structures most often affected, and many of the syndromes in this group have additional features which help to make the diagnosis.

The shortening of the long bones may primarily affect the proximal bones (the femur and humerus), called rhizomelic shortening or primarily affect the distal bones (the tibia/fibula and radius/ulna), called mesomelic. The long bones may be curved or so weak that multiple fractures occur, leading to angulation within the long bones.

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The diagnosis of skeletal dysplasia is usually first suspected by ultrasound measurement of the long bones. The femur is traditionally measured, although once a suspicion of an abnormality has been raised all the long bones and the foot length are helpful in establishing a diagnosis. The appearances of the chest are crucial to the outcome and the thoracic circumference should be measured and compared to standard charts and the abdominal circumference. The chest has a distinctive picture on longitudinal section with the fetal head, chest and abdomen together resembling a champagne cork image , with the chest being narrow and the abdomen relatively much larger. The heart, although normal sized, will appear large in the chest and this may be the first abnormality seen.

Once a diagnosis of skeletal dysplasia has been made the outcome must be discussed frankly with the parents. Many will opt for termination of pregnancy, and following delivery a careful inspection of the fetus, X-rays of the whole baby and postmortem examination by an experienced perinatal pathologist will be necessary to establish the diagnosis. If possible fetal blood or amniocytes should be cultures to allow DNA extraction to take place so that a genetic diagnosis can be made where possible. This will give the option of early prenatal diagnosis in a future pregnancy using chorion villus sampling at 11 weeks gestation. If DNA diagnosis is not possible the diagnosis may not be clearly established until 20 weeks, or even later in some cases.

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In the majority of skeletal dysplasias the outcome for the baby is governed by the length of the ribs. Most types will cause severe shortening of the ribs leading to a narrow malformed chest which causes the lung development to be so restricted that the baby will die immediately after birth. This pulmonary hypoplasia rather than the severity of the limb malformations makes the majority of these conditions lethal.

Whilst most skeletal dysplasias are evident at birth some cases of milder dysplasia present later in childhood (hypochondroplasia, mesomelic dwarfism). It is important to establish a diagnosis by clinical examination and radiological skeletal survey.

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To be added

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© Perinatal Institute 2011