Skeletal Dysplasias/Dwarfism

What are Skeletal Dysplasias?

Skeletal dysplasia is the medical term for what many people refer to as “dwarfism”. The term is an umbrella for a group of over 350 distinct disorders that affect bone and cartilage growth. Genetic defects have been identified for approximately 70% of SDs. A change or defect in a specific gene, called a mutation, causes the problem. The disease may run in families, but it can appear spontaneously, without a positive family history. A SD is present in approximately 1/4500 pregnancies.

What is the prognosis (outcome) for a fetus with CDH?

A fetus with SD may present with a range of findings. Poor bone mineralization, bone fractures, shortened limbs, abnormal ribs (which may affect development of the lungs and heart), and small size. In some cases, the SD may be lethal. Lethality is caused by small chest circumference that results in pulmonary hypoplasia (poor lung development). Two types of SD, thanatophoric dysplasia and achondrogenesis, account for over 60% of the lethal SDs.

The most common type of non-lethal SD is called achondroplasia. All people with achondroplasia have short stature. Features of achondroplasia include an average-size trunk, short arms and legs with particularly short upper arms and thighs, limited range of motion at the elbows, and an enlarged head (macrocephaly) with a prominent forehead. People with achondroplasia are generally of normal intelligence.

Osteogenesis imperfecta (OI) is another form of SD. There are multiple sub-types of OI, with a range in outcome from mild-to-severe bone fragility throughout life to disease which is lethal in utero.

As stated, specific symptoms of skeletal dysplasia vary with each type. Individuals with SDs may have abnormal shape and size of their arms, legs, trunk, and a skull. Other symptoms could include:

  • shortened stature
  • scoliosis
  • joint pain
  • fractured bones
  • limb absence
  • duplication of fingers or toes
  • club feet
  • developmental delays
  • mental retardation

What other tests should we consider?

Any fetus with a suspected SD should be referred to a specialized perinatal center such as the Fetal Medicine Unit at Mount Sinai. A detailed ultrasound should be performed to confirm the diagnosis and to exclude the presence of associated abnormalities. Ultrasound findings are not necessarily distinct for one specific disorder. Importantly, however, ultrasound can help differentiate lethal from non-lethal types of SD.

Genetics counseling is suggested to discuss the possible diagnosis (the genetic mutation), as well as to assess the risk of recurrence in a subsequent pregnancy.
Amniocentesis is a procedure that collects amniotic fluid from around the fetus for genetic testing. This can detect genetic causes of SD and help in counseling for the pregnancy and for recurrence risk.

Pediatrics consultation is also suggested to discuss the management of children with non-lethal SD after birth.

How are Skeletal Dysplasias managed?

Because of the wide range of SDs, the management of the fetus is determined on a case-by-case basis. Doctors from multiple specialties (MFM, genetics, pediatrics) review each case, and diagnosis, before deciding on the most appropriate treatment plan.

References
Superti-Furga A, Unger S. Nosology and classification of genetic skeletal disorders: 2006 revision. Am J Med Genet A 2007;143:1–18
Krakow, D et al. Guidelines for the prenatal diagnosis of fetal skeletal dysplasias. Genet Med. 2009 February ; 11(2): 127–133
Parilla, B et al. Antenatal Detection of Skeletal Dysplasias. J Ultrasound Med 2003;22:255–258

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