Thanatophoric Dysplasia Type 1

Thanatophoric dysplasia type 1 (TD1) is a form of TD (see this term) characterized by short, bowed femurs, micromelia, narrow thorax, and brachydactyly.

Epidemiology

The prevalence is unknown but it is more common than TD 2 (see this term).

Clinical description

TD1 presents in the prenatal period (in the first to second trimester) with growth deficiency of the limbs of less than 5%, bowed femurs (like a telephone receiver), shortened ribs, and platyspondyly of the vertebrae. Distinctive facial features include macrocephaly, large anterior fontanel, frontal bossing, proptosis and low nasal bridge. Neonates usually die shortly after birth due to respiratory insufficiency and/or spinal cord/brain stem compression. Platyspondylic lethal skeletal dysplasia, San Diego type (PTSD-SD) is now thought to be an earlier fetal phenotype of TD1, and is no longer characterized as a distinct dysplasia.

Etiology

TD1 is caused by one of several distinct missense mutations in the fibroblast growth factor receptor 3 (FGFR3) gene, located to chromosome 4p16.3.

Genetic counseling

TD1 is inherited autosomal dominantly but the majority of cases are due to a de novo mutation in the proband. Genetic counseling allows families who have already had one child with TD1 to know that recurrence rate is about 2%, so their risk of having a healthy child is high.