Hypochondroplasia is a developmental disorder caused by an autosomal dominant genetic defect in the fibroblast growth factor receptor 3 gene (FGFR3) that results in a disproportionately short stature, micromelia, and a head that appears large when compared with the underdeveloped portions of the body. It is also known as "achondroplasia tarda" and "atypical achondroplasia."
* Small stature * Large head * Short limbs * Short tubular bones * Mild metaphyseal flare
This disorder results from mutations in the proximal tyrosine kinase domain of the FGFR3 gene. This gene plays an important role in embryonic development, playing a part in regulating activities such as cell division, migration, and differentiation.
Hypochondroplasia is diagnosed by the recognition of characteristic clinical and radiologic findings that remain controversial. The diagnosis is difficult to make in children under the age of three years, as skeletal disproportion tends to be mild and many of the radiographic features are subtle during infancy. DNA-based testing is available and about 70% of affected individuals are heterozygous for a mutation in the FGFR3 gene. However, it is clear that locus heterogeneity exists because mutations in other as-yet-unidentified genes can result in similar, if not identical, phenotypes.