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What is hypochondroplasia?
The diagnosis is often made at 2-4 years of age when the child falls off the growth curve and the rhizomelic shortening is noted. Diagnosis is made by exam and X-rays.
Infants with hypochondroplasia have low normal birth weights and lengths. The forehead is often prominent. The extremities are short with milder shortening of the proximal portions as compared to achondroplasia. Mild bow-legs can occur. Range of motion at the elbows is often limited. The fingers are short. The back shows increased lumbar curve (lordosis). The final adult height is 125-160cm (49-63 inches).
Hypochondroplasia is inherited in an autosomal dominant condition. This means that each child born to a person with hypochondroplasia has a 50% or 1 in 2 chance of having hypochondroplasia. Many children with hypochondroplasia are born to average-sized parents. In these cases, the hypochondroplasia is due to a new mutation or genetic change.
The gene for hypochondroplasia has been found. It is the same gene as is responsible for achondroplasia and is called the fibroblast growth factor receptor 3 (FGFR3). Although the same gene causes these two conditions, different changes occur in that gene (FGFR3) to produce the two different disorders.