Hypochondroplasia

What is hypochondroplasia?

Hypochondroplasia is a form of short-limbed dwarfism. This condition affects the conversion of cartilage into bone (a process called ossification), particularly in the long bones of the arms and legs. Hypochondroplasia is similar to another skeletal disorder called achondroplasia, but the features tend to be milder.

All people with hypochondroplasia have short stature. The adult height for men with this condition ranges from 138 centimeters to 165 centimeters (4 feet, 6 inches to 5 feet, 5 inches). The height range for adult women is 128 centimeters to 151 centimeters (4 feet, 2 inches to 4 feet, 11 inches).

People with hypochondroplasia have particularly short arms and legs and broad, short hands and feet. Other characteristic features include a large head, limited range of motion at the elbows, and a sway of the lower back (lordosis). These signs are generally less pronounced than those seen with achondroplasia and may not be noticeable until early or mid-childhood. Some studies have reported that a small percentage of people with hypochondroplasia have mild to moderate mental retardation and learning disabilities, but other studies have produced conflicting results.

How common is hypochondroplasia?

The incidence of hypochondroplasia is unknown. Researchers believe that it may be about as common as achondroplasia, which affects 1 in 15,000 to 40,000 births. More than 200 people worldwide have been diagnosed with hypochondroplasia.

What genes are related to hypochondroplasia?

Mutations in the FGFR3 gene cause hypochondroplasia.

About 70 percent of hypochondroplasia cases are caused by mutations in the FGFR3 gene. The protein made by this gene is a receptor that plays a role in the development and maintenance of bone and brain tissue. It is not known how FGFR3 mutations lead to the disturbances in bone growth seen with this condition; however, it is likely that the mutations mildly overactivate the receptor, which interferes with normal ossification.

The causes of the other 30 percent of hypochondroplasia cases are unknown, but researchers suspect that mutations in other genes may be involved.

How do people inherit hypochondroplasia?

Hypochondroplasia is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. Most people with hypochondroplasia have average-size parents; these cases result from a new mutation in the FGFR3 gene. Other people with hypochondroplasia inherited an altered FGFR3 gene from a parent who has the condition.

What other names do people use for hypochondroplasia?

• HCH
• Hypochondrodysplasia