Hypochondroplasia has autosomal dominant inheritance. Most children born with hypochondroplasia have a new gene change in the FGFR3 gene. That means neither parent has hypochondroplasia.
Hypochondroplasia is caused by a mutation (gene change) in the fibroblast growth factor receptor-3 (FGFR3) gene. Typically, the FGFR-3 protein functions in a signaling pathway to slow linear growth in the growth plates of the long bones. The gene change that causes hypochondroplasia turns on the pathway more than it should, albeit to a lesser degree than achondroplasia, so bones grow slower and end up shorter than they typically would. This type of change associated with an increased ability is called a "gain-of-function" mutation.
Sometimes no gene change is found in FGFR3 testing even though someone has the signs and symptoms of hypochondroplasia. This doesn’t necessarily mean that someone does not have hypochondroplasia. Their gene change just might not be found by today’s technology. Or, they may have a change in a different gene that causes features similar to hypochondroplasia.
Hypochondroplasia causes disproportionate short stature. Like achondroplasia, the limbs are disproportionately small when compared to the trunk, but the disproportion can be subtle for hypochondroplasia. Overall height is usually two to three standard deviations below the mean during childhood, and adult heights range from 54" to 65" for men and 50" to 59" for women. Individuals with hypochondroplasia have average intelligence and can lead very rich and productive lives.
The diagnosis of hypochondroplasia is made based on clinical and radiographic features. Testing of the FGFR3 gene can also be done for confirmation although in some individuals with hypochondroplasia, a gene change cannot be identified.
For parents of typical stature, hypochondroplasia is rarely diagnosed prenatally or at birth. Clinical features tend to be subtle. In mild cases, it may remain undetected throughout life. The first sign of the condition may be a failure to achieve the normal pubertal growth spurt. Most typically however, hypochondroplasia is diagnosed in early childhood when growth differences are first noticed.
If there’s a family history of hypochondroplasia, an amniocentesis or CVS (chorionic villus sampling) during pregnancy can check the fetus for the gene change (if known).
Given the relationship between achondroplasia and hypochondroplasia, some medical issues associated with achondroplasia such as sleep apnea, foramen magnum stenosis, and lumbar spinal stenosis should be monitored for. However, they occur less frequently in hypochondroplasia than achondroplasia.
There are several medical problems which seem to occur more frequently in hypochondroplasia as compared to achondroplasia. Specifically, there appears to be an increased risk of temporal lobe epilepsy and learning disabilities for individuals with hypochondroplasia.
Signs and symptoms requiring orthopedic intervention are similar to those seen in achondroplasia including: genu varum (bowlegs) and spinal stenosis. In hypochondroplasia, these problems are relatively rare.
Most children with hypochondroplasia do not have every sign and symptom listed here. In fact, many children with hypochondroplasia do not have any of these medical problems. There is a lot of variability. People who have hypochondroplasia should get regular medical checkups. This helps doctors find and treat any medical problems right away.
Overall, we also recommend you talk about hypochondroplasia — both with your child and others — as a difference rather than a problem. Your attitude can help your child develop good self-esteem. Treat the child according to age and not size and encourage others to do the same. Also, find ways to adapt. For example, get a light switch extender and a stepstool so your child can turn the lights on and off and get to the sink independently to wash their hands. Encourage your child’s school to make modifications/accommodations for your child to maximize their independence in an age-appropriate way. Get involved with groups like the Little People of America. Getting to know other people with hypochondroplasia and other forms of dwarfism can help your child feel connected.
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