What is achondroplasia?
Achondroplasia is a genetic (inherited) bone disorder that occurs in one in 25,000 live births. Achondroplasia is the most common type of dwarfism, in which the child’s arms and legs are short in proportion to body length. Further, the head is often large and the trunk is normal size.
The average height of adult males with achondroplasia is 52 inches (or 4 feet, 4 inches). The average height of adult females with achondroplasia is 49 inches (or 4 feet, 1 inch).
What causes achondroplasia?
Achondroplasia is inherited by an autosomal dominant gene that causes abnormal cartilage formation. Autosomal dominant inheritance means that the gene is located on one of the autosomes (chromosome pairs 1 through 22). This means that males and females are equally affected. Dominant means that only one gene is necessary to have the trait.
When a parent has a dominant trait, there is a 50 percent chance that any child they have will also inherit the trait. So, in some cases, the child inherits the achondroplasia from a parent with achondroplasia. The majority of achondroplasia cases (80 percent), however, are the result of a new mutation in the family – the parents are of average height and do not have the abnormal gene.
As mentioned, persons with achondroplasia have a 50 percent chance to pass the gene to a child, resulting in the condition. If both parents have achondroplasia, with each pregnancy, there is a 50 percent chance to have a child with achondroplasia, a 25 percent chance that the child will not inherit the gene and be of average height, and a 25 percent chance that the child will inherit one abnormal gene from each parent, which can lead to severe skeletal problems that often result in early death.
Geneticists have found that fathers who are older than 45 have a higher chance of having children with certain autosomal dominant conditions such as achondroplasia, but no cause for the new mutations in sperm has been discovered at this time.
The gene responsible for achondroplasia was discovered in 1994 making accurate prenatal diagnosis available, in most cases.
What are the symptoms of achondroplasia?
The following are the most common symptoms of achondroplasia. However, each child may experience symptoms differently. Symptoms may include:
- Small vertebral canals (back bones) – may lead to spinal cord compression in adolescence. Occasionally children with achondroplasia may die suddenly in infancy or early childhood in their sleep due to compression of the upper end of the spinal cord, which interferes with breathing.
- Shortened arms and legs, with the upper arms and thighs more shortened than the forearms and lower legs
- Large head size with prominent forehead and a flattened nasal bridge
- Crowded or misaligned teeth
- Curved lower spine – a condition also called lordosis (or “sway-back”) which may lead to kyphosis, or the development of a small
- Hump near the shoulders that usually goes away after the child begins walking.
- Bowed lower legs
- Flat feet that are short and broad
- Extra space between the middle and ring fingers (Also called a trident hand.)
- Poor muscle tone and loose joints
- Frequent middle ear infections which may lead to hearing loss
- Normal intelligence
- Delayed developmental milestones such as walking (which may occur between 18 to 24 months instead of around one year of age)
The symptoms of achondroplasia may resemble other problems or medical conditions. Always consult your child’s physician for a diagnosis.
How is achondroplasia diagnosed?
Achondroplasia can be diagnosed before birth by fetal ultrasound or after birth by complete medical history and physical examination. DNA testing is now available before birth to confirm fetal ultrasound findings for parents who are at increased risk for having a child with achondroplasia. MRI is the study of choice to diagnose spinal cord or nerve root compression.
Treatment of achondroplasia
When present, spinal cord compression can be very successfully treated with various operations to decompress the spinal canal. Frequently these occur at the top of the spinal column or at the lower end. Both are commonly treated by our pediatric neurosurgeons.
Currently, there is no way to prevent or treat achondroplasia, since the majority of cases result from unexpected new mutations. Treatment with growth hormone does not substantially affect the height of an individual with achondroplasia. Leg-lengthening surgeries may be considered, in some very specialized cases.
Detection of bone abnormalities, particularly in the back, are important to prevent breathing difficulties and leg pain or loss of function.
Kyphosis (or hunch-back) may need to be surgically corrected if it does not disappear when the child begins walking. Surgery may also help bowing of the legs. Ear infections need to be treated immediately to avoid the risk of hearing loss. Dental problems may need to be addressed by an orthodontist (dentist with special training in the alignment of teeth.)
There is research into the family of genes called fibroblast growth factors, in which the gene that causes achondroplasia is included. The goal is to understand how the faulty gene causes the features seen in achondroplasia, in order to lead to improved treatment. These genes have been linked to many heritable skeletal disorders.