What is achondroplasia?

Achondroplasia is a genetic bone disorder, affecting one in 20,000 babies. It is the most common type of dwarfism in which the child's arms and legs are short in proportion to body length. 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), and the average height of adult females with achondroplasia is 49 inches (or 4 feet, 1 inch).

What are the symptoms of achondroplasia?

The following are the most common symptoms of achondroplasia, but each child may experience symptoms differently:

  • 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 
  • 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)
  • 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)

What causes achondroplasia?

In some cases, the child inherits the achondroplasia from a parent with the disorder but most cases — about 80 percent — are caused by a new mutation in the family. This means the parents are of average height and do not have the abnormal gene.

However, people with achondroplasia have a 50 percent chance to pass the gene to a child, resulting in the condition. If both parents have achondroplasia 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.

Fathers who are older than 45 years have a higher chance of having children with certain genetic disorders, including achondroplasia. At this time, researchers have not uncovered a particular mutations in sperm that is linked to the disorder. 

How we care for achondroplasia

Children with achondroplasia can be seen in the Genetics Program or the Maternal Fetal Care Program at Boston Children's Hospital.

Areas of innovation for achondroplasia

Researchers are looking into a family of genes called fibroblast growth factors. The gene that causes achondroplasia is included in this category. 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 other types of inherited skeletal disorders.