What is achondroplasia?

Achondroplasia is a bone growth disorder that causes disproportionate dwarfism. Dwarfism is defined as a condition of short stature as an adult. People with achondroplasia are short in stature with a normal sized torso and short limbs. It’s the most common type of disproportionate dwarfism.

The disorder occurs in approximately 1 in 25,000 live births, according to Columbia University Medical Center (CUMC). It’s equally common in males and females.

During early fetal development, much of your skeleton is made up of cartilage. Normally, most cartilage eventually converts to bone. However, if you have achondroplasia, a lot of the cartilage doesn’t convert to bone. This is caused by mutations in the FGFR3 gene.

The FGFR3 gene instructs your body to make a protein necessary for bone growth and maintenance. Mutations in the FGFR3 gene cause the protein to be overactive. This interferes with normal skeletal development.

In more than 80 percent of cases, achondroplasia isn’t inherited, according to the National Human Genome Research Institute (NHGRI). These cases are caused by spontaneous mutations in the FGFR3 gene.

About 20 percent of cases are inherited. The mutation follows an autosomal dominant inheritance pattern. This means that only one parent needs to pass down a defective FGFR3 gene for a child to have achondroplasia.

If one parent has the condition, the child has a 50 percent chance of getting it.

If both parents have the condition, the child has:

  • a 25 percent chance of normal stature
  • a 50 percent chance of having one defective gene that causes achondroplasia
  • a 25 percent chance of inheriting two defective genes, which would result in a fatal form of achondroplasia called homozygous achondroplasia

Infants born with homozygous achondroplasia are usually stillborn or die within a few months of being born.

If there’s a history of achondroplasia in your family, you may want to consider genetic testing prior to becoming pregnant so that you fully understand your future child’s health risks.

People with achondroplasia generally have normal intelligence levels. Their symptoms are physical, not mental.

At birth, a child with this condition will likely have:

  • a short stature that’s significantly below average for age and sex
  • short arms and legs, especially the upper arms and thighs, in comparison to body height
  • short fingers in which the ring and middle fingers may also point away from each other
  • a disproportionately large head compared to the body
  • an abnormally large, prominent forehead
  • an underdeveloped area of the face between the forehead and upper jaw

Health problems an infant may have include:

  • decreased muscle tone, which can cause delays in walking and other motor skills
  • apnea, which involves brief periods of slowed breathing or breathing that stops
  • hydrocephalus, or “water on the brain”
  • spinal stenosis, which is a narrowing of the spinal canal that can compress the spinal cord

Children and adults with achondroplasia may:

  • have difficulty bending their elbows
  • be obese
  • experience recurrent ear infections due to narrow passages in the ears
  • develop bowed legs
  • develop an abnormal curvature of the spine called kyphosis or lordosis
  • develop new or more severe spinal stenosis

The average height for adults with achondroplasia is 4 feet, 4 inches for males and 4 feet, 1 inch for females, according to The Children’s Hospital of Philadelphia (CHOP). It’s rare for an adult with achondroplasia to reach a height of 5 feet.

Your doctor may diagnose your child with achondroplasia while you’re pregnant or after your infant is born.

Diagnosis during pregnancy

Some characteristics of achondroplasia are detectable during an ultrasound. These include hydrocephalus, or an abnormally large head. If your doctor suspects achondroplasia, genetic tests may be ordered. These tests look for the defective FGFR3 gene in a sample of amniotic fluid, which is the fluid that surrounds the fetus in the womb.

Diagnosis after your child is born

Your doctor can diagnose your child by looking at his or her features. The doctor may also order X-rays to measure the length of your infant’s bones. This can help confirm a diagnosis. Blood tests may also be ordered to look for the defective FGFR3 gene.

There’s no cure or specific treatment for achondroplasia. If any complications arise, then your doctor will address those issues. For instance, antibiotics are given for ear infections and surgery may be performed in severe cases of spinal stenosis.

Some doctors use growth hormones to increase the growth rate of a child’s bones. However, their long-term effects on height haven’t been determined and are thought to be modest at best.

According to the NHGRI, most people with achondroplasia have a normal life span. However, there’s a slightly increased risk of death during the first year of life. There may also be an increased risk of heart disease later in life.

If you have achondroplasia, you may need to make some physical adaptations, such as avoiding impact sports that could damage the spine. However, you can still live a full life.