Key takeaways

  • Achondroplasia is a genetic bone growth disorder that causes disproportionate dwarfism, where individuals develop short limbs and a torso of typical size. It occurs in roughly 1 in 20,000 to 30,000 live births.
  • Most cases result from spontaneous genetic mutations rather than inheritance, though if one parent has the condition, their child has a 50% chance of inheriting it.
  • Individuals with achondroplasia usually have no cognitive impairment. There is no cure for achondroplasia, so treatment focuses on managing complications like ear infections or spinal stenosis.

Achondroplasia is a bone growth disorder that causes disproportionate dwarfism. Dwarfism is defined as a condition of short height as an adult.

People with achondroplasia are shorter than average, with a standard-sized torso and short limbs. It’s the most common type of disproportionate dwarfism.

The disorder occurs in approximately 1 in 20,000 to 30,000 live births, according to the National Organization for Rare Disorders (NORD). 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 typical skeletal development.

In roughly 80% of cases, achondroplasia isn’t inherited. These cases are caused by spontaneous mutations in the FGFR3 gene.

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

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

If both parents have the condition, the child has:

  • a 25% chance of normal stature
  • a 50% chance of having one defective gene that causes achondroplasia
  • a 25% 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.

Achondroplasia does not cause intellectual or cognitive impairment. Individuals with the condition develop a range of physical features.

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

  • a short height 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 at 22 weeks of gestation. These include bowing of the thighs and shortening of the femurs and upper arms.

If your doctor suspects achondroplasia, they may order genetic tests. These tests look for a specific variant of the 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

A doctor can diagnose achondroplasia after birth by looking at a child’s features. The doctor may also order X-rays to measure the length of an infant’s bones. This can help confirm a diagnosis. Blood tests may also be ordered to look for the FGFR3 gene variant.

There’s no cure for achondroplasia. Long-term management of the condition involves routine surveillance and treatment of any complications that may arise.

Common complications that may require treatment include:

  • marcrocephaly (large head)
  • foramen magnum stenosis (narrowing at the base of the skull near the brainstem and spinal cord)
  • hearing impairment
  • sleep apnea
  • spinal stenosis
  • bowed legs
  • obesity

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.

In recent years, the Food and Drug Administration (FDA) has approved a drug — Voxzogo (vosoritide) — to increase linear bone growth in children with achondroplasia. Studies suggest that it may increase growth by approximately 1.5 to 2.0 centimeters per year.

According to NORD, individuals with achondroplasia have a life span roughly 10 years shorter than average, owing to complications related to the heart and blood vessels.

If you’re living with achondroplasia, it’s best to receive regular checkups throughout your life to monitor for complications and receive specialty care.