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 is the most common type of disproportionate dwarfism.
The disorder occurs in approximately one in 25,000 live births, according to the Children’s Hospital of Philadelphia (CHOP). It is as common in males as females.
During early fetal development, much of the skeleton is made up of cartilage. Normally, most cartilage converts to bone. However, in people with dwarfism, a lot of the cartilage does not convert. This is caused by mutations in the FGFR3 gene.
The FGFR3 gene instructs the body to make a protein necessary for bone growth and maintenance. The mutations cause the protein to be overactive. This interferes with normal skeletal development.
In more than 80 percent of cases, achondroplasia is not 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, causing achondroplasia
- a 25 percent chance of inheriting two defective genes. This results in a fatal form of achondroplasia called homozygous achondroplasia. Infants with this condition are generally stillborn or die within a few months of birth.
If there is any 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 abnormalities are only physical.
At birth, a child with this condition will likely have:
- short stature (significantly below average for age and sex)
- short arms and legs (especially the upper arms and thighs) in comparison to body height
- short fingers (the ring and middle fingers may also point away from each other)
- disproportionately large head compared to the body
- abnormally large forehead
- underdevelopment of the mid-face region (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 (brief periods where breathing is slowed or stops)
- hydrocephalus (water on the brain)
- spinal stenosis (a narrowing of the spinal canal that can compress the spinal cord)
Children and adults with achondroplasia may:
- have difficulty bending their elbows
- struggle with obesity
- experience recurrent ear infections, because of 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 National Institutes of Health (NIH, 2011). It is rare for an adult with achondroplasia to reach 5 feet in height.
Your doctor may diagnose your child with achondroplasia while you are 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. This tests look for the defective FGFR3 gene in a sample of amniotic fluid.
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 is no cure or specific treatment for achondroplasia. If any complications arise, then your doctor will address those issues. For example, 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 have not yet been established.
According to NYU Langone Medical Center, most people with achondroplasia have a normal life span. However, there is a slightly increased risk of death during the first year of life (NYU, 2011. There may also be an increased risk of heart disease later in life.
People with achondroplasia may need to make some physical adaptations, such as avoiding impact sports that could damage the spine. However, they can still grow up and have a full and interesting life.