Achondroplasia is a form of short-limbed dwarfism. The word achondroplasia literally means "without cartilage formation." However, the problem is not in forming cartilage but in converting it to bone (a process called ossification), particularly in the long bones of the arms and legs.
All people with achondroplasia have short stature. The average height of an adult male with achondroplasia is 131 centimeters (4 feet, 4 inches), and the average height for adult females is 124 centimeters (4 feet, 1 inch).
People with achondroplasia are generally of normal intelligence. The typical appearance of achondroplastic dwarfism can be seen at birth.
Symptoms may include:
- Short height
- Short upper arms and thighs (compared to the forearms and lower legs)
- Large head and forehead with a flat bridge of the nose
- Dental problems, including crowded or crooked teeth
- Broad, flat feet, short toes and short fingers
- Trident hand, a condition in which you have an extra space between the middle and ring fingers
- Weak muscle tone. Babies with weak muscle tone may have delays in meeting developmental milestones, like sitting, standing and walking.
- Bowed legs. This is when legs curve outward between the thighs and ankles. Bowed legs can cause pain and trouble with walking. If the bowing or pain is severe, surgery can fix bowed legs.
Health problems commonly associated with achondroplasia include episodes in which breathing slows or stops for short periods (apnea), obesity, and recurrent ear infections. In adulthood, individuals with the condition usually develop a pronounced and permanent sway of the lower back (lordosis) and bowed legs. Older individuals often have back pain, which can cause difficulty with walking.
How common is achondroplasia?
Achondroplasia is the most common type of short-limbed dwarfism. The condition occurs in 1 in 15,000 to 40,000 newborns.
What genes are related to achondroplasia?
Mutations in the FGFR3 gene cause achondroplasia.
The FGFR3 gene provides instructions for making a protein that is involved in the development and maintenance of bone and brain tissue. Two specific mutations in the FGFR3 gene are responsible for almost all cases of achondroplasia. Researchers believe that these mutations cause the protein to be overly active, which interferes with skeletal development and leads to the disturbances in bone growth seen with this disorder.
How do people inherit achondroplasia?
Achondroplasia is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. About 80 percent of people with achondroplasia have average-size parents; these cases result from a new mutation in the FGFR3 gene. In the remaining cases, people with achondroplasia have inherited an altered FGFR3 gene from one or two affected parents. Individuals who inherit two altered copies of this gene typically have very severe problems with bone growth, and are usually stillborn or die shortly after birth from respiratory failure.
Achondroplasia and Pregnancy
During pregnancy, a prenatal ultrasound may show excessive amniotic fluid surrounding the unborn infant. Examination of the infant after birth shows increased front-to-back head size. There may be signs of hydrocephalus ("water on the brain"). X-rays of the long bones can reveal achondroplasia in the newborn.
There is no specific treatment for achondroplasia. Related abnormalities, including spinal stenosis and spinal cord compression, should be treated when they cause problems.