Achondroplasia is a disorder of bone growth. It is the most common form of disproportionate short stature. It occurs in one in every 15,000 to one in 40,000 live births.
Achondroplasia is a disorder of bone growth that prevents the changing of cartilage (particularly in the long bones of the arms and legs) to bone. It is characterized by dwarfism, limited range of motion at the elbows, large head size (macrocephaly), small fingers, and normal intelligence.
Achondroplasia is caused by a gene alteration (mutation) in the FGFR3 gene. The FGFR3 gene makes a protein called fibroblast growth factor receptor 3 that is involved in converting cartilage to bone. FGFR3 is the only gene known to be associated with achondroplasia. All people who have only a single copy of the normal FGFR3 gene and a single copy of the FGFR3 gene mutation have achondroplasia.
Achondroplasia can cause health complications such as interruption of breathing (apnea), obesity, recurrent ear infections, an exaggerated inward curve of the lumbar spine (lordosis). More serious problems include a narrowing of the spinal canal that can pinch (compress) the upper part of the spinal cord (spinal stenosis) and a buildup of fluid in the brain (hydrocephalus).
Some people with achondroplasia may have delayed motor development early on, but cognition is normal.
People who have achondroplasia have abnormal bone growth that causes the following clinical symptoms: short stature with disproportionately short arms and legs, short fingers, a large head (macrocephaly) and specific facial features with a prominent forehead (frontal bossing) and mid-face hypoplasia.
Infants born with achondroplasia typically have weak muscle tone (hypotonia). Because of the hypotonia, there may be delays in walking and other motor skills. Compression of the spinal cord and/or upper airway obstruction increases the risk of death in infancy.
Adults with achondroplasia may develop a pronounced and permanent sway of the lower back (lordosis) and bowed legs. The problems with the lower back can cause back pain leading to difficulty with walking.
Achondroplasia is diagnosed by characteristic clinical and X-ray findings in most affected individuals. In individuals who may be too young to make a diagnosis with certainty or in individuals who do not have the typical symptoms, genetic testing can be used to identify a mutation in the FGFR3 gene.
Genetic testing can identify mutations in 99 percent of individuals who have achondroplasia. Testing for the FGFR3 gene mutation is available in clinical laboratories.
No specific treatment is available for achondroplasia. Children born with achondroplasia need to have their height, weight and head circumference monitored using special growth curves standardized for achondroplasia. Measures to avoid obesity at an early age are recommended.
A magnetic resonance imaging (MRI) or CT scan may be needed for further evaluation of severe muscle weakness (hypotonia) or signs of spinal cord compression. To help with breathing, surgical removal of the adenoids and tonsils, continuous positive airway pressure (CPAP) by nasal mask, or a surgical opening in the airway (tracheostomy) may be needed to correct obstructive sleep apnea.
When there are problems with the lower limbs, such as hyperreflexia, clonus or central hypopnea, then surgery called suboccipital decompression is performed to decrease pressure on the brain.
Children who have achondroplasia need careful monitoring and support for social adjustment.
Most cases of achondroplasia are not inherited. When achondroplasia is inherited, it is inherited in an autosomal dominant manner.
Over 80 percent of individuals who have achondroplasia have parents with normal stature and are born with achondroplasia as a result of a new (de novo) gene alteration (mutation). These parents have a small chance of having another child with achondroplasia.
A person who has achondroplasia who is planning to have children with a partner who does not have achondroplasia has a 50 percent chance, with each pregnancy, of having a child with achondroplasia. When both parents have achondroplasia, the chance for them, together, to have a child with normal stature is 25 percent. Their chance of having a child with achondroplasia is 50 percent. Their chance for having a child who inherits the gene mutation from both parents (called homozygous achondroplasia – a condition that leads to death) is 25 percent.
Achondroplasia is the most common type of short-limbed dwarfism. The condition occurs in 1 in 15,000 to 40,000 newborns worldwide.
Some populations appear to have a higher incidence of achondroplasia. For instance, it is estimated to occur in about 1 case in 6400 births in Denmark and about 1 case in 10,000 births in Latin America. No particular race has been documented to be more commonly affected.