Achondroplasia is a disorder of bone growth that causes the most common type of dwarfism.
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.
Most people who have achondroplasia have average-size parents. In this situation, the FGFR3 gene mutation occurs in one parent’s egg or sperm cell before conception. Other people with achondroplasia inherit the condition from a parent who has achondroplasia.
Achondroplastic dwarfism results in the following:
- Unusually large head
- Large forehead and flat bridge of the nose
- Crowded or crooked teeth
- Short stature (well below the average height for a person of the same age and sex)
- Average size trunk with short arms and legs (especially the upper arms and thighs)
- Bowed legs
- Limited range of motion of the elbows
- Short stature (significantly below the average height for a person of the same age and sex)
- Spine curvatures called kyphosis and lordosis
- Short fingers with an extra space between the ring and middle finger (trident hand)
- Decreased muscle tone in infants
Almost all people with achondroplasia have normal intelligence and will have a normal lifespan. Children with the disorder may develop at a slower rate than other children the same age.
Exams and Tests
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. X-rays of the long bones can reveal achondroplasia in the newborn.
CT scan or MRI of the brain may be done if there are signs of hydrocephalus (“water on the brain”).
Neuroimaging is needed to check the base of the head to look for signs of spinal cord compression.
Sleep studies help detect problems with sleep apnea.
Children with achondroplasia will need ongoing monitoring as they grow to check for health problems that can develop.
What is the treatment for achondroplasia?
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.
Is achondroplasia inherited?
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.