Definition

Achondroplasia is a disorder of bone growth that causes the most common type of dwarfism.

Alternative Names

Dwarf; Dwarfism; Achondroplastic dwarfism

Causes

Achondroplasia is one of a group of conditions called chondrodystrophies or osteochondrodysplasias. These rare genetic disorders cause problems with the development of bone, cartilage, and connective tissue.

Achondroplasia is caused by a change (mutation) in the fibroblast growth factor receptor 3 (FGFR3) gene. This prevents bone growth and mainly affects the long bones in the arms and legs. People with achondroplasia have a short stature, with an average height under 4 feet 6 inches (137 centimeters).

Achondroplasia may be inherited as an autosomal dominant trait. This means that if a child gets the defective gene from one parent, the child will have the disorder. If one parent has achondroplasia, the infant has a 50% chance of inheriting the disorder. If both parents have the condition, the infant's chance of being affected increases to 75%.

However, most cases (about 80%) appear as spontaneous mutations. This means that two parents without achondroplasia may give birth to a baby with the condition.

Symptoms

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
• 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 "water on the brain" (hydrocephalus).

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.

Treatment

There is no specific treatment for achondroplasia. The disorder may cause certain health conditions, which need to be treated when they cause problems.

• Obstructive sleep apnea can be treated with surgery and continuous positive airway pressure (CPAP) devices.
• Children who have recurring ear infections may need surgery to place tubes in their eardrums.
• Obesity can put extra strain on the body, so learning to eat a healthy diet and avoid weight gain from a young age is important.
• Bracing may be needed to help correct upper spine curvature (kyphosis).
• Hydrocephalus can be treated with surgery (ventriculoperitoneal shunting).
• Physical therapy can help if gait is affected due to lordosis (curving of the lower back).
• Corrective surgery may be needed if there are problems walking due to bowed legs.
• Spine surgery may be needed to relieve pressure on the spinal cord due to spinal stenosis or spinal cord compression at the base of the skull.

Vosoritide (Voxzogo) is a new medicine approved for use in children age 5 years and older who are still growing. This once daily injection may slightly improve bone growth.

Children and adults will need adaptations to their home, school, and work environment to accommodate a shorter stature.

Support Groups

Being physically different from others can be difficult for children who may be made fun of or bullied. Even in adulthood, people with dwarfism may be stared at or treated differently. Connecting with others affected by the disorder can help children and their families feel less alone.

More information and support for people with achondroplasia and their families can be found at:

• Human Growth Foundation - www.hgfound.org/support-groups
• Little People of America - www.lpaonline.org
• MAGIC Foundation - www.magicfoundation.org

Outlook (Prognosis)

Adult heights typically are 46 to 57 inches (1.18 to 1.45 meters) for men and 44 to 54 inches (1.12 to 1.36 meters) for women. Intelligence is in the normal range. Infants who receive the abnormal gene from both parents do not often live beyond a few months.

People with achondroplasia should have regular checkups throughout their life, as the condition can cause other health problems. Most problems can be treated.

Possible Complications

Complications that may develop include:

• Breathing problems from a small upper airway and from pressure on the area of the brain that controls breathing
• Lung problems from a small ribcage
• Narrowing of the spinal column (spinal stenosis)
• Pressure on the spinal column at the base of the neck
• Hydrocephalus
• Back pain and joint problems

When to Contact a Medical Professional

If there is a family history of achondroplasia and you plan to have children, you may find it helpful to speak to your health care provider.

Prevention

Genetic counseling may be helpful for prospective parents when one or both have achondroplasia. However, because achondroplasia most often develops spontaneously, prevention is not always possible.

References

Hoover-Fong JE, Horton WA, Hecht JT. Disorders involving transmembrane receptors. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 716.

Krakow D. FGFR3 disorders: thanatophoric dysplasia, achondroplasia, and hypochondroplasia. In: Copel JA, D'Alton ME, Feltovich H, et al, eds. Obstetric Imaging: Fetal Diagnosis and Care. 2nd ed. Philadelphia, PA: Elsevier; 2018:chap 50.

Legare JM. Achondroplasia. 1998 Oct 12 [Updated 2023 May 11]. In: Adam MP, Feldman J, Mirzaa GM, et al, eds. GeneReviews [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2023. www.ncbi.nlm.nih.gov/books/NBK1152/.

National Organization for Rare Disorders website. Achondroplasia. rarediseases.org/rare-diseases/achondroplasia/. Updated November 17, 2023. Accessed November 18, 2023.